Nearshore Food Web Structure 

on the OCS in Cook Inlet 

Principal Investigators: 

Katrin Iken, Brenda Konar

College of Fisheries and Ocean Sciences, University of Alaska Fairbanks

Final Report 

OCS Study BOEM 2019-078 

January 2020
Contact Information:

email: UAF-CMI@alaska.edu

phone: 907.474.6782 

fax: 907.474.7204 

Coastal Marine Institute 

College of Fisheries and Ocean Sciences 

University of Alaska Fairbanks

P. O. Box 757220 

Fairbanks, AK 99775-7220 

This study was funded in part by the U.S. Department of the Interior, Bureau of Ocean Energy Management (BOEM) through 

Cooperative Agreement M17AC00003 between BOEM, Alaska Outer Continental Shelf Region, and the University of Alaska 

Fairbanks. This report, OCS Study BOEM 2019-078, is available through the Coastal Marine Institute, select federal 

depository libraries and can be accessed electronically at http://www.boem.gov/Alaska-Scientific-Publications. 

The views and conclusions contained in this document are those of the authors and should not be interpreted as representing 

the opinions or policies of the U.S. Government. Mention of trade names or commercial products does not constitute their 

endorsement by the U.S. Government.
iii 

Table of Contents 

List of Figures ................................................................................................................................ iv 

List of Tables ................................................................................................................................. iv 

Abstract ........................................................................................................................................... v 

Introduction ..................................................................................................................................... 1 

Methods........................................................................................................................................... 2 

Site description ............................................................................................................................ 2 

Sample collection ........................................................................................................................ 5 

Laboratory analyses ..................................................................................................................... 5 

Data analyses ............................................................................................................................... 6 

Results ............................................................................................................................................. 8 

Discussion ..................................................................................................................................... 18 

Acknowledgments......................................................................................................................... 23 

Study Products .............................................................................................................................. 23 

References ..................................................................................................................................... 25 
iv 

List of Figures 

Figure 1: Map of study area of Lower Cook Inlet with Kachemak Bay to the east and Kamishak Bay to 

the west ......................................................................................................................................................... 3 

Figure 2: Map of the Outer Continental Shelf (OCS) Cook Inlet Planning Area with lease sale areas in 

relation to the Kachemak Bay and Kamishak Bay study areas ..................................................................... 4 

Figure 3: Trophic niche space, represented by the convex hull area in δ13C – δ15N biplot space ................. 9 

Figure 4: Trophic niche space, represented by the convex hull area in δ13C – δ15N biplot space, for sites in 

Kachemak Bay and Kamishak Bay in 2017 ................................................................................................ 11 

Figure 5: Trophic niche space, represented by the convex hull area in δ13C – δ15N biplot space, for sites in 

Kachemak Bay and Kamishak Bay in 2018 ................................................................................................ 12 

Figure 6: Trophic niche space, represented by the convex hull area in δ13C – δ15N biplot space, for 

Kachemak Bay and Kamishak Bay compared between the two study years .............................................. 13 

Figure 7: POM values from all locations in Kachemak Bay and Kamishak Bay in 2017 and 2018 .......... 14 

Figure 8: POM isotope values taken in Kachemak Bay over several weeks in 2017 to assess short-term 

variability in POM isotope values ............................................................................................................... 15 

Figure 9: Results of a carbon source mixing model for Kachemak Bay and Kamishak Bay in 2017 and 

2018 ............................................................................................................................................................ 17 

List of Tables 

Table 1: Sampling site locations in each region and year ............................................................................. 4 

Table 2: Two-way ANOVA results for core measures of isotope space with factors region and year....... 14 

Table 3: Results of a two-way ANOVA for POM stable isotope values with factors region and year ...... 15 

Table 4: Regional values (for Kachemak and Kamishak Bays) of various isotope trophic diversity 

measures ...................................................................................................................................................... 16 

Table 5: Two-way ANOVA results for various isotope trophic diversity measures with factors region and 

year .............................................................................................................................................................. 16 

 
v 

Abstract 

Rocky intertidal communities in Lower Cook Inlet, Alaska, may be particularly vulnerable from 

impacts of planned and ongoing oil and gas extraction activities on the OCS in Cook Inlet. Here, 

food web structure of rocky intertidal communities in Kamishak Bay in lower western Cook Inlet 

was investigated and compared with food web structure of similar communities in Kachemak 

Bay, which comprises the same regional species pool but is hydrographically upstream of oil and 

gas activities. Common macroalgal and invertebrate members of three rocky intertidal 

communities were sampled in each region in 2017 and 2018. Particulate organic matter (POM) 

was collected offshore from the rocky intertidal at each site as it is a food source for intertidal 

invertebrates in addition to macroalgae. Food web structure was assessed using carbon and 

nitrogen stable isotope analyses. Carbon isotopes inform about the diversity at the base of the 

food web (endmembers) and the use of different carbon sources (here: POM versus macroalgae) 

by intertidal consumers. Nitrogen isotopes inform about trophic position of consumers and the 

length of the food web. Combined, these two metrics allow for the assessment of total trophic 

niche space for each region. The distribution of taxa within this niche space informs about 

resource use patterns in the respective regions. 

Trophic niche space in Kamishak rocky intertidal systems was larger, driven by larger ranges in 

both carbon and nitrogen isotope values, than in Kachemak Bay. The larger carbon isotope range 

in Kamishak Bay was mostly driven by the POM source being more depleted in 13C compared 

with POM in Kachemak Bay, likely because of the greater influence of freshwater and input of 

terrestrial matter in Kamishak Bay. A larger 15N range in Kamishak Bay intertidal pointed 

towards a longer food web, possibly driven by additional microbial processing steps being 

needed to render terrestrial matter components of POM labile to marine consumers. In both 

regions and years, consumers derived the majority of their carbon from macroalgae as opposed 

to POM. Generally, carbon sourcing from macroalgae was higher in Kamishak Bay compared 

with Kachemak Bay, possibly because of lower quality of POM in Kamishak Bay due to high 

levels of inorganic silt in the suspended matter. The results of this study suggest that intertidal 

organisms in Lower Cook Inlet have high feeding plasticity that can adapt to regionally different 

conditions as well as potentially different annual conditions. This plasticity, plus the ability to 

source carbon from multiple sources, increases the overall stability of these food webs against 

perturbations. However, it is not clear whether these taxa are at their limit of plasticity under 

current regional conditions, which could increase the potential for future climate-related or 

human-induced perturbations to exceed this plasticity.

 
1 

Introduction 

Rocky intertidal communities are complex systems strongly influenced by many static and 

dynamic environmental drivers as well as biological interactions (Menge 2001, Konar et al. 

2016). Top-down control of rocky intertidal communities, as from grazing and predation, has 

been the topic of intense and sometimes seminal studies (e.g., Paine 1966, Menge et al. 1986, 

Wootton 1995, Hori et al. 2006). Bottom-up factors have also garnered attention in rocky 

intertidal community studies examining how food supply from primary production can support 

these systems (e.g., Bustamante et al. 1995, Menge 2000, Wieters 2005). Combined, top-down 

and bottom-up forces create a network of energy flow through a system that is represented by its 

food web. 

Trophic relationships are an important component of community functioning, particularly as they 

relate to the use of main carbon sources. Intertidal communities and food webs are linked to both 

local and regional oceanographic conditions (Nielsen and Navarette 2004, Blanchette et al. 2006, 

2008). For example, productive upwelling conditions increase the carbon subsidies to intertidal 

food webs (Menge and Menge 2013). Estuarine systems provide another example of the impact 

of strong hydrographic gradients. In these environments, carbon inputs are not only influenced 

by environmental conditions, but allochthonous sediments contributed by ocean currents may 

add to the diversity of nearshore food webs. In addition to phytoplankton, primary producers in 

estuarine rocky intertidal systems include marine macroalgae, terrestrial plants, and benthic 

microalgae (Guerry et al. 2009). These sources, or carbon subsidies, can contribute variable 

amounts of carbon to rocky intertidal consumers (Bustamante and Branch 1996, Hill and 

McQuaid 2008). The availability of multiple food sources can contribute to the stabilization of 

nearshore food webs (Huxel et al. 2002, McMeans et al. 2013). Different primary producers vary 

in their vulnerability to environmental stressors; thus, if the availability of one primary 

production source is reduced, consumers could compensate with other food sources and maintain 

food web linkages. The number of species contributing to the food web in a specific system, their 

distribution across trophic levels, the links to different basal food sources, and how species are 

complementary or independent in their resource use are all important determinants of system 

resilience (Link 2002, Kondoh 2003, Hillebrand and Shurin 2005). 

In addition to influences from climate change or other anthropogenic activities, nearshore marine 

habitats are particularly vulnerable to the potential impacts of marine oil and gas extraction 

activities (Crowe et al. 2000). For example, catastrophic effects have been documented on all 

trophic levels of coastal ecosystems immediately after oil spills (e.g., Estes 1991, Newey and 

Seed 1995), but effects can also linger for decades (e.g., Thompson et al. 2002, Peterson et al. 

2003). These impacts include direct lethal effects of severe incidents such as oil spills and subtle 

effects, such as the reduction of resilience and health of species and the disruption of the energy 

flow among the various nearshore ecosystem components (Temara et al. 1999). Therefore, the 

nearshore food web structure can be a useful metric to assess not just the structure but also the 

function of coastal ecosystems. To understand possible changes in food web structure, 
2 

benchmark understanding has to be created so natural ecosystem functioning and variability can 

be distinguished from potential impacts (Underwood 1991). 

Here, the trophic structure of rocky intertidal communities was evaluated and compared in two 

regions of Cook Inlet, one of Alaska’s outer continental shelf regions of active and planned oil 

and gas extraction. Based on general hydrography, the western region of Cook Inlet is 

downstream of resource extraction activities, and the eastern, upstream side is mostly unaffected. 

These regions also differ hydrographically, with significant influxes from riverine and glacial 

melt discharges on the western side, which add cold, oligotrophic, and silt-laden waters into 

coastal habitats (Muench et al. 1978, Feely and Massoth 1982, Arendt et al. 2002, Hood et al. 

2009). The eastern side of Cook Inlet is mostly under oceanic influences and can serve as a 

useful comparative system. The specific questions asked here were (1) Does intertidal food web 

structure differ between the western and eastern regions of Cook Inlet? (2) Are intertidal food 

web structure characteristics consistent over two years of study? (3) Do intertidal organisms 

source carbon more from phytoplankton or macroalgae? 

Methods 

Site description 

Intertidal food webs were analyzed in two regions of Lower Cook Inlet; Kachemak Bay to the 

east and Kamishak Bay to the west (Figure 1). The northern Outer Continental Shelf Cook Inlet 

Planning Area reaches into the lower portions of the Inlet (Figure 2). Circulation in Cook Inlet is 

such that water from the northern Gulf of Alaska enters on the eastern side of the Inlet, where 

part of this water is diverted to the east into Kachemak Bay (Muench et al. 1978). Within 

Kachemak Bay, water flows along its southern shore, where riverine and glacial discharge in 

upper Kachemak Bay modifies the hydrography. Water exits Kachemak Bay on the northern 

shore and enters back into Lower Cook Inlet at Anchor Point (Burbank 1977). Currents in Cook 

Inlet then flow along the eastern side, again being modified by significant input of freshwater 

and sediments from rivers (peak discharge > 5000 m3

 s-1) and glacial discharge (sediment load of 

up to 2000 mg L-1) in upper Cook Inlet (Sharma and Burell 1970, Muench et al. 1978, Hein et al. 

1979, Feeley and Massoth 1982, Neal et al. 2010). Net southward outflow from Cook Inlet is 

then bathymetrically steered along the western side of the Inlet. Part of this water passes through 

Kamishak Bay in western lower Cook Inlet before exiting into Shelikof Strait in the Gulf of 

Alaska. Sites in Kamishak Bay are, thus, downstream from potential oil and gas exploration 

activities in the BOEM Cook Inlet Planning Area (Figure 2). The large tidal range in Cook Inlet 

(~ 9 m) causes strong tidal currents upwards of 4 m s-1 that relax and enforce this general flow on 

a twice-daily basis (Muench et al. 1978, Nelson and Whitney 1996). 
3 

Figure 1: Map of study area of Lower Cook Inlet with Kachemak Bay to the east and Kamishak 

Bay to the west. The three study sites per region are marked. Glacial and riverine input into the 

study systems can be seen as green-brown coloration of the water. 

Six sites were chosen to represent regional food webs; Port Graham, Outside Beach, and Cohen 

Island in Kachemak Bay, and Douglas Reef, Augustine, and Nordyke Island in Kamishak Bay 

(Figure 1, Table 1). The Nordyke site was slightly different between the two years because of 

accessibility problems in 2018 due to rough weather, but the locations were in close vicinity (~ 2 

km apart) on Nordyke Island. Sites in Kachemak Bay were in the outer regions of the bay (i.e., 

outside of Homer Spit) and mostly influenced by oceanic water properties, while sites in 

Kamishak Bay were located in the outflow of waters modified from upper Cook Inlet glacial and 

river discharge (Figure 1). Sites in Kachemak Bay typically had a steeper slope and more 

heterogeneous substrate of cobble and boulder than sites in Kamishak Bay, which typically had a 

lower slope and more uniform substrate, often bedrock or uniform grain size. Collections for 

intertidal food web structure occurred in late April/early May (Kachemak Bay) and June 

(Kamishak Bay) in both years. 
4 

Figure 2: Map of the Outer Continental Shelf (OCS) Cook Inlet Planning Area with lease sale 

areas in relation to the Kachemak Bay and Kamishak Bay study areas. Map from 

https://www.boem.gov/Map-of-Blocks-Receiving-Bids/. 

Table 1: Site locations in each region and year. 

Site Year Region Latitude Longitude

Port Graham 2017, 2018 KACH 59.37333 -151.8941 

Outside Beach 2017, 2018 KACH 59.46458 -151.7094 

Cohen Island 2017, 2018 KACH 59.53913 -151.4769 

Douglas Reef 2017, 2018 KAMI 59.1052 -153.716 

Nordyke Island 2017 KAMI 59.1814 -154.082 

Nordyke Reef 2018 KAMI 59.2236 -154.121 

Augustine Island 2017, 2018 KAMI 59.4064 -153.455 
5 

Sample collection 

Common macroalgal, invertebrate, and suspended particulate organic matter (POM) samples 

were collected at all sites. Surface water samples for POM stable isotope analysis were collected 

by hand offshore (at least 100 m from shore) at each site. These water samples were filtered onto 

Whatman GF/F filters (0.7 μm pore size); approximately 100 mL were filtered in 2017, and 200 

mL were filtered in 2018 to ensure sufficient material for analysis. Understanding that POM is an 

extremely heterogeneous source (consisting of various heterotrophic protists, phytoplankton 

taxa, and detritus) and that stable isotope composition can change based on composition, POM 

was collected several times over a 6-week period in Kachemak Bay. This was not logistically 

possible in Kamishak Bay. Macroalgal endmembers collected by hand at each site and year 

included the green alga Acrosiphonia sp., the brown algae Fucus distichus and Saccharina spp., 

and the red alga Palmaria hecatensis. Intertidal consumers were suspension-feeding mussels 

(Mytilus trossulus) and barnacles (Balanus spp.), grazing limpets (Lottia scutum), periwinkles 

(Littorina spp.) and chitons (Katharina tunicata and Tonicella sp.), as well as predatory whelks 

(Nucella spp.) and sea anemones (Urticina crassicornis). These taxa were chosen because they 

were typical and abundant members of the rocky intertidal communities and occurred in both 

regions, facilitating comparisons. Samples were identified to the lowest possible taxonomic level 

in the field and frozen (dissected or whole, depending on the organism) on the day of collection. 

Macroalgae were collected whole, and muscle tissue, body wall, or whole organisms were used 

for invertebrates, depending on organism size. 

Laboratory analyses 

Invertebrate samples of sufficient size were dissected to remove soft tissue, avoiding hard parts 

such as shells and lipid-rich material such as reproductive tissue. When animals were too small 

to dissect, they were kept whole. Macroalgal and invertebrate tissue samples were dried at 60C 

for approximately 24 hours (or until stable weight was achieved). Samples with carbonate 

structures, such as barnacles and echinoderms, were acidified using 1N HCl until carbonates had 

dissolved, indicated by the ceasing of bubbling. Samples were then rinsed in deionized water and 

dried as described above. POM filters were exposed to saturated HCl fumes for at least four 

hours to dissolve any carbonates present. Dried macroalgal, invertebrate, and POM samples were 

analyzed for bulk δ13C and δ15N values. Carbon stable isotopes are indicative of basal resource 

uses, and nitrogen stable isotopes are a good indicator of trophic level because of the stepwise 

enrichment in this isotope between trophic levels (Peterson and Fry 1987). Stable isotope ratios 

of individual bulk tissue samples were measured using continuous-flow isotope ratio mass 

spectrometry (IRMS) at the Alaska Stable Isotope Facility. Stable isotope ratios will be reported 

in conventional δ notation as ‰ deviation from the international standards PDB (carbon) and air 

(nitrogen) according to the following equation: 

δ X = [(R sample/R standard) – 1] • 1000 
6 

where X is 13C or 15N of the sample, and R is the corresponding ratio 13C/12C or 15N/14N with 

standards of Vienna Pee Dee Belemnite for δ13C and atmospheric N2 for δ15N. 

Lipids tend to be isotopically lighter than other compounds in their 13C values (DeNiro and 

Epstein 1977), so samples were corrected for variable lipid content to maximize comparability of 

samples. However, chemical extraction of lipids can have an unwanted effect on δ15N values, 

which needed to be accounted for (Mintenbeck et al. 2008). To assess the effect of lipid 

extraction (LE) on invertebrate samples and to determine the most appropriate approach of 

correcting for these lipids, a subset of samples from a variety of common species were analyzed 

in duplicate, one with LE and a second set without LE. Lipids were chemically extracted three 

times per sample (or more if the solvent still contained color) using 2:1 chloroform:methanol 

(after Folch et al. 1957). Samples were then dried and prepared as described above for stable 

isotope analysis. LE did affect the δ15N values in invertebrate samples; therefore, the non-LE 

13C values (denoted as 13C’) for invertebrate consumers were mathematically corrected based 

on the C:N ratio of the bulk tissue sample using the following equation (after Post et al. 2007): 

 δଵଷCൌδଵଷC′ െ 3.44 ൅ 0.99 ൈ C: N௕௨௟௞ 

Where 3.44 is the assumed trophic enrichment step between trophic levels and C:Nbulk refers to 

the carbon:nitrogen (both by volume) of the invertebrate tissue. 

Data analyses 

A number of community-wide trophic metrics were calculated from the stable isotope data to 

characterize and compare community trophic structure between the regions and assess species 

interactions (Parnell et al. 2010, Layman et al. 2007, Cucherousset and Villeger 2015). The range 

of δ13C values is a representation of trophic niche diversification at the base of the food web; it 

includes all endmembers and consumers and is the distance between the two species with the 

lowest and highest δ13C values. The δ15N range represents the overall food chain length, or 

vertical food web structure, and is the maximum range of δ15N values of all endmembers and 

consumers. Isotope values (δ13C and δ15N) were averaged among sites within each region and 

year for overall region descriptions. Differences in δ13C and δ15N ranges between regions and 

years were determined by two-way ANOVA with region and year as independent factors, using 

the range in δ13C or δ15N values at each of the three sites within each region as replicates for 

each of the years. Significance for this and all following comparisons was set at α = 0.05. 

Total trophic niche area, or trophic richness, is the area in trophic space occupied by a 

community, measured as the convex hull of all species with δ13C and δ15N as coordinates 

(Layman et al. 2007, Cucherousset and Villeger 2015, Rigolet et al. 2015). As such, the trophic 

niche area is closely linked to the δ13C and δ15N ranges at each site. Convex hull area was 

calculated from the δ13C–δ15N values of macroalgae, POM, and invertebrates for the same taxa at 

each site in Kachemak and Kamishak Bays; regional convex hull areas were calculated from 

averaged δ13C and δ15N values across the three sites per region for each taxon. The same taxa 

were analyzed at all sites in both regions. However, in rare cases where a taxon was not available 
7 

at all sites, data for that taxon and site were substituted with an average of the values of the same 

taxon from the other two sites in that region so that an overall balanced data design could be 

preserved. Again, differences in convex hull areas between regions and years were determined 

from convex hull areas determined for each site and region in both years using two-way 

ANOVA. 

Trophic niche area measures only one facet of the trophic diversity, trophic richness (the total 

amount of trophic space filled by the community, independent of how taxa are distributed within 

this space). Therefore, measures are needed that detail how organisms utilize trophic space. 

Isotopic divergence (IDiv) measures how species diverge in their distances from the center of 

gravity in trophic space (i.e., the convex hull) (Cucherousset and Villeger 2015). IDiv ranges from 

0 to 1, where low values reflect that most species are close to the center of gravity, while higher 

values reflect more taxa at more extreme (boundary) areas of total trophic niche space. Isotopic 

dispersion (IDis) is similar to IDiv, but the latter measures the mean distance of individual species 

within trophic space (convex hull) to the centroid of all species, not the center of gravity of the 

convex hull (Laliberté and Legendre 2010, Cucherousset and Villeger 2015). IDis also ranges 

from 0 to 1, where 0 means all organisms have the same stable isotope values, and larger values 

mean that most of the taxa’s isotope values are far from the centroid of all points. Therefore, 

while IDiv relates taxa to the overall niche space and resources used, IDis is a better representation 

of taxa to each other. 

Isotopic evenness (IEve) and isotopic uniqueness (IUni) are metrics that describe the regularity of 

species distribution within isotopic space (Cucherousset and Villéger 2015). IEve is calculated as 

the sum of branch lengths of the minimum spanning tree linking organisms in isotopic space. IUni

is the average of the nearest distance neighbor distances divided by the maximal distance 

between the two nearest neighbors. Both metrics range from 0 to 1, where small values indicate 

the close position of organisms in isotopic space, reflecting a high level of redundancy in trophic 

resource niche use. In contrast, large values indicate communities that are dominated by more 

specialist species that exhibit high niche diversification. All trophic isotope metrics were 

calculated in Matlab after equations given in Cucherousset and Villeger (2015). Differences in 

all metrics between regions and years were tested with two-way ANOVA. 

To characterize carbon sourcing of intertidal organisms in both regions, a stable isotope mixing 

model (Stable Isotope Mixing Models in R, simmr) was used to determine the relative 

contributions of different endmembers to consumer diets in each region. Endmembers used were 

the average values of POM and average combined macroalgae per region and year. A species 

must have at least four replicates to undergo analysis in simmr, so mixing models were only 

calculated at the regional level (Parnell et al. 2013). Trophic enrichment factors used in the 

model were based on literature values, with enrichment of 1.63 ± 0.63 ‰ for 13C and 3.54 ± 

0.63 ‰ for 15N per trophic level (Caut et al. 2009). 
8 

Results 

In 2017, the range in δ13C values as a representation of trophic niche diversification at the base 

of the food web was smaller in Kachemak Bay (5.4 ‰) than in Kamishak Bay (9.9 ‰). In 2018, 

the food web base was more similar between the two regions with 6.7 ‰ in Kachemak Bay and 

8.7 ‰ in Kamishak Bay, but the range in Kachemak Bay was still smaller (Figure 3). These 

differences in δ13C range were significant for the factor region (two-way ANOVA, p=0.002) but 

not for the factor year or the interaction term (Table 2). In both regions and years, the lowest 

δ13C values were always associated with POM. The highest δ13C values in 2017 were associated 

with the periwinkle Littorina spp. in both regions. In 2018, the highest δ13C values were from the 

red alga Palmaria hecatensis (Kachemak Bay) and the whelk Nucella spp. (Kamishak Bay). 

Ranges in δ15N values representing overall food chain length were smaller in Kachemak Bay 

than in Kamishak Bay in both years; 7.9 ‰ in Kachemak Bay and 9.7 ‰ in Kamishak Bay in 

2017, and 8.0 ‰ in Kachemak Bay and 10.2 ‰ in Kamishak Bay in 2018 (Figure 3). The 

differences in δ15N range were significant by region (two-way ANOVA, p<0.0001) but not by 

factor year. The interaction effect was marginal (p=0.052), indicating some influence of year on 

regional patterns (Table 2). The lowest δ15N values were associated with POM in both regions 

and years, except for Kamishak Bay in 2018, where the lowest δ15N value was obtained for the 

kelp, Saccharina spp. The highest δ15N values in both regions and years were consistently 

associated with the anemone, Urticina crassicornis. 
9 

Figure 3: Trophic niche space, represented by the convex hull area in δ13C – δ15N biplot space. 

Each point represents the carbon and nitrogen stable isotope value for a species, averaged from 

values across the three replicate sites per region. Convex hull area values are given in the legend 

for both regions and years. The range in δ13C and δ15N values is given for each region and year 

as arrows outside the axes, color-coded by region. Starred data points represent the particulate 

organic matter samples, data points with a black outline represent macroalgal samples. 
10 

Total convex hull area from the distribution of species in isotopic space (in δ13C – δ15N biplots) 

represents the total trophic niche space of the system and was larger in Kamishak Bay compared 

with Kachemak Bay in both years (Figure 3). These differences were again significant for the 

factor region (two-way ANOVA, p<0.0001) but not for the factor region or the interaction term 

(Table 2). The larger trophic niche space in Kamishak Bay was also reflected in the site-specific 

niche space measures in both study years (Figures 4 and 5). The particularly small overall trophic 

niche space in Kachemak Bay in 2017 was mostly driven by a very small niche space at Cohen 

Island that year (Figure 4). While Cohen Island's niche space was larger in 2018, it was still 

smaller than other sites in Kachemak Bay (Figure 5). Trophic niche space in Port Graham and 

Outside Beach in Kachemak Bay were more similar to each other, and there was no consistent 

trend in niche space area between the two years (Figure 5). Trophic niche space at the three sites 

in Kamishak Bay was overall more similar without any trend among sites in the two years 

(Figures 4 and 5). In an annual comparison within each region, the average niche space in 

Kachemak Bay was smaller in 2017 than in 2018, but the overall area and patterns of distribution 

of taxa within that niche space were still relatively similar (Figure 6). The average niche space 

and distribution of taxa in Kamishak Bay were also similar across the two years (Figure 6). 
11 

Figure 4: Trophic niche space, represented by the convex hull area in δ13C – δ15N biplot space, 

for sites in Kachemak Bay (top panel) and Kamishak Bay (bottom panel) in 2017. Each point 

represents the carbon and nitrogen stable isotope value for a species, averaged from three 

replicate samples per taxon and site. Convex hull area values are given in the legend for sites in 

both regions. Crossed data points represent the particulate organic matter samples. 
12 

Figure 5: Trophic niche space, represented by the convex hull area in δ13C – δ15N biplot space, 

for sites in Kachemak Bay (top panel) and Kamishak Bay (bottom panel) in 2018. Each point 

represents the carbon and nitrogen stable isotope value for a species, averaged from three 

replicate samples per taxon and site. Convex hull area values are given in the legend for sites in 

both regions. Crossed data points represent the particulate organic matter samples. 
13 

Figure 6: Trophic niche space, represented by the convex hull area in δ13C – δ15N biplot space, 

for Kachemak Bay (top panel) and Kamishak Bay (bottom panel) compared between the two 

study years. Crossed data points represent the particulate organic matter samples. 

2

4

6

8

10

12

14

16

‐25 ‐20 ‐15 ‐10

KBAY ‐ 2017

KBAY ‐ 2018

2

4

6

8

10

12

14

16

‐25 ‐20 ‐15 ‐10

KAMI ‐ 2017

KAMI ‐ 2018

δ13C (‰)

δ15N (‰)

δ13C (‰)

δ15N (‰)

x

x

x

x
14 

Table 2: Two-way ANOVA results for core measures of isotope space with factors region and 

year. Significant factors in bold. 

Metric Source SS df MS F Sig. 

δ13C range Region 13.534 1 13.534 21.032 0.002 

 Year 0 1 0 0 0.993 

 Reg x Year 0.019 1 0.019 0.03 0.867 

 Error 5.148 8 0.643 

 

 δ15N range Region 32.323 1 32.323 36.534 <0.0001 

 Year 0.062 1 0.062 0.07 0.799 

 Reg x Year 4.594 1 4.594 5.192 0.052 

 Error 7.078 8 0.885 

 

Convex hull Region 1,653.96 1 1,653.96 36.259 <0.0001 

 Year 58.96 1 58.96 1.293 0.288 

 Reg x Year 121.973 1 121.973 2.674 0.141 

 Error 364.916 8 45.615 

Inspection of the isotopic data showed that regional isotopic niche space was, in part, driven by 

differences in the POM values. POM δ13C values in Kachemak Bay were, on average, 2–3 ‰ 

higher than in Kamishak Bay, while δ15N values were similar between the two regions (Figure 

7). For carbon stable isotope POM values, there was a significant effect of the factor region but 

not of the factor year and no interaction effect; no significant effect was detected for either factor 

or the interaction for nitrogen stable isotope POM values (Table 3). Temporal variation in POM 

δ13C values over a 6-week period in Kachemak Bay varied by 1.3 ‰ and by 2.6 ‰ for δ15N 

values (comparisons for both isotopes significant at p=0.01, t-test) (Figure 8). 

Figure 7: POM values from all locations in Kachemak Bay and Kamishak Bay in 2017 (closed symbols) 

and 2018 (open symbols). Higher δ13C values occurred in Kachemak Bay. 

δ13C (‰)

2.0

3.0

4.0

5.0

6.0

7.0

8.0

‐25.0 ‐24.0 ‐23.0 ‐22.0 ‐21.0 ‐20.0

δ15N (‰)

KACH‐2017

KAMI‐2017

KACH‐2018

KAMI‐2018
15 

 

Figure 8: POM isotope values taken in Kachemak Bay (close to Cohen Island site) over several weeks in 

2017 to assess short-term variability in POM isotope values. Dates of collection are given as 

M/DD/YYYY. 

Table 3: Results of a two-way ANOVA for POM stable isotope values with factors region and year. 

Significant factors in bold. 

Metric Source SS df MS F Sig. 

POM δ13C Region 10.237 1 10.237 8.895 0.018 

 Year 0.638 1 0.638 0.554 0.478 

 Reg x Year 0.678 1 0.678 0.59 0.465 

 Error 9.207 8 1.151 

 

POM δ15N Region 0.666 1 0.666 0.965 0.355 

 Year 2.548 1 2.548 3.695 0.091 

 Reg x Year 0.044 1 0.044 0.064 0.807 

 Error 5.518 8 0.69 

Isotopic divergence (Idiv) was smaller in Kachemak Bay compared with Kamishak Bay in 2017, 

but the pattern was reversed for 2018 (Table 4). This reflects a significant region x year 

interaction effect while neither the factor region nor the factor year was significant (Table 5). 

The smaller IDiv values in Kachemak Bay in 2017 are reflective of the more clustered 

concentration of taxa around the center of gravity in the convex hull, or the center of total trophic 

niche space (Figure 3). In 2018, IDiv values in both regions were intermediate to the values of 

both regions in 2017, reflecting the more similar clustering of taxa around the respective center 

of gravity of their convex hulls (Table 4, Figure 3). None of the other isotope trophic diversity 

measures showed any significant effects (Table 5). Isotopic dispersion (IDis) was slightly smaller 

3.0

4.0

5.0

6.0

7.0

‐23.0 ‐22.5 ‐22.0 ‐21.5 ‐21.0 ‐20.5 ‐20.0

6/4/2017

5/24/2017 4/28/2017

5/21/2017

δ13C (‰)

δ15N (‰)
16 

in Kachemak Bay compared with Kamishak Bay in 2017, while values for 2018 were 

intermediate to 2017 values in both regions (Table 4). IEve and IUni were both slightly larger in 

Kamishak Bay than Kachemak Bay in 2017, indicating more evenly distributed and more unique 

consumer species than in Kachemak Bay, where consumers had a higher level of redundancy 

(Table 4). These trends were not noticeable in 2018. 

Table 4: Regional values (for Kachemak and Kamishak Bays) of various isotope trophic 

diversity measures. IDiv = isotopic divergence, IDis = isotopic dispersion, IEve = isotopic evenness, 

IUni = isotopic uniqueness. 

Year Region IDiv IDis IEve IUni

2017 KACH 0.604 0.310 0.604 0.241 

2017 KAMI 0.688 0.377 0.799 0.375 

2018 KACH 0.648 0.360 0.612 0.245 

2018 KAMI 0.605 0.339 0.558 0.265 

Table 5: Two-way ANOVA results for various isotope trophic diversity measures with factors 

region and year. Significant factors in bold. IDiv = isotopic divergence, IDis = isotopic dispersion, 

IEve = isotopic evenness, IUni = isotopic uniqueness. 

Metri

c Source SS df MS F Sig. 

IDiv Region 0.001 1 0.001 2.273 0.17 

 Year 0 1 0 0.437 0.527 

Reg x Year 0.01 1 0.01 18.801 0.002 

 Error 0.004 8 0.001 

 

IDis Region 0.007 1 0.007 1.774 0.22 

 Year 0.001 1 0.001 0.318 0.588 

 Reg x Year 0.004 1 0.004 1.083 0.328 

 Error 0.031 8 0.004 

 

IEve Region 0.006 1 0.006 0.923 0.365 

 Year 0.008 1 0.008 1.22 0.301 

Reg x Year 0.008 1 0.008 1.236 0.299 

 Error 0.055 8 0.007 

 

IUni Region 0.011 1 0.011 1.212 0.303 

 Year 0.007 1 0.007 0.774 0.405 

 Reg x Year 0.005 1 0.005 0.517 0.493 

 Error 0.07 8 0.009 
17 

Carbon sourcing from POM versus macroalgae was calculated for intertidal taxa in both regions. 

The average percent sourcing from POM was 40 % in Kachemak Bay and 22 % in Kamishak 

Bay in 2017 (Figure 9), indicating considerable sourcing from macroalgae in both regions. In 

2018, however, POM sourcing in Kachemak Bay (34%) was only marginally larger than the 32% 

in Kamishak Bay (Figure 9). This sourcing was relatively consistent across trophic levels, from 

filter feeders to grazers to predators, in both regions and years. 

Figure 9: Results of a carbon source mixing for Kachemak Bay (left panels, orange) and 

Kamishak Bay (right panels, blue) in 2017 (top panels) and 2018 (bottom panels). Carbon source 

input into the model was POM and macroalgae. Shown here is the percent contribution of POM 

to consumer diets, with macroalgae providing the remaining percent. Consumers are abbreviated 

as Myt – Mytilus, Barn – barnacles, Poly – Polyplacophora, Lott – Lottia, Litt – Littorina, Nuc – 

Nucella, Urt – Urticina. 

6

7

8

9

10

11

12

13

14

15

0 0.2 0.4 0.6 0.8 1

relative POM contribution

δ15N (‰)

6

7

8

9

10

11

12

13

14

15

0 0.2 0.4 0.6 0.8 1

relative POM contribution

δ15N (‰)

6

7

8

9

10

11

12

13

14

15

0 0.2 0.4 0.6 0.8 1

6

7

8

9

10

11

12

13

14

15

0 0.2 0.4 0.6 0.8 1

relative POM contribution

δ15N (‰)

relative POM contribution

δ15N (‰)

filter

grazer

predator

filter

grazer

predator

Litt

Myt

Barn

Nuc

Urt

Poly

Lott Litt Myt

Barn

Nuc

Urt

Poly

Lott

Litt

Myt

Barn

Nuc

Urt

Poly

Lott

Litt

Myt

Barn

Nuc

Urt

Poly

Lott

2017

2018
18 

Discussion 

Rocky intertidal food web structure in two hydrographically different regions of Cook Inlet was 

generally similar, although there were some differences in trophic niche space metrics and 

isotopic divergence; both were larger in the Kamishak Bay intertidal than in Kachemak Bay. 

Some of these differences were driven by the more depleted carbon isotope signatures of the 

POM source in Kamishak Bay, which may be linked to the hydrographic conditions in the two 

regions. There was higher carbon sourcing from POM in Kachemak Bay compared with 

Kamishak Bay, but consumers in both regions derived the majority of their primary carbon 

source from macroalgae. The use of several carbon sources in both regions points to an increase 

in food web stability and plasticity in consumers to capitalize on multiple carbon sources. 

The rocky intertidal systems in Kamishak Bay covered larger ranges in δ13C and δ15N, ultimately 

resulting in a larger trophic niche space (isotope convex hull) utilized by the Kamishak 

communities compared with Kachemak Bay. The δ13C range, an indication of the diversity of 

carbon sources, reflected a broader range in carbon sources in Kamishak Bay than in Kachemak 

Bay. Given that the same taxa were analyzed in both regions, the higher diversity at the base of 

the food web in Kamishak Bay stems to a large part from differences in the carbon isotope 

values of some of the primary producers. Specifically, the δ13C values of POM in Kamishak Bay 

were about 2 ‰ lower than in Kachemak Bay. POM is a heterogeneous source that combines 

cell components (phytoplankton, bacteria, ciliates) with detrital components as well as different 

particle sizes (Benner et al. 1997, Lee et al. 2004, Bianchi and Bauer 2011). Phytoplankton δ13C 

values are driven by a network of biosynthetic reactions from photosynthesis to the production of 

nucleic acids, proteins, carbohydrates, and lipids that form biomass during growth (Hayes 1993, 

2001). For these processes, isotopic fractionation in primary producers depends on the isotopic 

composition and abundance of dissolved inorganic carbon (DIC) utilized by the cells (Boschker 

and Middelburg 2002, Vuorio et al. 2006) and nutrient availability (Brutemark et al. 2009). In 

addition, biological factors, such as identity, size and morphology of the cells (Popp et al. 1998, 

Sato et al. 2006, Vuorio et al. 2006), carbon fixation pathways (Leboulanger et al. 1995), and 

growth rates (Burkhardt et al., 1999), and environmental factors, such as light/irradiance 

(Thompson and Calvert 1994), temperature (Sackett et al. 1965), salinity (Leboulanger et al. 

1995), and pH (Hinga et al. 1994; Thompson and Calvert 1994) play a role. Considerable 

amounts of DIC are discharged into the coastal ocean from rock weathering in high-latitude 

watersheds (Tank et al. 2012), influencing the DIC availability and, in combination with the 

drivers noted above, influencing the stable isotope composition of coastal phytoplankton in 

estuaries such as Cook Inlet. Estuarine phytoplankton in low-salinity regions are typically 

depleted in 13C, with subsequent strong gradients of increasing POM δ13C values indicative of 

oceanic conditions at the mouth of estuaries (Middelburg and Nieuwenhuize 1998, Fry 2002). 

This fits the POM carbon isotope patterns observed in Cook Inlet, where POM δ13C values in the 

more freshwater-influenced Kamishak Bay region were depleted compared with more enriched 

values in the more oceanic-influenced Kachemak Bay. 
19 

In addition to freshwater effects on DIC, estuaries such as Cook Inlet receive variable, often 

considerable, amounts of terrestrial detritus from river and glacial discharge (Schlünz and 

Schneider 2000, Mannino and Harvey 2000, Fellman et al. 2014). This terrestrial material is 

isotopically lighter than marine primary production because atmospheric CO2 (δ13C about −8‰) 

used for photosynthesis by terrestrial vegetation is more depleted in 13C than the dissolved 

inorganic carbon (δ13C about 0‰) used by marine phytoplankton (Farquhar et al. 1989). The 

addition of terrestrial detritus modifies the isotopic composition of the POM pool that provides a 

food source to nearshore consumers (Hedges et al. 1997, Raymond and Bauer 2001). Freshwater 

and terrestrial matter input are larger in Kamishak Bay as it is downstream from the major river 

and glacier discharges in upper Cook Inlet, while sites in Kachemak Bay are only marginally 

influenced by discharge and mostly receive input from the Gulf of Alaska (Muench et al. 1978). 

The Alaska Stream hugging the coastal Gulf of Alaska and turning into lower Cook Inlet, 

including Kachemak Bay, is also influenced by freshwater sources (Royer 1981), but this 

freshwater (and related terrestrial matter) loading is much smaller than inputs in upper Cook 

Inlet. This difference in freshwater and terrestrial matter inputs explains the consistently lower 

δ13C values of POM in Kamishak Bay compared with Kachemak Bay. 

Characteristics of the POM source, including contributions from terrestrial matter, will influence 

the POM chemical composition and δ13C values on seasonal to shorter time scales (Mayzaud et 

al. 1989, Sato et al. 2006). For example, high variability in POM δ13C values was visible in 

POM measurements taken in Kachemak Bay over a 6-week period in 2017, which varied over 

about 1 ‰ in δ13C. The lower POM δ13C values in Kamishak Bay may be part of this strong 

temporal variability; however, their consistency, especially in 2017, points beyond variability 

towards a more systematic difference. The most likely explanation for this difference is the 

higher terrestrial component in Kamishak Bay, as discussed above. This pattern in POM δ13C 

values was similar but not as pronounced in 2018, which may indicate that greater inputs of 

freshwater and terrestrial matter influencing the POM in 2017 compared with 2018. 

The range of δ13C values was also larger because some species were more enriched in 13C in 

Kamishak Bay than Kachemak Bay. Differences on the higher end of the δ13C range were less 

pronounced among the primary producers but were driven by some of the grazers. Specifically, 

the grazer Littorina was the most enriched in both regions, with higher values in Kamishak Bay. 

Littorina, a common grazer in rocky intertidal systems, can feed on filamentous green algae but 

most commonly scrapes off benthic microalgae from the biofilm covering rocks (Sommer 2000, 

Van Alstyne et al. 2009). The high Littorina δ13C values in both regions could indicate that this 

species feeds almost exclusively on the most enriched macroalgae (brown algae), which would 

be consistent with an approximate 1 ‰ enrichment between producer and consumer (Vander 

Zanden and Rasmussen 2001). Alternatively, the systems can have an unacknowledged carbon 

source channeling into the food web, specifically benthic microalgae (Takai et al. 2004). Benthic 

microalgae are much enriched in 13C compared with phytoplankton (here: POM) because of the 

higher concentration gradients of the CO2 substrate across the boundary layers for benthic 

compared with pelagic algae (France 1995). Diffusion of CO2 or HCO3- in water is slow, 
20 

especially across stagnant boundary layers surrounding benthic algae. The slow replenishment of 

the carbon sources for photosynthesis across these boundary layers leads to a deficit of the 

typically metabolically preferred 12C atom and increased use of the 13C atom (Osmond et al. 

1981, Keeley and Sandquist 1992). This applies not only to benthic macroalgae but also benthic 

microalgae, and their high growth rates (equaling high CO2 demand) can lead to highly enriched 

δ13C values (Fry and Wainright 1991). Benthic microalgae were not included in this study, but 

high δ13C values in consumers such as in Littorina would be consistent with these assumptions. 

The overall larger trophic niche space in Kamishak Bay compared with Kachemak Bay was also 

driven by larger nitrogen isotope ranges in Kamishak Bay. The POM sources with the lowest 

δ15N values were nearly identical in both regions and years. While there was variability in the 

nitrogen isotope values of the different macroalgal endmembers, these differences were not 

systematic (i.e., not consistently higher in one over the other region). Therefore, the larger 

nitrogen isotope range in Kamishak Bay must have been driven by longer trophic transfer 

pathways to predators. While δ15N undergoes stepwise enrichments between trophic levels (3–4 

‰ per step, Vander Zanden and Rasmussen 2001), this enrichment is influenced by temporal 

variation in endmembers (Vander Zanden and Rasmussen 1999), trophic level of the consumers 

(Buchheister and Latour 2010, Malpica-Cruz et al. 2012, Hussey at al. 2014), ontogenetic status 

of consumers (Scharf et al. 2000), and the level of omnivory in consumers (Svanbäck et al. 

2015). Therefore, some of the differences in δ15N positions of top consumers may have been 

driven by differences in the transfer of energy modulated by differences in consumer physiology 

and biology in the two study regions. For example, consumers of the same species could have 

been of different ages or sizes in the two regions. In addition, the terrestrial component in POM 

may require additional microbial degradation before the material becomes labile for marine 

consumers (Moran and Hodson 1989, Garneau et al. 2009). Energetically and isotopically, these 

microbial processes present additional trophic steps, increasing the δ15N values or trophic level 

of subsequent consumers (Middelburg and Herman 2007, Bell et al. 2016). This additional 

trophic step could be responsible for the higher δ15N values of top consumers in Kamishak Bay, 

where POM presumably contained higher levels of terrestrial materials (see above), resulting in 

the overall larger trophic niche space in Kamishak Bay. 

Total trophic niche space, as discussed above, provides a quantitative measure of the extent of 

resources available to the system (Rigolet et al. 2015). Total trophic niche space does not, 

however, describe whether and how these resources are being utilized. Measures of the 

distribution of taxa within this trophic niche space are indicative of trophic functioning and 

resource use. Those isotopic measures were not significantly different between the two regions, 

except for isotopic divergence. This index measures how species diverge in their distances from 

the center of gravity in trophic space, that is, the extent to which species are located towards the 

center (low IDiv) or towards the border (high IDiv) of the trophic niche space (Cucherousset and 

Villeger 2015). Ecologically speaking, high IDiv means that most of the resources in trophic space 

are utilized by the assemblage, indicative of resource use specialization and niche differentiation 

(Rigolet et al. 2015). In contrast, low IDiv indicates that some resources are likely to be 
21 

underutilized, pointing to a system of more generalist feeders and less diversified ecosystem 

functioning. 

In this study, there was a significant interaction effect between region and year for IDiv, with 

much larger values in Kamishak than Kachemak Bay in 2017. This larger distribution across 

trophic space in Kamishak Bay was due to both primary producers and consumers. For primary 

producers, the larger divergence indicates that a broader array of resources (i.e., carbon substrate 

and nutrients) is being utilized during photosynthesis in the Kamishak Bay system, which could 

be due to a difference in available inorganic carbon and nutrients in the two regions. Glacial and 

riverine runoff influence nearshore dissolved organic matter, inorganic carbon sources, and 

nutrients in high latitude systems, but the direction and scale of these influences depend on 

erosional characteristics of the glaciers, vegetation in the watershed, and bacterial mineralization 

processes (Hodson et al. 2005, Hood and Scott 2008). Barring any carbon and nutrient data in the 

two regions, it can only be inferred that the strong glacial and riverine discharge upstream from 

Kamishak Bay heavily influences carbon and nitrogen sources in that region, creating different 

characteristics than in Kachemak Bay. The significant interaction effect for the IDiv metric 

suggests that these influences are variable across years, possibly due to regional and local 

weather conditions in any given year, influencing discharge rates from rivers and glaciers, 

mixing of the water column, etc. In terms of consumers contributing to higher divergence in 

Kamishak Bay, this could stem from broader utilization of the more diverse primary production 

base, or include the potentially unacknowledged endmembers (e.g., benthic microalgae) 

discussed above. 

Rocky intertidal consumers in both systems derived the majority of their carbon from macroalgal 

as opposed to phytoplankton (POM) sources. This utilization of macroalgae occurs not only in 

the form of direct grazing but also as consumption of the macroalgal detritus formed when algae 

senesce and thallus tissue is sloughed off and broken into small particles via hydrodynamics and 

bacterial degradation (Rieper-Kirchner 1989). Consumers of all feeding types can make use of 

the macroalgal subsidy, which was obvious in this study where grazers and filter feeders had 

similar proportions of macroalgal contributions to their diets. Predators are unlikely to consume 

macroalgae directly (although it is a possibility); it is more likely that macroalgal carbon is 

transferred into these higher trophic levels through their prey items. The dependence of a large 

number of consumers on macroalgal carbon is not uncommon in many rocky intertidal systems. 

For example, both mussels and barnacles are known to consume a mixture of organic matter 

derived from phytoplankton as well as macroalgal production in many rocky intertidal systems 

worldwide (Bustamante and Branch 1996, Tallis 2009), including Alaska (Duggins et al. 1989). 

The use of multiple food subsidies (i.e., diversification of the food base) in a system is 

considered important for overall food web stability (Huxel et al. 2002). This may be particularly 

important in high-latitude systems where phytoplankton production is extremely seasonal 

(McMeans et al. 2013, Renaud et al. 2015). While macroalgae have seasonal peak growth cycles, 

many are perennial and provide a more temporally-stable food supply than phytoplankton 

(Leclerc et al. 2013). This overall pattern of diversified endmember use applied to both regions 
22 

in this study. However, the slightly larger dependence of consumers on POM in Kachemak Bay 

may indicate that the POM source there is of higher quality and/or more accessible. Due to high 

glacial discharge upstream, The POM in Kamishak Bay contains a large portion of inorganic, 

silty glacial flour. Filter feeders can get clogged by this fine and non-nutritious material, and the 

excessive production of mucus required to capture and shed this inorganic material is 

energetically costly (Riisgård and Larsen 1995). Hence, a larger reliance on macroalgae could be 

more energetically beneficial for consumers in Kamishak Bay. 

Differences between resource use and utilized trophic niche space were discussed here for the 

two rocky intertidal systems in Kachemak and Kamishak Bays. It should be noted that the same 

taxa were considered in both systems, which increased the overall comparability of the two 

systems but does not describe either system in its entirety. A benefit of the direct comparison is 

that comparisons could be made at the individual species level and in larger metrics such as 

niche space. The variability in trophic position of the same species in the two systems suggests 

high feeding plasticity in these intertidal producers and consumers (Padilla and Savedo 2013). 

Plasticity is not a new concept but bears discussing here as it can inform about the resilience of 

ecosystem functioning against disturbances. For example, in different systems, the mudsnail 

Hydrobia can switch food sources from macroalgae and marine detritus to terrestrial marsh plant 

detritus, depending on organic matter availability in their estuarine environment (Riera 2010). 

Similarly, filter-feeding oysters (Crassostrea gigas) can change the proportions of various food 

sources along an estuarine gradient (Marchais et al. 2013). Plasticity in estuarine systems also 

occurs in biological aspects other than feeding. Macroalgae can show plasticity in photosynthesis 

and growth over a wide range of estuarine conditions, as documented for the kelp Saccharina 

latissima in locations of differing salinity and nutrients within Kachemak Bay (Spurkland and 

Iken 2011a, 2011b, 2012). Fucus is known to thrive over a large range of temperature and 

salinity conditions in estuarine settings (Takolander et al. 2017). Barnacles are adaptable in 

morphology, growth, reproduction, and body condition to a wide range of salinities (Wrange et 

al. 2014). Hence, plasticity to a broad range of environmental conditions is likely an inherent 

trait of these intertidal, estuarine taxa, even though adaptation to extreme conditions can be 

energetically costly (Sanford 2002). This plasticity is a tremendous contributor to the long-term 

persistence of rocky intertidal communities, but it can also place these communities close to their 

physiological limits (Dahlhoff et al. 2002). While the taxa included in this study seem to be 

adaptable to a range of environmental conditions, it is unknown how these conditions relate to 

their physiological performance (i.e., Are some taxa operating at their limits?). Also, this 

plasticity may not apply to the overall community and some differences in the taxonomic 

composition of the rocky intertidal systems (see Konar et al. 2009 and BOEM final report for 

M15PG00037) may stem from low plasticity of certain taxa to the prevalent conditions in a 

region. 

In summary, trophic niche space was larger in the Kamishak Bay assemblage due to a higher 

diversity in food sources but also due to longer food chains. In both regions, food webs were 
23 

stabilized by the use of multiple carbon sources and greater use of persistently available 

macroalgae use over more seasonal POM by all consumer taxa. The question remains: Which 

system is more resilient to changes or disturbances? The larger niche space and more efficient 

resource use in Kamishak Bay may suggest this system is more resilient; however, the opposite 

argument can be made. The larger niche space and higher divergence in Kamishak Bay could be 

a result of a more stressed system (overall lower nutrient availability in the glacially-influenced 

system), which may stretch species more towards the limits of their physiological and trophic 

plasticity. In that case, disturbances or changes could be more devastating to the Kamishak than 

Kachemak rocky intertidal food webs. It is not clear if the current regional conditions push these 

taxa to the limit of their plasticity, nor whether future climate-related or human-induced 

perturbations would overextend this plasticity. Regardless, the high plasticity of common rocky 

intertidal taxa observed in this study suggests that, in general, these systems can thrive in the 

“typical” range of conditions for lower Cook Inlet. 

Acknowledgments 

This study was funded by the Coastal Marine Institute (University of Alaska Fairbanks and 

Bureau of Ocean Energy Management) under Cooperative Agreement Number M17AC00003. 

We are grateful to Cathy Coon for her guidance on this project as the Program Manager. Field 

collections of this work were leveraged through the Cook Inlet Characterization Study (AK-15-

08 Subtidal and intertidal habitats and invertebrate biota in Lower Cook Inlet, Alaska funded by 

the Bureau of Ocean Energy Management) and the Gulf Watch Alaska program (funded by the 

EXXON Valdez Oil Spill Trustee Council, project 18120114-H). Danielle Siegert, who will be 

using part of this work in her MS degree in Marine Biology at the University of Alaska 

Fairbanks, was instrumental during field collection and laboratory analyses. We are grateful to 

Susan Saupe, Mandy Lindeberg, and Lauren Sutton for assistance during field collections in 

Kamishak Bay. We thank Tim Howe at the Alaska Stable Isotope Facility, UAF, for his 

assistance with the isotope analyses. 

Study Products 

Siegert D, Iken K, Saupe S, Lindeberg M. 2019. Comparing intertidal food web and community 

structure across two regions of lower Cook Inlet. Alaska Marine Science Symposium, 

Anchorage, AK. 28–31 January 2019 (poster presentation) 

Siegert D, Iken K, Saupe S, Lindeberg M. 2019. Comparing intertidal food web and community 

structure across two regions of lower Cook Inlet. CMI Annual Review, Anchorage, AK. 

1 February 2019 (oral presentation) 

Siegert D, Iken K, Konar B, Saupe S, Lindeberg M. 2018. Nearshore food web structure in two 

contrasting regions of Cook Inlet. Alaska Marine Science Symposium, Anchorage, AK. 

22–25 January 2018 (poster presentation) 
24 

Siegert D, Iken K, Konar B, Suape S, Lindeberg M. 2018. Nearshore food web structure in two 

contrasting regions of Cook Inlet. CMI Annual Review, Anchorage, AK. 26 January 

2018 (oral presentation) 

Siegert D, Iken K, Konar B, Saupe S, Lindeberg M. 2018. Nearshore food web structure in two 

contrasting regions of Cook Inlet. Kachemak Bay Science Conference, Homer, AK. 8–9 

March 2018 (poster presentation) 
25 

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