# Importance of Serpulid Reef to the Functioning of a Hypersaline Estuary

Terence Palmer, Natasha Breaux, Benoit Lebreton, Gaël Guillou, Jennifer

Beseres Pollack

► To cite this version:

Terence Palmer, Natasha Breaux, Benoit Lebreton, Gaël Guillou, Jennifer Beseres Pollack. Importance of Serpulid Reef to the Functioning of a Hypersaline Estuary. Estuaries and Coasts, 2022, 10.1007/s12237-021-00989-0. hal-03362606

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## Importance of Serpulid reef to the functioning of a hypersaline estuary

Terence A. Palmerª; Natasha Breauxª; Benoit Lebretonᵇ; Gaël Guillouᵇ; Jennifer Beseres Pollackª*

ª Harte Research Institute for Gulf of Mexico Studies, Texas A&M University-Corpus Christi, Corpus Christi, TX 78412, USA.

ᵇ UMR Littoral, Environment et Societies, CNRS - University of La Rochelle, Institut du Littoral et de l'Environnement, 17000 La Rochelle, France.

E-mail: [jennifer.pollack@tamucc.edu](mailto:jennifer.pollack@tamucc.edu), Telephone: +1 361 825 2041, Fax: +1 361 825 2050

## Acknowledgements

This project was made possible with funding from the Building Conservation Trust, Coastal Conservation Association, and Coastal Bend Bays and Estuaries Program. We would like to thank Beau Hardegree, whose interest and research initially sparked our efforts, and members of the Coastal Conservation and Restoration Ecology Lab at the Harte Research Institute of Gulf of Mexico Studies at Texas A&M University-Corpus Christi, particularly Auria Avalos, Hannah Ehrmann, Danielle Downey, Meghan Martinez, Abe Margo, and Abby Williams, for their hard work on the project. We are grateful to Scott Murray and Dr. Mike Wetz for championing recent efforts in Baffin Bay and to the anglers of Baffin Bay for their input and donation of tissue samples.

We would like to acknowledge the research of Dr. David McKee, and Dr. Wes Tunnell, whose findings helped to guide this project. Lastly, we thank two anonymous reviewers for providing useful suggestions and comments.

<footnote>* Corresponding author contact details:</footnote>
23 **Abstract**

24 Biogenic reefs provide important ecological functions and services to coastal and marine
25 environments, supplying high levels of biodiversity, providing refuge and foraging habitat, and
26 supporting fisheries. Serpulid reefs are a relatively uncommon habitat in coastal ecosystems
27 globally that provide habitat for a diverse community of organisms, and have become a target for
28 conservation and management efforts due to habitat degradation and loss. Baffin Bay, Texas, USA,
29 is known for exceptionally productive commercial and recreational fisheries that are thought to be
30 supported by Serpulid reef benthic macrofauna, particularly during regular periods of
31 hypersalinity. This study compared the functioning of Serpulid reef habitats with nearby soft
32 sediment areas using quantitative faunal and food web analyses. Serpulid reefs support a unique
33 benthic macrofaunal community with 191 times greater abundance, 97 times greater biomass, and
34 twice the number of species than in soft sediments. In contrast to soft-sediment macrofauna,
35 Serpulid reef macrofaunal abundance and biomass were not correlated with any measured water
36 quality variables. Isotope compositions of both suspension and deposit-feeding macrofauna from
37 both habitats (i.e. Serpulid reefs and soft sediments) were close to organic matter from the
38 sediment, demonstrating connectivity and the importance of primary production in the sediment
39 to both habitat types. Abundant macrofauna inhabiting Serpulid reefs likely serve as an important
40 food source for sport fish and other higher trophic-level fauna, particularly in hypersaline periods
41 when soft-sediment macrobenthic food resources are scarce. Given the substantial loss of Baffin
42 Bay's Serpulid reef habitat compared to historic levels, conservation actions may be warranted to
43 protect and restore Serpulid habitat and food resources. The results of this study can be used to
44 increase the success of such efforts.
45 **Key Words**

46 salinity; stable isotopes; trophic relationships, Baffin Bay, Texas, Gulf of Mexico

47 **1. Introduction**

48 Estuaries across the Gulf of Mexico are dominated by soft-sediment habitats, yet the bulk of
49 benthic faunal biomass is associated with biogenic reefs (Enochs 2012, Davis 2017, La Peyre et
50 al. 2019). A relatively uncommon reef type in coastal ecosystems is built by Serpulid worms
51 (Serpulidae, Polychaeta) that settle on hard substrates and secrete a calcareous tube around
52 themselves as they grow (Bastida-Zavala et al. 2017). In rare circumstances, millions of small (<
53 2 cm) Serpulid worms can aggregate via gregarious larval settlement (Ippolitov et al. 2014),
54 forming extensive reefs (Andrews 1964; Bosence 1973; Bianchi and Morri 2001). These Serpulid
55 reefs supplement marine complexity by increasing the three-dimensional relief of the bottom,
56 providing refuge and foraging habitat for fish and invertebrate species (Haines and Maurer 1980,
57 Zühlke et al. 1998; Obenat et al. 2001), and influencing sediment and organism transport processes
58 (Obenat and Pezzani 1994). Through their suspension feeding activities, Serpulid worms can also
59 improve water quality and minimize algal blooms (Leung and Chueng 2017 and references
60 therein).

61 Baffin Bay, a secondary bay within the Upper Laguna Madre Estuary, Texas, USA, is famous
62 locally for its extensive Serpulid reefs. The predominantly hypersaline conditions in Baffin Bay
63 (Orlando et al. 1993) make it uninhabitable for the Eastern Oyster (*Crassostrea virginica*), the
64 dominant reef-building species in northern Gulf of Mexico estuaries. *Hydroides dianthus*, a
65 common Serpulid (Bastida-Zavala et al. 2017), is the only species currently building reef structures
66 in Baffin Bay. *H. dianthus* has a wide recorded tolerance to both salinity (1-51.7) and temperature
67 (3-30 °C and higher; maximum summer temperature for Baffin Bay (May 2013-March 2021) is
31.2 °C (Wetz unpublished data)), enabling it to successfully invade disparate waters of the world (e.g., Northern Yellow Sea, China, salinity 33, Dong et al. 2018; Thyrrenian Sea, Italy, salinity 13-48, Bianchi and Morri 2001; Delaware Bay, USA, salinity 23-28, Maurer et al. 1979; Haines and Maurer 1980). However, sizeable Serpulid reef structures to the extent of those found in Baffin Bay (last estimated at 16 km²; Brown et al. 1977) are rare and relatively understudied (Ardbear Lough, Ireland, <1 km² (Bosence 1973); Ellis Fjord, Antarctica, >8 km long (Kirkwood and Burton 1988); Delaware Bay, USA, 1 km² (Haines and Maurer 1980); see ten Hove and van den Hurk 1993 for review).

The diverse and abundant benthic macrofauna that occur on Baffin Bay's Serpulid reefs are potential forage for higher trophic-level animals. Therefore, the Serpulid reefs may serve as an important nursery area for Spotted Seatrout (*Cynoscion nebulosus*), with Baffin Bay hosting the highest occurrence of juveniles (Froeschke and Froeschke 2011), and high densities of large "trophy"-sized adults (McKee 2008) relative to elsewhere along the Texas coast, making Serpulid reefs a unique habitat that is targeted by local anglers. Similarly, commercially important Black Drum (*Pogonias cromis*), are three to six times more abundant in Baffin Bay than elsewhere along the Texas coast (Martinez-Andrade et al. 2005; Olsen 2014). On the contrary, benthic macrofauna in Baffin Bay's soft sediments are sparse, particularly during periods of hypersalinity (Montagna and Kalke 1995; Rubio et al. 2018; Breaux et al. 2019), and Serpulid reefs are thought to contribute much of the biomass critical to supporting the exceptionally productive commercial and recreational fisheries (McKee 2008; Bohannon et al. 2015). However, evidence indicates that Serpulid reefs have been reduced in size and distribution compared to historic levels, primarily due to wave action, prop damage from boats, and trampling from fishermen (Hardegree 1997;
Tunnell et al. 2001; Smith et al. 2005; Chapman et al. 2012), and conservation actions may be needed to protect and restore habitat.

Given their limited distribution and evidence of decline, there is a need to better understand Serpulid reef ecological structure and function to inform conservation, restoration, and management efforts. This study aimed to assess the importance of Serpulid reef as habitat for benthic macrofauna and to investigate the role of these reef-resident species as prey for commercially and recreationally important fish species. Comparing carbon and nitrogen isotope compositions of primary producers, macrofauna of known feeding modes, and fish allows for determining the origin of the main food sources and assessing the structure and functioning of the food web. Specifically, we sought to: (1) quantify differences in benthic macrofaunal communities between Serpulid reefs and soft sediments, (2) determine the influence of varying hydrological conditions on Serpulid reef benthic macrofaunal communities, and (3) investigate the role of Serpulid reef benthic macrofauna in the Baffin Bay food web using stable isotopes (δ¹³C and δ¹⁵N), with a focus on commercially and recreationally important fish species.

## 2. Methods

### 2.1 Study Area

Baffin Bay is a shallow (0.9 - 2.3 m), 248-km² secondary bay of the Upper Laguna Madre Estuary, located in the northwestern Gulf of Mexico in a semi-arid region of South Texas, USA (Figure 1; Diener 1975; Tunnell et al. 2001). Serpulid reefs were last estimated in 1977 to cover approximately 6% (16 km²) of Baffin Bay's otherwise soft bottom (Breuer 1957; Brown et al. 1977). Freshwater enters the bay through several small ephemeral streams at an average rate that does not keep up with average evaporation rates. Residence time in Baffin Bay and the Upper Laguna Madre exceeds one year, much longer than any other major estuary in Texas (< 0.5 yr;
Solis and Powell 1999; Wetz et al. 2017). Salinity conditions in the Upper Laguna Madre, including Baffin Bay, are on average hypersaline (mean ± standard deviation = 35.9 ± 7.0; Montagna et al. 2011). Salinity in Baffin Bay specifically is generally higher and has more spatiotemporal variability (2013-2016 mean ± standard deviation = 41.2 ± 10.4; Montagna et al. 2018) than the Upper Laguna Madre as a whole (2015-2017 = 39.1 ± 6.4; Breaux et al. 2019).

## 2.3 Study Design

Three Serpulid reefs (R1-R3) and three soft sediment stations (SS1-SS3) were sampled quarterly from June 2018 to March 2019 (Figure 1; Table S1). A fourth soft sediment station (SS4) was also sampled in December 2018 and March 2019 to increase spatial coverage of soft sediment communities. Samples were collected during sampling events for quantitative faunal community analysis and assessment of food web structure. Soft sediment stations SS3 and SS4 were previously sampled for benthic macrofaunal community analysis from 1989 to 1993 (as stations 24 and 6, Montagna et al. 1993; Montagna and Kalke 1995). All soft sediment stations (SS1-SS4) were previously sampled from 2014 to 2017 (as stations BB40, AL1, BB24, BB6, Rubio et al. 2018; Breaux et al. 2019).

## 2.4 Water Quality

During each sampling event, water temperature, salinity, dissolved oxygen (DO) concentration, and pH were measured at each station using a YSI Pro DSS multiparameter instrument (YSI Incorporated 2014). Measurements were taken from approximately 0.1 m below the water surface and 0.2 m above the sediment bottom.
133

## 2.5 Quantitative Fauna Analyses

134

### *Sampling*

135

Three replicate pieces (approximately 3 cm³) of Serpulid reef were collected by hand at each of the three Serpulid reef stations. Reef pieces were immediately transferred to a 500 µm mesh bag to prevent loss of benthic macrofauna (henceforth “macrofauna”) that live within and between the worm tubes. At soft sediment stations, macrofauna were collected using a 6.7-cm diameter core tube (35.4 cm² area) to a depth of 3 cm (modified from Street et al. 1997). Three replicate cores were collected from each station on each sampling date. All samples taken for quantitative analysis were preserved in 10% buffered formalin.

142

### *Laboratory analysis*

143

In the laboratory, the volume of Serpulid reef samples were determined by water displacement, then molluscs and other hard-shelled organisms were counted and identified. Serpulid reef samples were then placed in 12 mol L⁻¹ HCl for 24 hours to dissolve the calcium carbonate reef substrate. Organisms from Serpulid reef and soft-sediment samples were extracted on a 500 µm sieve, sorted using a dissecting microscope, identified to the lowest practical identifiable level (usually species), and enumerated. A Folsom plankton splitter was used to obtain a subsample of reef samples when macrofaunal abundance was too great to be practically counted. Dry-weight biomass measurements for both Serpulid reef and soft-sediment organisms were obtained for individual species or family groups after drying at 55 °C for 24 hours. Mollusc shells were removed with 1 mol L⁻¹ HCl prior to drying and weighing.

144

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147

148

149

150

151

152

153

Organisms were classified by feeding mode (suspension feeder, deposit feeder, or predator/scavenger; Bruggeman 2020) and taxa group (e.g., Crustacea, Bivalvia, Polychaeta). Suspension feeders were defined as macrofauna possessing filter feeding structures that generally

154

155
feed on particles in the water column. Deposit feeders were defined as organisms that consume particulate matter in or on the sediment. Predator/scavengers were defined as carnivorous organisms that consume other fauna.

## Statistical Analysis

Spearman rank correlation analyses were run between water quality variables (salinity, temperature, DO, pH) and both Serpulid reef and soft sediment macrofaunal abundance and biomass. Spatio-temporal differences in biomass-based macrobenthic community composition were determined using non-metric multi-dimensional scaling (nMDS) and cluster analysis (group average method; Clarke 1993). Multivariate analyses were performed on a Bray-Curtis similarity matrix on log₁₀(x+1) transformed data using Primer 7 software (Clarke and Gorley 2015).

## 2.6 Stable Isotope Analyses

### Sampling

Surface sediment organic matter (SSOM) and suspended particulate organic matter (SPOM) were collected at all stations via benthic coring (3 cm deep, 35.4 cm² area) and surface water collection, respectively, during each sampling event. Samples were stored on ice until returning to the laboratory. Triplicate Serpulid reef pieces and sediment cores were sampled at each station during each sampling event to collect macrofauna for stable isotope analyses using the same methods as for quantitative fauna analysis. The stable isotope cores were sieved in the field using a 500 µm mesh bag. Retained items and reef pieces were stored in containers filled with local water inside a cooler until the end of the field day, when they were transported to the laboratory.

Fish were collected in two ways: Reef-dwelling fish (Pinfish [*Lagodon rhomboides*] and Sheepshead [*Archosargus probatocephalus*]) were collected from Serpulid reef stations using
hook and line, or speargun. Sport fish (Black Drum [*Pogonias cromis*], Red Drum [*Sciaenops ocellatus*], and Spotted Seatrout [*Cynoscion nebulosus*]) were collected through partnerships with commercial and recreational fishermen. Up to 15 sport fish (including at least five *P. cromis*) were collected from Baffin Bay six to eight weeks after each benthic sampling event. *Pogonias cromis* were purchased from local trotline fishermen, while tissue samples from other species (particularly the non-commercial species *S. ocellatus* and *C. nebulosus*) were collected from anglers at a recreational boat launch. Sport fish were caught in Baffin Bay, but not necessarily around Serpulid reefs. All fish tissue was stored on ice until returning to the laboratory to be stored at -20 °C.

## Laboratory Sample Preparation

Water samples for SPOM analysis were sieved through a 250 µm-mesh to remove large zooplankton and detritus, then filtered through pre-combusted (4 hours at 450 °C) glass fiber filters (Whatman GF/F glass fiber filters, 0.7 mm porosity) which were then freeze dried, generally for 24-72 hours, until dry (Labconco Freezone). Carbonates were removed from filters for δ¹³C measurements by contact with HCl fumes for 4 hours in a vacuum-enclosed system. Nitrogen isotope compositions were analyzed using raw filters.

Samples of SSOM were sieved through a 500-µm mesh to remove macrofauna, large detritus, and large pieces of shell hash, then freeze dried and ground using a mortar and pestle. Samples to be analyzed for carbon were decarbonated by adding 1 mol L⁻¹ HCl drop by drop until cessation of bubbling. These samples were then dried in a dry-block heater, rinsed with deionized water, freeze-dried and manually ground again.

Macrofauna collected for stable isotope analyses were sieved live on a 500 µm mesh within 24 hours of sample collection, separated by taxa, and placed in aquaria in artificial seawater for 36 hours to allow evacuation of gut contents. For each species identified, three individuals of differing
size classes were reserved when possible. Soft tissues were collected from macrofauna. Mollusk shells were manually removed prior to stable isotope analysis. Lipids were not extracted from macrofauna because entire prey items are ingested by fish and therefore all tissues (including lipids) can be assimilated into predators (Newsome et al. 2014; Wolf et al. 2015). Mathematical corrections to δ¹³C values to account for variations in prey lipid concentrations were not used in this study because corrections on a large range of prey (with a large range of tissue compositions; Table S2) in habitat-scale assessments such as these can lead to larger biases than when not using any correction (Arostegui et al. 2019). Epaxial fish muscle tissue samples were removed from the anterior portion of the fillet. Any adhering skin, scales, or bones were removed from the tissue samples. Lipid removal was not needed on fish tissues prior to stable isotope analysis as C/N ratios (by mass) were lower than 3.5 (Table S2; Post et al. 2007). All fish and macrofauna samples were stored at -20 °C, freeze-dried and then ground (Retsch MM 400 ball mill) to a homogenous fine powder.

Appropriate amounts of each sample type were encapsulated in tin capsules, except acidified sediment samples. Those were encapsulated in silver capsules (to prevent corrosion of the capsule by acidified material) and then packed into tin capsules (to improve their combustion during the elemental analysis). The surface of the filters (i.e. acidified and non-acidified) was scraped using flat tweezers to collect the maximum amount of SPOM and the smallest amount of glass fibers, to improve the quality of combustion in the elemental analyzer. Samples were prepared in Texas A&M University-Corpus Christi laboratories for stable isotope analysis before sending off to be analyzed with a Flash EA 1112 elemental analyzer equipped with the Smart EA option (Thermo Scientific, Milan, Italy), coupled with a Delta V Advantage isotope ratio mass spectrometer with a Conflo IV interface (Thermo Scientific, Bremen, Germany) at the University
of La Rochelle, France (Littoral, Environment and Societies Joint Research Unit stable isotope facility, LIENSs). Isotope compositions are reported in the δ notation as deviations from standards: atmospheric N₂ for δ¹⁵N and Vienna Pee Dee Belemnite for δ¹³C using the formula:

$$ \delta^{13}\text{C} \text{ or } \delta^{15}\text{N} = \left[ \left( \frac{R_{\text{sample}}}{R_{\text{standard}}} \right) - 1 \right] \times 1000 $$

where R is ¹³C/¹²C or ¹⁵N/¹⁴N. Calibration was conducted using reference materials (USGS-24, -61, -62, IAEA-CH6, -600 for carbon; USGS-61, -62, IAEA-N2, -NO-3, -600 for nitrogen). The analytical precision of the measurements was <0.15‰ for carbon and nitrogen based on analyses of USGS-61 and USGS-62 used as laboratory internal standards.

## Statistical Analyses

Kruskal-Wallis rank sum tests followed by multiple comparisons were used to investigate seasonal or spatial-related fluctuations in isotope compositions of SPOM, SSOM, macrofauna, and fish muscle tissue. All tests were carried out with the R software (R Core Team 2013). Multiple comparisons of means were completed using the *pgirmess* package (Giraudoux 2011).

# 3. Results

## 3.1 Water Quality

Water quality was similar at the Serpulid reef and soft sediment stations (Figure 2). Mean salinity decreased from 45.6 ± 3.8 (mean ± standard deviation) in June 2018 to 27.8 ± 0.7 in December 2018 before slowly increasing back to 28.9 ± 0.3 in March 2019. Temperature followed a typical seasonal pattern with a high of 29.1 ± 0.3 °C in June and a low of 14.0 ± 0.6 °C in December. Trends in DO concentration were also seasonal, and inversely related to temperature (5.7 ± 0.2 and 9.6 ± 1.1 mg L⁻¹ in June and December, respectively). Mean pH was highest during
246 the study period in September 2018 (8.5 ± 0.0) and then decreased to a low of 7.8 ± 0.1 in March
247 2019. Water quality measurements were within the range of historic averages (Bugica et al. 2020).

248 **3.2 Quantitative Fauna Analyses**

249 Mean abundance of macrofauna was 191 times greater per volume of Serpulid reef than
250 the same volume calculated from the top 3 cm of soft sediment (Figure 3). Forty-three species
251 occurred in Serpulid reef samples, as opposed to twenty-two taxa in the soft sediment samples
252 (Table S3). Serpulid reef macrofauna were dominated in abundance by crustaceans and
253 polychaetes. The most abundant taxa were the amphipod *Apocorophium louisianum* (formerly
254 *Corophium louisianum*; 18,837 n L⁻¹, 62.9% of total abundance), the polychaetes *Polydora ligni*
255 (1601 n L⁻¹, 5.4%), *Syllis cornuta* (1565 n L⁻¹, 5.2%) and *Nainereis laevigata* (1203 n L⁻¹, 4.0%),
256 the tanaid *Hargeria rapax* (1227 n L⁻¹, 4.1%) and the barnacle *Balanus eburneus* (1218 n L⁻¹,
257 4.1%). The serpulid *H. dianthus* (539 n L⁻¹, 1.8%) was most abundant and temporally variable at
258 station R1 (966 n L⁻¹), with peaks occurring in September 2018 (2272 n L⁻¹) and March 2019 (1214
259 n L⁻¹). The most abundant taxa in the soft sediment were the polychaete *Mediomastus ambiseta*
260 (55 n L⁻¹, 35.2%), the bivalve *Mulinia lateralis* (49 n L⁻¹, 31.2%) and unidentified amphipods from
261 the Ampeliscidae family (27 n L⁻¹, 17.3%).

262 Mean macrofauna biomass was 97 times greater per volume of Serpulid reef than the same
263 volume calculated from the top 3 cm of soft sediment (Figure 3). The most dominant taxa by
264 weight on the Serpulid reefs were *B. eburneus* (3968 mg L⁻¹, 46.7% of total biomass), *A. louisianum*
265 (1100 mg L⁻¹, 12.9%), the bivalve *Brachidontes exustus* (992 mg L⁻¹, 11.7%), unidentified
266 Actinaria (485 mg L⁻¹, 5.7%) and the polychaete *Nereis riisei* (433 mg L⁻¹, 5.1%) (Table S4). The
267 most dominant taxa by weight in the soft sediment were the bivalves *M. lateralis* (67 mg L⁻¹,
268 77.3%) and *Lyonsia hyalina* (3 mg L⁻¹, 3.7%), the polychaetes *Pectinaria gouldii* (8 mg L⁻¹, 9.3%)
269 and *M. ambiseta* (3 mg L⁻¹, 3.1%), and unidentified Ampeliscidae (3 mg L⁻¹, 3.1%).

270 Overall, Serpulid reef macrofauna abundance increased down-estuary toward the mouth of
271 Baffin Bay from R1 (18,984 n L⁻¹) to R3 (42,132 n L⁻¹) (Figure 3 and Figure 4). Abundance
272 increased down-estuary for crustaceans (from 10,993 to 32,598 n L⁻¹) and polychaetes (from 6,432
273 to 8,068 n L⁻¹), while bivalves decreased in abundance (from 736 to 170 n L⁻¹; Table S3). There
274 was no consistent spatial trend in macrofauna biomass among Serpulid reef stations. No spatial
trend was observed in soft-sediment macrofauna abundance or biomass. The highest abundance of
275 crustaceans (11 n L⁻¹) was at station SS4, located closest to the mouth of Baffin Bay near the
276 Laguna Madre.
277

278 Deposit feeders were the most abundant feeding type, contributing 69,873 n L⁻¹ (78%) of
279 total abundance for Serpulid reef and 386 n L⁻¹ (62%) for soft sediments (Table S3). Suspension
280 feeders were the next most abundant in both Serpulid reef (9,289 n L⁻¹; 10%) and soft-sediment
281 (225 n L⁻¹; 36%) stations. Macrofauna biomass was dominated by suspension feeders in both
282 Serpulid reef (17.3 mg L⁻¹; 68%) and soft-sediment (0.32 mg L⁻¹; 91%) stations (Figure 5, Table
283 S4). Deposit feeders were the next most dominant, constituting 5.8 mg L⁻¹ (22%) of total biomass
284 for Serpulid reef and 0.03 mg L⁻¹ (9%) for soft sediments.
285

285 Macrofauna communities differed greatly between Serpulid reef and soft sediment stations
286 (Figure 6). Macrofauna community composition from all stations was at least 3% similar to one
287 another. Among Serpulid reef stations, macrofauna community composition was 53% similar.
288 There was a change in community composition and increase in diversity at Serpulid reef stations
289 moving down-estuary from R1 to R3. Soft sediment macrofauna community composition had
higher variability among stations and over time compared to macrofauna from Serpulid reef stations.

Macrofauna abundance in soft sediments was negatively correlated with salinity ($r = -0.499, p \le 0.069$), temperature ($r = -0.473, p \le 0.088$) and pH ($r = -0.550, p \le 0.042$; Table 1). Soft sediment biomass was also negatively correlated with pH ($r = -0.682, p \le 0.007$). Abundance and biomass of Serpulid reef macrofauna were not correlated with any measured water quality variables ($r \le |0.459|, p \ge 0.134$).

## 3.3 Isotope compositions

The isotope compositions of SPOM ($\delta^{13}$C: -24.5‰, $\delta^{15}$N: 3.5‰) were more depleted than SSOM ($\delta^{13}$C: -20.8‰, $\delta^{15}$N: 4.4 ‰) (paired t-tests, $\delta^{13}$C: $p < 0.001$; $\delta^{15}$N: $p = 0.029$; Table 2). Isotope compositions of SPOM and SSOM were similar among sampling stations (Kruskal-Wallis tests, $p \ge 0.063$; Figure 7). $\delta^{13}$C and $\delta^{15}$N values of SSOM were stable throughout the study (Kruskal-Wallis tests, $\delta^{13}$C: $p = 0.652$; $\delta^{15}$N: $p = 0.491$), while SPOM $\delta^{13}$C values decreased over time (Kruskal-Wallis test, $p < 0.001$; Table S5; Figure S1). $\delta^{15}$N values of SPOM peaked in December 2018 (Kruskal-Wallis test, $p = 0.009$). Algae presented relatively large ranges of $\delta^{13}$C, with means ranging from -20.5 (March 2019) to -16.9‰ (June 2018) and $\delta^{15}$N values, ranging from 3.9 (September 2019) to 6.2‰ (March 2019; Table S5).

Serpulid reef suspension feeders had mean $\delta^{13}$C values ranging from -21.6 (*B. exustus*) to -19.3‰ (*B. eburneus*) and mean $\delta^{15}$N values from 5.3 (*B. exustus*) to 7.9‰ (*B. eburneus*, Table 2). $\delta^{13}$C values of *B. eburneus* were lower in December 2018 compared to June 2018 and March 2019 (Kruskal-Wallis test, $p < 0.001$; Table S5). Otherwise, carbon isotope compositions of the three major species of suspension feeders (*B. eburneus*, *B. exustus* and *H. dianthus*) in the reefs were stable over time (Figure S1) and did not follow any spatial pattern. The $\delta^{15}$N values of these three
consumers were similar between stations (Kruskal-Wallis tests, *p* ≥ 0.391) with no clear temporal pattern. Serpulid deposit feeding species had δ¹³C and δ¹⁵N values similar to those of suspension feeders; mean δ¹³C values ranged from -20.4 (*A. louisianum*, Terebellidae) to -18.7‰ (*Cerapus tubularis*) and mean δ¹⁵N values ranged from 6.4 (*H. rapax*) to 7.6‰ (Terebellidae; Table 2).
Carbon isotope composition of the major species of deposit feeder (*A. louisianum*) was stable over time (Kruskal-Wallis test, *p* = 0.182; Figure S1). Predators/scavengers had mean δ¹³C values ranging from -20.8 (Cnidaria) to -17.9‰ (Xanthidae) and mean δ¹⁵N values from 8.4 (Nereididae) to 10.0‰ (Cnidaria), with the upper limits of these ranges being higher than those of suspension and deposit feeders.

Soft sediment suspension and deposit feeders' δ¹³C and δ¹⁵N values were similar over space and time. Mean δ¹³C values of soft sediment suspension feeders ranged from -21.1 (Pectinariidae) to -20.5‰ (*M. lateralis*), and mean δ¹⁵N values ranged from 5.8 (*M. lateralis*) to 7.1‰ (Pectinariidae; Table 2). Deposit feeders had mean δ¹³C values ranging from -22.1 (Ampharetidae) to -15.0‰ (*N. vibex*) and mean δ¹⁵N values ranged from 5.6 (Ampeliscidae) to 9.7‰ (Goniadidae). Soft sediment predators/scavengers had mean δ¹³C values ranging from -21.8 (Nemertea) to -19.7‰ (*A. canaliculata*) and mean δ¹⁵N values from 7.5 (Turbellaria) and 10.1‰ (Nemertea).

Comparisons between habitats demonstrated that suspension and deposit feeders collected at Serpulid reef and soft sediment stations had similar δ¹³C values (Kruskal-Wallis test, *p* = 0.180; Figure 7). δ¹⁵N values for these two groups of primary consumers were also similar, except suspension feeders from the Serpulid reef which were slightly more ¹⁵N-enriched than suspension feeders from the soft sediment (means of 6.8 vs. 6.0‰; Kruskal-Wallis test, *p* = 0.026).
Pogonias cromis collected by commercial fishermen throughout Baffin Bay had similar isotope compositions to L. rhomboides and A. probatocephalus collected at Serpulid reef sampling stations (Kruskal-Wallis tests, δ¹³C: *p* = 0.690, δ¹⁵N: *p* = 0.883; Figure 7). Mean fish δ¹³C values ranged from -18.9 (A. probatocephalus) to -15.7‰ (S. ocellatus). No difference was observed between δ¹³C values of fish species (Kruskal-Wallis test, *p* = 0.359). Mean fish δ¹⁵N values ranged from 9.9 (L. rhomboides) to 13.5‰ (C. nebulosus), with C. nebulosus being more enriched in ¹⁵N than other fish species.

The range of fish δ¹³C values overlapped those of suspension and deposit feeders from soft sediment and Serpulid reefs (Figure 7). The mean δ¹³C values of suspension (-20.7‰) and deposit feeders (-20.1‰) from both habitats were in the range of mean SSOM δ¹³C values (-21.0 to -20.8‰) and were much more ¹³C-enriched than SPOM (-24.7 to -24.4‰; Table 2).

## 4. Discussion

### 4.1 Biomass enhancement and Serpulid reef macrofauna

Baffin Bay's Serpulid reefs increase habitat complexity and support specific macrofauna assemblages with enriched abundance, biomass, and diversity. Macrofauna inhabiting Serpulid reefs occur in considerably higher densities (191x), have substantially higher biomass (97x), and are more diverse (2x) than those occurring in soft sediments. Macrofaunal community composition differed between habitat types, with crustaceans and polychaetes dominating at Serpulid reef habitat and bivalves and polychaetes dominating in soft sediments. While the population dynamics and community composition of Baffin Bay's soft sediment macrofauna has fluctuated greatly over time (Montagna and Kalke 1995; Rubio et al. 2018; Breaux et al. 2019), Serpulid reef macrofauna communities characterized in the current study are similar to those recorded historically, in particular compared to the most recent and probably most comprehensive study of Serpulid reefs
in Baffin Bay, conducted two decades ago (Hardegree 1997). The deposit-feeding amphipod *A. louisianum* was the most abundant reef organism observed in both studies (Hardegree 1997: 71% of total abundance; current study: 60% overall, 70% at station R3), and the eight most abundant macrofauna collected in 1997 were among the most numerous (8 of the 10 most abundant species) in the current study.

Whereas soft sediment macrofauna decreased in abundance with increases in salinity, temperature and pH, Serpulid reef macrofauna abundance and biomass were stable over time, indicating that they are more consistently available to higher-level consumers than soft sediment communities. Indeed, Baffin Bay experiences large and frequent fluctuations in water quality variables including salinity and pH, as well as frequent algal blooms (Cira and Wetz 2019; Cira et al. 2021), and numerous studies have documented the effects on soft sediment macrofauna (Montagna et al. 1993; Montagna and Kalke 1995; Street et al. 1997; Rubio et al. 2018; Breaux et al. 2019). While the role of salinity and temperature in structuring macrofauna communities is generally well understood (Ritter et al. 2005; Beseres Pollack et al. 2009; Palmer and Montagna 2015; Van Diggelen and Montagna 2016), the mechanism for the response to pH is less well known. Fluctuations in pH can affect marine organisms in various ways including reductions in metabolism and growth, solubility of calcium carbonates, and bioavailability of metals (Knutzen 1981; Guinotte and Fabry 2008), which may have contributed to variability in soft sediment macrofauna community.

Although Serpulid reef macrofaunal abundance and biomass were unrelated to salinity fluctuations in the current study, persistent hypersalinity could have a negative effect on Serpulid reef macrofaunal richness, as occurs in nearby soft sediment communities (Simmons 1957; Montagna et al. 2002). A greater number of reef taxa were identified in the current study (43 in
this study versus 29 in Hardegree 1997) despite similar sampling efforts, which may be related to
the relatively moderate salinity range in this study (monthly salinity range of 28 to 47) as compared
to the consistently hypersaline conditions (salinity range of 40 to 65) occurring during the
Hardegree study. In contrast, the number of reef taxa in this study (43) is comparable to that found
in a limited study of Baffin Bay under similar salinity conditions conducted three decades ago (38
species, Cole 1981). The proposed link between hypersalinity and decreasing Serpulid reef
macrofauna richness is speculative, as factors such as other changes in water quality could be
responsible. There are likely to be long-term fluctuations in Serpulid reef macrofauna communities
over time, but community stability relative to the surrounding soft sediment macrofauna is evident
and important to the functioning of this dynamic system.

## 4.2 Role of macrofauna from Serpulid Reefs in the Food Web

Baffin Bay supports a uniquely large *P. cromis* population (Olsen 2014), and Serpulid reef macrofauna are likely a key contributor to fish success in this environmentally stressed system. Isotope analyses indicate that *P. cromis* and other sport fish likely rely on macrofauna from both Serpulid reef and soft sediment habitats (directly for *P. cromis*, *A. probatocephalus* and *L. rhomboides*, and indirectly for higher-trophic level consumers like *C. nebulosus*). Although it is not possible to distinguish Serpulid reef macrofauna from macrofauna in soft sediments based on their isotope compositions, quantitative analyses from this and other studies demonstrate that these food sources are present in much higher numbers, with much greater biomass, and have less variability, at Serpulid reefs than in soft sediments (Montagna et al. 1993; Montagna and Kalke 1995; Hardegree 1997; Rubio et al. 2018; Breaux et al. 2019).

During hypersaline periods, when soft sediment macrofauna become scarce, Serpulid reefs
likely become an essential source of macrofaunal prey for higher trophic level consumers. Breaux
et al. (2019) found that *P. cromis* diets in Baffin Bay consist primarily of deposit-feeding macrofauna, which were rare or absent in soft sediment habitats during high salinity conditions. *Pogonias cromis* have limited motility during hypersaline conditions, foraging primarily within the boundaries of the bay rather than migrating to adjacent systems (Ajemian et al. 2018; Rubio et al. 2018; Breaux et al. 2019). In these situations, Serpulid reefs are the primary source of macrofauna prey. An increased understanding of the *P. cromis* diet is important because up to 76% of annual commercial landings for the region are harvested from Baffin Bay (Olsen et al. 2018), and because periodic emaciation events with significantly underweight *P. cromis* have plagued this relatively small estuarine system (Olsen 2014; 2016). Additional research to estimate *P. cromis* population size and carrying capacity would help further elucidate the importance of Serpulid reef macrofauna to maintaining fish populations in Baffin Bay.

While Baffin Bay's Serpulid reefs host considerably more macrofauna than do soft sediments, there is a clear and strong connection between the two habitat types, with soft sediments playing an important trophic role. Macrofauna biomass was dominated by suspension feeders in both Serpulid reef (68%) and soft sediment (91%) habitats. Suspension feeders generally rely on food sources suspended in the water column, but in the current study their isotope compositions were close to those of SSOM, indicating that a large part of their food resources originate from the sediment, likely due to the shallow depth (average ~1 m) and predominant wind-driven mixing (Tunnell 2001). The role of SSOM as a food source for suspension feeders has been demonstrated in similarly shallow, well-mixed estuaries (Blomberg et al. 2017), and in other estuarine habitats such as seagrass beds, and intertidal and subtidal mudflats (Riera and Richard 1996; Lebreton et al. 2011; Kang et al. 2015). Our results highlight the connectivity between these two major
habitats: soft sediments where primary production and/or storage of this production occurs, and
Serpulid reefs where macrofaunal consumers make this resource available to higher trophic levels.

## 4.3 Implications for Conservation and Restoration

Serpulid reefs are limited in their global distribution and at risk from anthropogenic
disturbance yet are highly valued for their ecological and conservation importance owing to their
high levels of biodiversity (Haines and Maurer 1980). Loss of biogenic reef structure has negative
implications for both reef-associated macrofauna and for higher trophic level organisms foraging
in these habitats (Kaiser et al. 1999). The only Serpulid found on the reef in the current study, *H. dianthus*, is known to exhibit gregarious settlement (Haines and Maurer 1980) and rapid growth
(up to 66 mm yr⁻¹; Grave 1933). However, it is plausible that *H. dianthus* reef-building activities
are affected by the predominantly hypersaline conditions (Cole 1981), reducing the rate of
biogenic reef recovery from physical disturbances, e.g., those associated with fishing activities
(Kaiser et al. 1999). Biochemical mechanisms may also contribute to reef decline, with common
and extreme changes in pH in the bay potentially altering calcification rates and changing the
mechanical properties of Serpulid tubes (Chan et al. 2012; Smith et al. 2013; Díaz-Castañeda et
al. 2019) and reducing their effectiveness against external attacks and bioerosion by predators
(Waldbusser et al. 2011; Wisshak et al. 2012; Hossain and Rahman 2017). Additional research is
warranted to understand how Serpulids will respond to environmental change.

Restoration is increasingly being used as a tool to ameliorate the effects of habitat loss for
biogenic reef builders (Jaap 2000; De Santiago et al. 2019). For habitats that are rare or limited in
distribution, ecological knowledge is needed to promote efficient, effective, and successful
restoration efforts (Cook et al. 2021). Although the current extent and size of Baffin Bay's Serpulid
reefs are unknown, observations indicate substantial reductions in habitat have occurred from the
16 km² reported from surveys conducted in the 1970s (Brown et al. 1977; Hardegree 1997; Tunnell et al. 2001). Additional effort is warranted to update maps of Serpulid reef in Baffin Bay using approaches that have been successful in other systems, including lidar (Noernberg et al. 2010) and side-scan sonar surveys (Moore et al. 2009; Raineault et al. 2012). Our results indicate that Serpulid reef macrofauna abundance increased up-estuary, and that *H. dianthus*, the only reef-builder in Baffin Bay, was most numerous at the furthest up-estuary station. Updated maps could be used in combination with these results to help select suitable locations to create restored reefs or protected areas to reestablish lost Serpulid habitat and food resources. Also, restoration success may be enhanced by selecting locations where *H. dianthus* are found in abundance because *H. dianthus* settlement relies on chemical cues associated with adults as indicators of habitat suitability (Toonen and Pawlik 1996). Additional information on hydrodynamics and larval dispersal could further help improve restoration planning and protected area design. The creation of short-term (< week) hydrodynamic transport models would be particularly useful for Serpulid management because *H. dianthus* larvae settle approximately five days after fertilization in the water column (Scheltema et al. 1981).

## 5. Conclusion

We have shown that Serpulid reefs play a critical role in the functioning of the Baffin Bay ecosystem, supporting specific macrofauna assemblages and providing increased macrofauna biomass. In contrast to macrofauna from soft sediments, Serpulid reef macrofauna abundance and biomass were stable over time, indicating they are more consistently available to higher-level consumers. In periods of hypersalinity common to the bay, when soft sediment macrofauna are limited, Serpulid reefs likely become an essential source of prey supporting *P. cromis* and other
472 sport fish populations. Stable isotope analysis results demonstrate trophic connectivity between
473 Serpulid reef and soft sediment habitats and highlight the importance of sediment organic matter
474 as a food source for the dominant suspension feeders. Previous evidence has shown substantial
475 reductions of Serpulid reef habitat in Baffin Bay compared to historic levels. Our results indicate
476 that further reef degradation and loss will undoubtedly have cascading effects for higher trophic
477 level organisms. Conservation actions may be needed to reestablish lost Serpulid habitat and food
478 resources, and results from the current study could help in selecting suitable locations for
479 restoration or protection. An improved understanding of the ecological structure and function of
480 Serpulid reef habitats is important because it will increase the probability of restoration success of
481 this rare and vulnerable habitat.

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<table><thead><tr><th>Habitat</th><th>Variable (unit)</th><th></th><th>Salinity</th><th>Temperature (°C)</th><th>pH</th><th>Dissolved Oxygen (mg L<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="4">Serpulid Reef</td><td rowspan="2">Abundance (n L<sup>-1</sup>)</td><td>rho</td><td>0.112</td><td>0.046</td><td>0.098</td><td>0.056</td></tr><tr><td>p</td><td>0.729</td><td>0.888</td><td>0.762</td><td>0.863</td></tr><tr><td rowspan="2">Biomass (mg L<sup>-1</sup>)</td><td>rho</td><td>-0.238</td><td>-0.459</td><td>-0.203</td><td>0.385</td></tr><tr><td>p</td><td>0.457</td><td>0.134</td><td>0.527</td><td>0.217</td></tr><tr><td rowspan="4">Soft Sediment</td><td rowspan="2">Abundance (n L<sup>-1</sup>)</td><td>rho</td><td>-0.499</td><td>-0.473</td><td>-0.550</td><td>0.389</td></tr><tr><td>p</td><td>0.069</td><td>0.088</td><td>0.042</td><td>0.169</td></tr><tr><td rowspan="2">Biomass (mg L<sup>-1</sup>)</td><td>rho</td><td>-0.341</td><td>-0.292</td><td>-0.682</td><td>0.134</td></tr><tr><td>p</td><td>0.233</td><td>0.311</td><td>0.007</td><td>0.648</td></tr></tbody></table>
<table><caption>**Table 2.** Stable isotope compositions (δ¹³C and δ¹⁵N, ‰) of taxa and organic matter samples collected from June 2018 to March 2019. SPOM = suspended particulate organic matter, SSOM = surface sediment organic matter.</caption><thead><tr><th rowspan="2">Location<br>Collected</th><th rowspan="2">Sample Type</th><th rowspan="2">Species Name</th><th rowspan="2">n</th><th colspan="2">δ¹³C</th><th colspan="2">δ¹⁵N</th></tr><tr><th>Mean</th><th>Std.<br>Dev.</th><th>Mean</th><th>Std.<br>Dev.</th></tr></thead><tbody><tr><td rowspan="10">Soft<br>Sediment</td><td rowspan="2">Organic matter</td><td>SPOM</td><td>25</td><td>-24.7</td><td>1.6</td><td>3.5</td><td>2.4</td></tr><tr><td>SSOM</td><td>14</td><td>-21.0</td><td>0.7</td><td>4.6</td><td>0.3</td></tr><tr><td rowspan="2">Suspension<br>feeder</td><td>Mulinia lateralis</td><td>19</td><td>-20.5</td><td>0.8</td><td>5.8</td><td>1.0</td></tr><tr><td>Pectinariidae</td><td>10</td><td>-21.1</td><td>0.5</td><td>7.1</td><td>1.2</td></tr><tr><td rowspan="6">Deposit feeder</td><td>Armandia agilis</td><td>1</td><td>-21.6</td><td></td><td>7.9</td><td></td></tr><tr><td>Ampeliscidae</td><td>2</td><td>-19.6</td><td>1.0</td><td>5.6</td><td>0.5</td></tr><tr><td>Ampharetidae</td><td>3</td><td>-22.1</td><td>2.2</td><td>7.7</td><td>0.3</td></tr><tr><td>Amphipoda</td><td>1</td><td>-20.8</td><td></td><td>7.3</td><td></td></tr><tr><td>Goniadidae</td><td>4</td><td>-20.9</td><td>1.1</td><td>9.7</td><td>1.1</td></tr><tr><td>Nassarius vibex</td><td>1</td><td>-15.0</td><td></td><td>6.1</td><td></td></tr><tr><td rowspan="3">Predator/<br>scavenger</td><td>Acteocina canaliculata</td><td>2</td><td>-19.7</td><td>1.0</td><td>7.6</td><td>2.2</td></tr><tr><td>Nemertea</td><td>1</td><td>-21.8</td><td></td><td>10.1</td><td></td></tr><tr><td>Turbellaria</td><td>2</td><td>-20.9</td><td>0.4</td><td>7.5</td><td>0.0</td></tr><tr><td rowspan="3">Primary<br>producer /<br>Organic matter</td><td>Algae</td><td></td><td>20</td><td>-18.4</td><td>4.0</td><td>5.0</td><td>1.6</td></tr><tr><td>SPOM</td><td></td><td>20</td><td>-24.4</td><td>1.6</td><td>3.4</td><td>1.7</td></tr><tr><td>SSOM</td><td></td><td>12</td><td>-20.8</td><td>2.0</td><td>4.2</td><td>0.7</td></tr><tr><td rowspan="3">Suspension<br>feeder</td><td>Balanus eburneus</td><td></td><td>36</td><td>-19.3</td><td>1.1</td><td>7.9</td><td>0.9</td></tr><tr><td>Brachidontes exustus</td><td></td><td>29</td><td>-21.6</td><td>0.6</td><td>5.3</td><td>0.6</td></tr><tr><td>Hydroides dianthus</td><td></td><td>29</td><td>-21.1</td><td>0.9</td><td>7.0</td><td>0.9</td></tr><tr><td rowspan="6">Serpulid<br>Reef</td><td rowspan="5">Deposit feeder</td><td>Apocorophium<br>louisianum</td><td></td><td>33</td><td>-20.4</td><td>1.2</td><td>6.5</td><td>0.7</td></tr><tr><td>Cerapus tubularis</td><td></td><td>2</td><td>-18.7</td><td>1.5</td><td>7.2</td><td>1.6</td></tr><tr><td>Hargeria rapax</td><td></td><td>1</td><td>-19.8</td><td></td><td>6.4</td><td></td></tr><tr><td>Orbiniidae</td><td></td><td>4</td><td>-20.3</td><td>0.5</td><td>7.0</td><td>0.3</td></tr><tr><td>Terebellidae</td><td></td><td>3</td><td>-20.4</td><td>0.2</td><td>7.6</td><td>0.1</td></tr><tr><td rowspan="3">Predator/<br>scavenger</td><td>Cnidaria</td><td></td><td>6</td><td>-20.8</td><td>0.3</td><td>10.0</td><td>0.9</td></tr><tr><td>Nereididae</td><td></td><td>29</td><td>-19.7</td><td>1.2</td><td>8.4</td><td>1.0</td></tr><tr><td>Xanthidae</td><td></td><td>4</td><td>-17.9</td><td>0.8</td><td>8.5</td><td>0.7</td></tr><tr><td rowspan="5">Baffin<br>Bay</td><td rowspan="5">Fish</td><td>Archosargus<br>probatocephalus</td><td></td><td>1</td><td>-18.9</td><td></td><td>11.0</td><td></td></tr><tr><td>Cynoscion nebulosus</td><td></td><td>13</td><td>-17</td><td>1.5</td><td>13.5</td><td>0.7</td></tr><tr><td>Lagodon rhomboides</td><td></td><td>8</td><td>-16.4</td><td>2.5</td><td>9.9</td><td>1.1</td></tr><tr><td>Pogonias cromis</td><td></td><td>23</td><td>-16.8</td><td>2.8</td><td>10.2</td><td>1.7</td></tr><tr><td>Sciaenops ocellatus</td><td></td><td>4</td><td>-15.7</td><td>2.8</td><td>12.2</td><td>0.6</td></tr></tbody></table>
<figure><img src="image_2.png" /></figure>

**Figure 1.** Map of the Baffin Bay complex showing reef (R) and soft sediment (SS) sampling locations. Approximate locations of reef are derived from NOAA electronic navigation charts (NOAA 2018). Almost all reefs in the map are Serpulid reefs (some in the Laguna Madre are rock reefs).

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<figure><img src="image_3.png" /><figcaption>**Figure 2.** Salinity and pH measured at soft sediment (blue) and Serpulid reef (red) stations. Shaded regions represent minimum and maximum salinities in each sampling period. Values before 2018 are plotted for reference only.</figcaption></figure>

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<figure><img src="image_4.png" /><figcaption>**Figure 3.** Abundance (top), and biomass (bottom) of fauna (n L⁻¹) in the top 3 cm of soft sediment (blue) and 3 pieces (approximately 3 cm³) of Serpulid reef (red) from June 2018-March 2019. In each boxplot, the line represents the median, x symbol represents the mean, the upper and lower box limits represent the upper and the lower quartile, upper and lower whiskers represent the minimum and maximum values (excluding outliers).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Mean abundance (top) and biomass (bottom) of major faunal groups occurring in soft sediment and Serpulid reef stations from June 2018-March 2019. Pie graph size is indicative of relative macrofaunal abundance and biomass, respectively.</figcaption></figure>

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<figure><img src="image_6.png" /><figcaption>**Figure 5.** Mean biomass of macrofauna by feeding type in soft sediment and Serpulid reef stations from June 2018-March 2019. Pie graph size is indicative of relative macrofaunal biomass.</figcaption></figure>

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<figure><img src="image_7.png" /><figcaption>**Figure 6.** Nonmetric multidimensional scaling plot of macrofaunal communities from Serpulid reef (R1-R3; triangles) and soft sediment stations (SS1-SS4; circles) from June 2018-March 2019. Ellipses represent 3% similarity groupings determined by cluster analysis.</figcaption></figure>

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<figure><img src="image_8.png" /><figcaption>**Figure 7.** Stable isotope compositions (δ¹³C and δ¹⁵N, ‰) of potential food sources and fish. Fish are shown as individual samples. All other samples are averaged by season (mean ± standard deviation). Sources from the Serpulid reef are represented by solid symbols and those from soft sediment by hollow symbols. SPOM = suspended particulate organic matter, SSOM = surface sediment organic matter.</figcaption></figure>

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