# TAMPA BAY

Numeric Nutrient Criteria:

Tidal Creeks

Letter Memorandum

Prepared for:

Tampa Bay Estuary Program

Prepared by:

Janicki Environmental, Inc.

16 February 201116 February 2011 1 

## Foreword

This letter memo was produced in partial fulfillment of Purch ase Order #6584, TBEP Contract T-07- 01 - Development of Numeric Nutrient Criteria for Tampa Bay.16 February 2011 2 

## Acknowledgements

We wish to thank the Partners of the Tampa Bay Estuary Program an d members of the Tampa Bay Nitrogen Management Consortium for the numerous conversations pro viding direction and insight into concerns regarding numeric nutrient criteria establishmen t and appropriate methodology for developing the proposed criteria. Ms. Holly Greening and Mr. Ed Sherwood were particularly helpful in their insights to this issue.16 February 2011 3 

## EXECUTIVE SUMMARY

Tidal creeks are relatively small coastal tributaries (<1-20 km in length) that lie at the transition
zone between terrestrial uplands, freshwater streams, and the open estuary, and serve as a link
between terrestrial and estuarine systems. Despite their close connec tion to these systems, tidal
creeks play a unique and integral role in the ecological fun ction of coastal estuaries as:

• a source of high primary and secondary production,

• a site of nutrient cycling, and

• a source of food for small-bodied fishes and crustaceans, as w ell as a foraging area for larger piscivorous fishes, wading birds, snakes, and alligators, and nu rsery habitat for juvenile fishes and crustaceans of economic value, including the common sno ok ( Centropomus

undecimalis ).

Tidal creeks possess water quality characteristics that differ from freshwater systems and from the open estuary. As a result of their direct connection and cl ose proximity to watershed sources of nutrient inputs and their smaller volumes and shallower depths relative to the open estuary, tidal creeks have relatively high nutrient and chlorophyll concentra tions and low dissolved oxygen (DO) levels compared to downstream waterbodies. DO levels from 2-4 mg/L ar e commonly observed in tidal creeks in the southeastern United States, including creeks fro m undeveloped watersheds. Higher nutrient concentrations and lower DO levels in tidal c reeks relative to the greater estuary may be required to support the higher levels of primary and secon dary production in these systems. Nutrient inputs from the surrounding watershed supply much o f the fuel that drives primary production in tidal creeks, in the form of benthic microa lgal communities and phytoplankton. The algal stocks, in turn, support upper trophic levels and drive secondary production by benthic macroinvertebrates, fishes, and decapod crustaceans. Despite po ssessing water quality conditions that would otherwise be considered impaired in freshwater an d estuarine systems, tidal creeks have been shown to support higher densities of many species of small-bod ied fishes compared to the adjacent estuary and tidal rivers. Many of these species have acqu ired physiological and behavioral adaptations, including aquatic surface respira tion (ASR) and air-gulping, which allow them to persist under the low-DO conditions that often occur in tidal creeks and to take advantage of the forage and refuge value of these systems.

In the Tampa Bay estuary, there are approximately sixty tidal creeks th at are terminal tributaries to the bay or to smaller embayments within the bay. Most tidal creeks i n Tampa Bay are relatively small (<10 km in length) and narrow (spanning only 25-50 m fr om bank to bank) in contrast to the tidal rivers which range from 40-100 km in length and 100-300 m wide on average. Many of Tampa Bay’s tidal creeks have been developed for urban, industri al, or agricultural land uses, though some tidal creeks remain relatively undeveloped and are pred ominantly found in mangrove wetlands. Unlike the open estuary of Tampa Bay which possesses expa nsive seagrass beds, tidal creeks are typically devoid of seagrass. For this reason, the develop ment of nutrient criteria based on water clarity and seagrass persistence is not appropriate.

Given the current state of knowledge for tidal creeks, four appr oaches to develop numeric nutrient criteria in tidal creeks are available. Each of these approach es has previously been considered for16 February 2011 4

development of nutrient criteria for the Tampa Bay estuary. These i nclude: 1) stressor-response models which examine quantitative relationships between nutrient c oncentrations and either chlorophyll a or DO concentrations within a specific waterbody of concern (i.e., tidal creek); 2) reference condition methods which use available data for selected reference periods in creeks to derive numeric nutrient criteria for the creeks; and 3) dow nstream protective values which are based on the relationship between water quality in the creek a nd that of the downstream receiving estuary such that the water quality in the tidal creek does not result in water-quality exceedances in the downstream estuary.

The most desirable approach to establish numeric nutrient cri teria would be to develop stressorresponse models. Stressor-response models require the identificati on of an indicator variable that

can be used to evaluate the condition of the tidal creek. Mo reover, stressor-response models require identification of a threshold value above (or below) which the system would no longer fully support its designated use. Due to a current lack of data with which to develop stressor-response models for Tampa Bay’s tidal creeks, numeric nutrient criteria for these systems are currently not

recommended.

Numeric nutrient criteria established for tidal creeks must co nsider the different ecological processes and functions that distinguish them from both the fresh water systems upstream and the open estuary downstream. It is important that the established criteria for tidal creeks also account for the fact that these systems by nature are more variable than t heir upstream or downstream counterparts. This variability is in part what makes these systems so productive and also so difficult to generalize. Implementation of criteria for tidal creeks shoul d also rely heavily on quantifying the

uncertainties in both the derivation of criteria and in t he evaluation of potential remediation efforts associated with failure of the criteria. Only with careful consideration of these factors can criteria be developed that will maintain the function of tidal creek s in support of the greater estuarine

ecosystem.

Based on the recognized need to define distinct biological endpoi nts for tidal tributaries and water
quality criteria to support them, TBEP staff recommends the foll owing:

• Recognize tidal tributaries as a separate waterbody class; and

• Consider setting a schedule (i.e., within 3 years) by which ti me endpoints and criteria will be proposed, but do not attempt to set interi m or final criteria with

insufficient data.

TBEP has dedicated funds to continue work in tidal tributa ries in Tampa Bay and will commit to
work with EPA to develop recommendations by September 2014.16 February 2011 5



### 1.0 Background

The Florida Department of Environmental Protection (FDEP) b egan development of numeric

nutrient standards in December 2001. The FDEP formed a technic al advisory committee and an agency work group to assist in identifying appropriate nutrien t standards. FDEP conducted a number of workshops and meetings as well as several studies that were conducted since 2002.

In 2008, several environmental groups filed suit against the U. S. Environmental Protection Agency (EPA) in Federal Court alleging that EPA had determined in 19 98 that Florida’s current narrative nutrient standard did not comply with the Clean Water Act and that EPA had not established numeric nutrient standards pursuant to Section 303(c)(4)(B) of the Clean Water Act. As a consequence of this lawsuit, EPA sent FDEP a letter on Janua ry 14, 2009 finding that FDEP’s narrative nutrient standard did not comply with the Clean Water Act and directing the State of Florida to develop its own numeric nutrient standards for ri vers and lakes by January 2010 and estuarine and coastal waters by January 2011 or EPA would a dopt its own nutrient standards. In August 2009, these groups and EPA agreed to a Consent Decree formally establishing these deadlines and EPA will be responsible for establishing these criteri a.

Currently, EPA is developing numeric nutrient criteria for fo ur water body types in Florida (EPA,

2010):

• Estuaries,

• South Florida flowing waters,

• South Florida coastal waters, and

• Other coastal waters.

The definition used by EPA for estuaries is similar to that of Pritchard (1967) and incorporates the State of Florida definition of a “predominantly marine water” and is as presented by Hagy (2010):

“An estuary is a semi-enclosed body of water, connected to the o pen sea, defined at the upstream limit by average salinity equal to 2.7 and at the seaw ard margin by the natural

limits of the semi-enclosed basin.”

Questions have been raised as to whether the criteria to be proposed for the estuary proper should apply to tidal creeks that drain to the estuary. The objec tive of this document is to provide support for the recommendation that unique numeric nutrient criter ia be developed for tidal creeks.

Tidal creeks play an integral role in the ecological functio n of coastal estuaries. The treatment of tidal creeks in the implementation of the estuarine numeric nut rient criteria is, therefore, a significant issue. A thorough understanding of the ecological elements (e.g., faunal and floral species and communities), processes (e.g., primary productivity, nu trient cycling, secondary production), dynamics of tidal creeks (e.g., temporal fluctua tions in dissolved oxygen) and function in exporting energy to estuarine and coastal ecotones is paramount to the establishment of ecologically appropriate nutrient criteria. Numeric nutrien t criteria established for tidal creeks must consider the different ecological processes and functions t hat distinguish them from both the freshwater systems upstream and the open estuary downstream. Only wit h careful consideration of these attributes can criteria be developed that will maintai n the function of tidal creeks in support of the greater estuarine ecosystem. The objective of this task is to:16 February 2011 6

• Provide a definition of tidal creeks,

• Provide a generalized overview of the ecological function of tidal creeks in relation to the

estuary,

• Identify factors to be considered when establishing criteria for tidal creeks, and

• Discuss potential methods for data evaluation directed at esta blishing nutrient criteria for these systems.

1.1 Definition of a Tidal Creek

Located at the transition zone between terrestrial uplands and the open estuary, tidal tributaries deliver freshwater and nutrients from the surrounding watershed to the estuary. Tidal tributaries can be classified based on size, with larger tidal rivers often a prominent feature in the estuarine landscape. Smaller tidal tributaries (herein referred to a s “tidal creeks”) include natural and manmade creeks, canals, navigational channels, and ditches creat ed for stormwater drainage or mosquito control. The geomorphological and physicochemical features of the tidal creeks distinguish them from the non-tidal, freshwater tributaries, springs, and lacustrine systems that are found elsewhere in the watershed and determine zonation patterns for the flora and fauna that inhabit these systems. Tidal creeks in peninsular Florida may reach well upstream of the mouth and may be distinguished on the basis of elevation; tidal creeks ext ending above sea-level are often greater in length and drain larger watershed areas than creeks restricted to elevations below sealevel. Those tidal creeks draining only intertidal areas are li kely dominated by tidal fluctuations and are less influenced by stormwater runoff than creeks origin ating above sea-level which often have well-developed freshwater reaches and are more sensitive to sto rmwater inputs.

The differences in physiographic and water quality attributes observed along the gradient from headwater streams to larger rivers are the result of processes rel ated to the flow of water and have been termed the “River Continuum Concept” by Vannote et al. (1 980). This concept is based on the idea that first-order tributaries are more strongly linked t o terrestrial processes and inputs and, as a result, are inherently different from downstream reaches. Flow -related changes to the geomorphology of the tributary (e.g., stream width, bank slope, ch annel depth) along the river continuum translate to differences in the composition of flor al and faunal communities, trophic structure and ecological processes. There is evidence that thi s concept can be applied to coastal systems, as well, from freshwater tributaries to tidal tribut aries to the estuary (Greathouse and Pringle, 2006).

1.2 Hydrological and Water Quality Processes in Tidal Creeks

Tidal creeks are expected to possess water quality characterist ics that differ from freshwater systems and from the open estuary; this should be a key consideration w hen developing numeric nutrient criteria for transitional systems like tidal creeks. As a result of their direct connection and close proximity to watershed sources of nutrient inputs and their sma ller volumes relative to the open estuary, tidal creeks have relatively high nutrient and chlorophyl l a concentrations and low dissolved oxygen (DO) levels (Holland et al., 2004; Sherwood, 200 8) in comparison to downstream waterbodies where nutrient loads are rapidly diluted by the greater water volumes. Flushing time in unmodified tidal creeks is relatively rapid (Buzzell i et al., 2007), but retention time and concentration of nutrient inputs from the watershed i ncreases in developed watersheds as the hydrology is impaired by sediment deposition, water-control stru ctures, etc. Tidal creeks with16 February 2011 7

extended flushing times or high nutrient inputs, in particular , have the potential to become hypoxic as nutrients are metabolized by the system and oxygen is consumed. For this reason, it is necessary that land use considerations be included when developing wa ter-quality criteria for these systems. Even then, the complexity of the landscape, extent of directly con nected impervious areas and the spatial arrangement of land use types within the watershed may l imit the ability to use the same

criteria for different tidal creeks.

DO levels and biochemical oxygen demand in tidal creeks are tightly c oupled to nutrient inputs via algal biomass which responds quickly to increased nutrients, o ften consuming oxygen in the process (Mallin et al., 2004). A graphical interpretatio n of the linkages among factors influencing dissolved oxygen levels is provided in Figure 1. Linkages among these f actors are consistent across aquatic systems, though the nature of the relationships varies as a result of multiple factors. The relative importance of allochthonous carbon (i.e., detritus from vascular plants, such as mangrove leaf litter, saltmarsh grasses, terrestrial vegetation) versus autoch thonous carbon (i.e., phytoplankton and benthic algae produced within the system) has a large influen ce on the rate of nutrient cycling and fluctuations in DO in tidal creeks. In the open estuary, autochthonous carbon is more important to nutrient, chlorophyll a . and DO dynamics, but in the transitional waters of the tid al creeks, the contribution of allochthonous inputs may be more i mportant. For any system, including tidal creeks, it is necessary to have sufficient knowledge to relate th ese factors to the DO response prior to establishing appropriate carbon-supply rates commensu rate with desired DO conditions. Typically, information on freshwater inflows, nutrient sup plies, the associated autochthonous carbon response (i.e., via phytoplankton), and the biotic i ntegrity of the system are more readily available than the supply rate of allocthonous organic car bon, re-aeration rates, and sediment oxygen demand that influence DO concentrations. Uncertainties related to the effects of these lessdefined impacts act as confounding factors in the development o f relationships between nutrients, phytoplankton responses, and DO.

Nutrients Flow

Re-aeration

Dissolved
Oxygen

Allochthonous

carbon Autochthonous

Carbon

- Phytoplankton

- Benthic Algae Biotic Integrity

Groundwater Production

& Respiration

Figure 1. Conceptual diagram depicting the relatio nships among water-quality parameters and physical factors that influence the biotic integrit y of aquatic systems. Large arrows identify the ke y relationships that can be used to develop numeric n utrient criteria for estuaries and tidal creeks.16 February 2011 8

Low DO is not uncommon in tidal creeks in the southeastern US, p articularly during the warmer spring and summer months and at night when primary producers swi tch from oxygen production to oxygen consumption (MacPherson et al., 2007). Several tidal c reeks from watersheds with varying land use characteristics in North Carolina all experienced lo w DO (2.0-4.0 mg/L) during the warmer, wetter months between May and September, in contrast to co oler, drier months when considerably higher (4.0-8.0 mg/L) DO levels were observed (MacPh erson, et al., 2007). Groundwater inflows, common in coastal areas where the surfic ial aquifer is in close contact with surface waters, can also be a significant driver of DO condit ions in tidal creeks and may need to be accounted for when developing water-quality relationships. A s anoxic groundwater infiltrates the tidal creek, greater oxygen demand results (MacPherson, et al., 2007).

1.3 Value of Tidal Creeks as Habitat for Estuarine Benthos and Nek ton

Though the ecological role of tidal creeks in the coastal ecosy stem is not yet fully understood, their value as a source of primary and secondary production and th eir contribution as habitat for juveniles of many species of marine, estuarine and freshwater fishes and crustaceans is becoming clear (Mallin and Lewitus, 2004; Holland et al., 2004; Krebs et al., 2007; Greenwood et al., 2008a; Sherwood et al., 2008). Nutrient inputs from the surroun ding watershed supply much of the fuel that drives primary production in tidal creeks, in the form of benthic microalgal communities and phytoplankton. The algal stocks, in turn, support small-bod ied fishes such as killifishes, sailfin mollies, and mosquitofish that reside permanently in the shallow waters of the tidal creeks (Nordlie, 2000) as well as schooling species like silversides and anchovies that feed on planktonic blooms.

The gently sloping banks of unmodified tidal creeks allow large exp anses of intertidal habitat to be inundated by rising tides and provide access to resident fishes wh ich use the intertidal areas adjacent to tidal creeks for spawning, feeding, and refuge from predators. These conditions allow many of the populations of resident fishes and crustaceans to reach densities that exceed those observed in the open estuary (Tukey and DeHaven, 2006; Sherwood , 2008; Stevens et al., 2010a) and provide an abundant food source which is consumed by up per trophic levels including largebodied fishes, wading birds, mangrove-saltmarsh snakes, alligato rs, and raccoons that forage in tidal creeks. In addition to resident taxa, relatively high abundanc es of juvenile transient fishes and blue crabs are found in tidal creeks (Krebs et al., 2007; Yeager et a l., 2007; Greenwood et al., 2008a,b; Brame, 2010) compared to adjacent habitats, suggesting that t idal creeks serve as nursery habitat for some estuarine and coastal marine species during their early life history.

Tidal creeks along the mid-Atlantic coast of the U.S. support a large number of nekton species. At least 100 taxa have been identified in tidal creeks from New Jerse y to Georgia many of which also occur in Florida’s tidal creeks. Schooling species, including Atlantic silversides ( Menidia menidia ), bay anchovy (

Anchoa mitchilli ) and several species of herring and shad, mullet ( Mugil spp.),

Fundulus killifishes and palaemonid grass shrimp are the numerically domina nt taxa in many of these systems. Penaeid and crangonid (sand) shrimp, spot ( Leiostomus xanthurus ), summer flounder ( Paralichthys dentatus ) and blue crab ( Callinectes sapidus ) are among the dominant economically important taxa found in tidal creeks(Cain and Dean. 1976; Hackney et al., 1976; Rozas and Hackney, 1984; Rountree and Able, 1992; Holland et al., 2004).

In temperate New Jersey estuaries, greater nekton densities have been doc umented for saltmarsh tidal creeks compared to adjacent seagrass and macroalgae habit ats demonstrating the importance of tidal creeks as habitat for fish and crustaceans and suggesti ng relatively high secondary production of small-bodied forage fishes in tidal creeks compar ed to adjacent habitats (Sogard and16 February 2011 9

Able, 1991). The value of tidal creeks in terms of their aqua tic-life support function is exemplified by higher growth rates and lower mortality of some juvenile fishes relative to downstream habitats in North Carolina (Ross, 2001).

Macrobenthic invertebrates serve as an important component o f the faunal community in tidal creeks, particularly as a food source for higher trophic levels. In South Carolina, 97 macroinvertebrate taxa were collected from tidal creeks, though n early half of these were rarely collected (Lerberg et al., 2000). Annelid worms, specifically o ligochaetes and polychaetes were the dominant taxa with nine species representing 90% of the communit y in terms of abundance, though nemertean worms were also more abundant than most taxa. In Tampa Bay tidal creeks, at least 44 taxa have been documented (Sherwood et al., 2007) with annelid worms, amphipods and mysid crustaceans among the dominant macrobenthos. The latter t wo taxa may be particularly important as trophic intermediates between primary production and upper-level consumers (Sherwood, 2008).

In terms of habitat value for macrobenthos, tidal creeks in Sou th Carolina have relatively low Shannon diversity (1.6-3.0) compared to adjacent estuarine h abitats (1.9-4.0, Chesapeake Bay; Lerberg et al., 2000). The range of macrobenthic diversity in Ta mpa Bay tidal creeks (1.5-3.0) was very similar to that observed for creeks in South Carolina for many of the tidal creeks, but very low diversity (0.5-1.0) was observed for several Tampa Bay creeks (Sher wood et al., 2007). Compared to the adjacent estuary, median diversity in Tampa Bay tidal cr eeks was generally lower (<2.0 vs. approximately 2.5; Karlen et al., 2008).

1.4 Southwest Florida Tidal Creeks

Tidal creeks in Florida are known to support a diverse fish c ommunity by providing habitat for numerous species and by maintaining high abundances of forage f ish. Approximately 150 taxa of fishes and decapods crustaceans have been collected from almost 80 tidal creeks from Cedar Key to Naples (Adams, 2005; Krebs et al., 2007; Greenwood et al., 20 08a,b; Stevens et al., 2008; Stevens et al., 2010a,b). Among these taxa are at least twenty-four sp ecies of economic value including spot, mullet, red drum, penaeid shrimp, blue crabs (Yeager et al. , 2006) and common snook, many of which use tidal creeks as a nursery during their juvenile stage. In comparison, approximately 200 taxa were collected from the Tampa Bay estuary (including th e tidal rivers) during 2008 fisheries-monitoring efforts (FWC, 2008). Similar species numbers were recorded for the Charlotte Harbor and Cedar Key estuaries (FWC, 2008).

Relative to adjacent bay and tidal river habitats, equivalent o r higher fish densities have been documented in tidal creeks from Cedar Key to Charlotte Harbor (Tukey and DeHaven, 2006; Krebs et al., 2007; Greenwood et al., 2008a; Stevens et al., 2010a, b). One species in particular, common snook, were observed as juveniles in tidal creeks at densities 2-36 t imes greater than shoreline habitat just outside the mouth of the creeks in Tampa Bay (Greenw ood et al., 2008a) and 6.5 times greater in tidal tributaries of the Caloosahatchee River compa red to the mainstem river (Stevens et al., 2010a). Fish densities in Gulf coast tidal creeks typical ly range from several hundred to several

thousand fish/100 m 2 (Adams, 2005; Tuckey and DeHaven, 2006; Krebs et al., 2007; Greenwood et al., 2008a, b; Dixon and Adams, 2010) though fish densi ties in some of these study creeks were

<100 fish/100 m 2 . Some of the most diverse fish assemblages in Tampa Bay tidal creeks were observed to have densities of 1,000-2,000 fish/100 m 2 and >30 taxa (Krebs, unpubl. ms). In comparison, average nekton densities from adjacent estuarine habit ats, including tidal rivers, seagrass and mangrove shorelines, are typically <1,000 fish/100 m 2 (Tuckey and DeHaven, 2006),16 February 2011 10

but have been shown to reach 2,500 fish/100 m 2 just outside the mouth of Tampa Bay tidal creeks (Greenwood et al., 2008a).

1.5 Dissolved Oxygen and Tidal Creek Fishes

Dissolved oxygen levels in tidal creeks are often lower than those observed in the receiving waters. For example, a study by Stevens et al. (2010a) reported average DO levels for the Caloosahatchee River to range from 7.0-7.9 mg/L in contrast to smaller tidal cr eeks which ranged from 6.4-6.8 mg/L and were 1.3 mg/L less than DO levels in the first 30 km of the mainstem river. Despite lower DO levels in the tidal creeks, total fish densities did not differ bet ween the tidal creeks and mainstem Caloosahatchee River and were, in fact, slightly greater in the tidal creeks (mean 502 vs. 554

fish/100 m 2 ). Species-level differences in abundance between river and creeks were observed, with higher densities of many resident taxa in the tidal creeks. As has b een documented in several previous studies (Greenwood et al., 2008; Sherwood et al., 20 08; Brame, 2010), higher densities of juvenile common snook were observed in tidal creeks, reinforcing the idea that tidal creeks serve as a nursery for this economically important species. The species co mposition of the fish assemblage was distinctly different between tidal creeks and adjacent areas, as well. Of the 33 taxa collected during the Stevens et al. (2010a) study, nearly half (n=14 ta xa) were collected in greater abundance in the tidal creeks, while only 9 taxa were collected in greater abundance in the mainstem river. The remaining taxa were equally abundant in both habitats.

Seasonally, fish abundances in Tampa Bay tidal creeks are highest near the end of the summer months when water temperatures are highest and dissolved oxygen levels are lowest. The trend is reversed during the cooler winter months when DO levels are highest a nd fish densities are lowest (Adams, 2005; Greenwood et al., 2008b). Although no rel ationship was found between land use and the community structure of macrobenthos in Tampa Bay tidal creeks (Sherwood et al., 2007), there was a clear pattern for tidal creek benthos in coastal South Carolina where pollution-tolerant taxa dominated the assemblage in urban watersheds (Lerberg et al., 2000). Lack of a relationship in Tampa Bay tidal creeks may have been related to very low rainfalls (i.e., low connectivity) during the study year and less runoff from the watershed.

Nekton species that commonly occur in tidal creeks have adapted to the physiologically stressful conditions of these systems. The often low DO conditions in tid al creeks have been suggested to provide a physiological refuge from predation for small fishes and crustaceans. For example, juvenile snook have been shown to have a much greater tolerance to low DO levels than adult snook, which has been proposed as a way to segregate smaller fro m larger individuals and reduce cannibalism (Peterson and Gilmore, 1991). Atlantic tarp on and Mayan cichlids are also able to persist at low DO levels by gulping air and storing it until oxygen is absorbed into the bloodstream (Geiger et al., 2000; Schofield et al., 2009). Similar physio logical and behavioral adaptations have been observed for highly abundant prey species such as the poecili id, cyprinodontid and fundulid fishes which include sailfin mollies, gulf killifish, and sheepsh ead minnows, all of which are capable of aquatic surface respiration (Nordlie, 2006), an adaptation that allows these species to utilize dissolved oxygen at the air-water interface when DO levels a re otherwise low in the tidal creek. Abundances of palaemonid grass shrimp and juvenile stripe d mullet, also very common prey for many species including juvenile snook and tarpon, were nega tively correlated with DO, with the highest abundances observed between 3-6 mg/L and very low ab undances at DO >6 mg/L (Greenwood et al. 2008a).16 February 2011 11



### 2.0 Establishing a Foundation for the Development of Numeric Nut rient Criteria in Tampa Bay Tidal Creeks

The following sections provide a summary of information that sho uld serve as the foundation for developing meaningful and relevant numeric nutrient criteria tha t will preserve the ecological function and habitat value of Tampa Bay tidal creeks. Furthe r summary of recent and historical studies on the ecology of tidal creeks in the Tampa Bay area is provi ded by Krebs et al. (2010), MacDonald et al. (2010), and Sherwood (2010).

2.1 Tampa Bay Tidal Creeks

There are approximately sixty tidal creeks that are terminal tribut aries to Tampa Bay or to smaller embayments within the bay (Figure 2). Tampa Bay tidal creeks differ substantially in scale from the larger tidal rivers and these differences in relative channel ge omorphology result in disparate hydrological and physicochemical characteristics from Tampa Ba y’s tidal rivers. Some of the larger tidal creeks extend far enough into the watershed that they have l ower order, freshwater tributaries that feed into them (e.g., Bullfrog Creek, Double Branch Creek, Frog Creek). Tidal creeks also differ from freshwater tributaries of the same size primarily due to their connection to the estuary. Small freshwater tributaries do not experience the semidiurnal tides which cause the daily and even hourly fluctuations in water level, flow direction, salinit y, water temperature and dissolved oxygen (DO) often recorded in tidal creeks (Buzzelli et al., 2007). Delineation of estuarine and freshwater tributaries to Tampa Bay is provided in Figure 3.

Unmodified tidal creeks are characterized by sinuous, meandering channels with average water depths <1.0 m, while those creeks modified for drainage, mosquito control, or navigation often have straightened channels with steeper, more uniform banks than unmodified creeks. Tidal creeks altered for navigation are typically deeper than other creeks (>2.0 m in depth) and often have hardened shorelines that have been cleared of vegetation. Most t idal creeks in Tampa Bay are relatively narrow, spanning only 25-50 m from bank to bank, i n contrast to the tidal rivers which are 100-300 m wide on average, although some of the larger tidal creeks reach 100 m or more in width near the mouth. The bathymetry of tidal creeks consists of alternating areas of deep, erosional and shallow, depositional bottom, unless the creek h as been channelized, in which case, it is often uniformly deep.

2.1.1 Riparian vegetation

Shoreline vegetation in many of Tampa Bay’s tidal creeks consists l argely of red mangrove

( Rhizophora mangle ) or white mangrove ( Laguncularia racemosa ), especially in the more mesohaline to polyhaline reaches and transitions. Black needleru sh ( Juncus roemerianus ) and cordgrass ( Spartina spp.) are also found along the banks in the higher salinity reaches, but are not nearly as common as mangroves. In the larger tidal tributaries with large watersheds, freshwatertolerant and upland vegetation such as cattails ( Typha spp.), leather fern ( Acrostichum

danaeifolium ), buttonwood ( Conocarpus erectus ) and oak ( Quercus spp.) occur as the tributary moves further into the upland areas.

2.1.2 Submerged aquatic vegetation

Unlike shallow embayments and open estuarine areas, submerged aqua tic vegetation is typically absent from tidal creeks, perhaps due to the proximity to freshwat er pulses and the resulting lower16 February 2011 12

salinities found in tidal creeks. Occasionally, ephemeral beds of widgeon grass ( Ruppia maritima ) have been observed in Tampa Bay’s tidal creeks but seagrass beds co nsisting of turtle grass

( Thalassia testudinum ), manatee grass ( Syringodium filiforme ) and shoal grass ( Halodule wrightii ) are not typically found in Tampa Bay tidal creeks.

Figure 2. Named tidal creeks within the Tampa Bay watershed.16 February 2011 13 Figure 3. Approximate location of the upstream limi t of tidal creeks and rivers as defined by
empirical salinity data analysis of data from tribu taries within the Tampa Bay watershed.

2.1.3 Dissolved oxygen and fishes

The relationship between fish abundance and species richness wit h DO has been examined from a number of Tampa Bay tidal creeks (Figures 4 and 5; Krebs et al., 2007; Greenwood et al., 2008a, 2008b). Both fish abundance and species richness in a number of tidal creeks have been shown to be similar at DO levels between 2-10 mg/L. Below 1 mg/L, however, abund ance and richness were much lower than observed at levels >2 mg/L. Fish abundance alo ne may not be a clear indicator of the DO in tidal creeks, as many taxa from these system s are eurytolerant to DO conditions and have adapted to persist at lower DO levels. Thi s is exemplified by the high abundance and low richness between 1-2 mg/L (Figures 4 and 5). Species richness of the fish and16 February 2011 14

decapods crustaceans is probably a more sensitive indicator of t he aquatic-life support function of tidal creeks as less tolerant taxa are more likely to occur at hi gher dissolved oxygen levels, thus increasing species richness. As DO levels decline, less tolerant taxa are less likely to be found and species richness declines. These results emphasize the unique nature o f tidal creeks and deserve consideration when the eventual nutrient criteria are prop osed.

2.1.4 Phytoplankton as measured by chlorophyll a

There is currently a paucity of data on in-stream chlorophyll a concentrations for Tampa Bay tidal creeks. The majority of creeks in Tampa Bay are not routinely moni tored and therefore there is little information from which to build stressor-response models. Yet, the available information does suggest that these creeks are highly productive systems and that benthic algae are an extremely important indicator of overall productivity in tidal cr eeks (Sherwood et. al., 2007). While benthic algae are not unique to tidal creeks, their relative contribu tion to system productivity is greater in

tidal creeks than in most other estuarine environments. Further research is needed to understand characteristics of in-stream chlorophyll a concentrations and to establish benthic algae as an indicator for developing nutrient criteria for tidal cr eeks.

2.1.5 Nutrients

Tidal creeks are ecologically distinct in many ways: both from t he freshwater streams that drain into them and from the downstream estuarine waters to which they drai n. Of particular note, the expectations for water quality in tidal creeks differ from both upstream and downstream waters. Specifically, chlorophyll a concentrations (including plan ktonic and benthic forms) needed to provide full aquatic-life support in tidal creeks are higher t han in the upstream or downstream waters. Similarly, the DO concentrations needed to provide ful l aquatic-life support in tidal creeks are lower than those required in the upstream or downstream wate rs. As such, the endpoints used to establish numeric nutrient criteria should be unique to t hese tidal creeks. Therefore, it is

recommended that the eventual numeric nutrient criteria for ti dal creeks be based on:

• stressor-response relationships between TN and TP and either c hlorophyll a or DO concentrations, and

• chlorophyll a thresholds and DO standards that reflect the unique nature o f these systems.

The potential options for developing numeric nutrient criteri a, including the stressor-response approach are detailed below.16 February 2011 15 Figure 4. Relationship between mean nekton abundan ce and DO in Tampa Bay tidal creeks. The numbers above each bar is the number of samples.

Figure 5. Relationship between mean species richne ss and DO for nekton in Tampa Bay tidal

creeks. The numbers above each bar is the number of samples.16 February 2011 16



### 3.0 Potential Approaches for the Development of Nutrient Crit eria for Tidal Creeks

Given the current state of knowledge on tidal creeks, three approa ches to develop numeric nutrient criteria in tidal creeks are available. Each of these approach es has previously been considered for development of nutrient criteria for the Tampa Bay estuary (Janic ki Environmental, Inc. 2011a,b).

• Stressor-response models – Examining quantitative relationships betwee n nutrient concentrations and either chlorophyll a or DO concentrations within a specific waterbody of concern (i.e., tidal creek);

• Reference condition methods - Using available data from selected ref erence periods in creeks to derive numeric nutrient criteria for the creeks;

• Downstream protective values - Based on the relationship between the water quality in the creek and that of the downstream receiving estuary such that th e water quality in the tidal creek does not result in water-quality exceedances in the down stream estuary.

3.1 Stressor-Response Method

The stressor-response modeling approach to establish numeric nutr ient criteria in tidal creeks relies on the development of a quantitative relationship between known i ndicators of system health (e.g. chlorophyll a concentrations and DO) and anthropogenic stressor variables (e.g., TN or TP). Using these relationships, the goal is to first identify the threshold r esponse beyond which adverse conditions are observed. Once this threshold value is determined, the relationship between stressors and response can be used to set limits on the magnitude of the stressor variable that is expected to maintain adequate water quality and avoid adverse conditions.

3.2 Reference Condition Method

The reference condition method uses available data for the system of interest to establish numeric nutrient criteria. The process involves the identification o f ambient water-quality conditions during periods when the system was meeting full aquatic-life support and establishing the criterion values for stressors and response indicators based on these condit ions. Often it is advantageous to establish both target values and threshold values using this app roach. Target values are those that represent a desired management endpoint for the system while threshold values are those beyond which the system is likely to exhibit adverse effects.

3.3 Downstream Protective Value Method

The goal of the downstream protection method would be to use th e estuarine nutrient criteria as a target to establish criteria for the tidal creek that is protec tive of downstream water quality. For example, relationships between total nitrogen concentration in the tidal creek compared to that in the adjacent bay segment could be used to determine how increases o r decreases in nitrogen in the tidal creek might be related to nitrogen concentrations in the bay segment, and to identify the nitrogen concentration in the tidal creek that would be commen surate with the downstream estuarine nitrogen criterion. Using this approach is less desi rable, however, as it is not based on maintaining the ecological function within the tidal creek and may result in nutrient criteria for the tidal creek that are insufficient to protect the biological integrity of the waterbody, including the high levels of primary production that are characteristic of these systems.16 February 2011 17



### 4.0 Recommendations

Studies of Tampa Bay tidal creeks have revealed compelling evidence that these systems represent unique ecotones within the greater Tampa Bay estuary. Tidal c reeks play an integral role in the ecological function of coastal estuaries as sites of high pr imary and secondary production, nursery and refuge habitat for several species of economically important fish and decapods crustaceans, and foraging areas for large-bodied fishes, wading birds, a nd other piscivorous species. Higher nutrient concentrations in tidal creeks relative to the greater estuary may be required to support the higher levels of primary and secondary production in these systems.

Analysis of fish collections in tidal creeks suggests that fishes inh abiting tidal creeks appear to be very tolerant to the typical DO conditions found in these systems. Both fish abundance and species richness data indicate that fish communities are relatively in variant to DO levels between 2-10 mg/L. There are indications that at DO concentrations below 2 mg/L, both fish abundance and species richness decline. Species richness of fish and decapods c rustaceans may be a more sensitive indicator of the aquatic-life support function of tidal creeks; however, these need further quantification to eliminate the possibility that seasonal recruitment patterns of estuarine-dependent fishes are not correlated with seasonal variation in dissolved oxyg en concentrations due to

temperature.

This report has provided a foundation from which further research can be conducted to establish scientifically sound and ecologically meaningful numeric nutri ent criteria for Tampa Bay tidal creeks. Future research into developing criteria for these systems sho uld recognize that:

• Tidal creeks represent a unique habitat in Tampa Bay, one th at serves a different ecological function than both freshwater tributaries, tidal rivers a nd downstream estuarine

environments

• Tidal creeks are generally highly colored systems with reduced water clarity and generally are devoid of seagrass

• Tidal creek productivity (measured as chlorophyll a concentration) is linked to both benthic algal production and water-column phytoplankton

• Dissolved oxygen concentrations are routinely below the current state standard of 4 mg/L for marine waters and evidence from analysis of fish collections suggests that DO does not appear to limit fish abundance or richness until DO concent rations reach levels <2 mg/L

• There is currently a paucity of empirical data from which to establish stressor-response relationships or reference condition approaches for Tampa Bay tidal creeks

The most desirable approach to establish numeric nutrient cri teria would be to develop stressorresponse models. Stressor-response models require the identification of an indicator variable that

can be used to evaluate the condition of the tidal creek. Mo reover, stressor-response models require identification of a threshold value above (or below) which the system would no longer fully support its designated use.

Based on current available data it will be difficult to selec t a reference condition for many of Tampa Bay’s tidal creeks due to the paucity of empirical data in these systems. Much of the available data were generated from short duration studies that were intended a s investigational and to serve as16 February 2011 18

baseline information. More effort is needed to identify a repres entative period of time when the systems were fully supporting aquatic uses to confidently establish r eference condition criteria.

Because of these constraints, numeric nutrient criteria for th ese systems are currently not recommended. However, the following recommendations can be made f or future efforts to define criteria for these systems.

It is important that any established criteria for tidal c reeks also account for the fact that these systems by nature are more variable than their upstream or downstrea m counterparts. This variability is in part what makes these systems so productive and a lso so difficult to generalize. The timing and volume of freshwater inflows are physical drivers that exert a great deal of control on tidal creeks. Inflows are deterministic of salinity regimes, nutrien t delivery, water depths, temperatures and the potential for salinity stratification in these systems. Inflows also may control access to these systems for both small recruit species looking for ref uge and for large-bodied predators. Therefore, the quantification of the effects on inflows on these systems will be necessary both to determine appropriate criteria and in the evaluatio n process.

Implementation of criteria for tidal creeks should rely heavily on quantifying the uncertainties in

both the derivation of the criteria and in the evaluation o f potential remediation effort associated with failure of the criteria. Only if the criteria are act ually relevant to the ecological function of the system will the criteria be meaningful in protecting full aquati c-life support in these systems. As such, there are many considerations in the implementation proce ss. These considerations are provided in detail in Janicki Environmental (2011c).

It is recommended that the assessment of compliance with the propo sed numeric nutrient criteria be performed in a manner similar to that which has been propo sed by TBEP for compliance with both the Tampa Bay Reasonable Assurance and TMDL (TBEP and Jani cki Environmental, 2010). The goal of the estuarine numeric nutrient criteria is to prov ide full aquatic-life support within the estuary. The TBEP has determined that seagrasses are important ind icators of desirable conditions in the bay and has defined the water-quality conditions (i. e., chlorophyll a concentrations) that allow for the maintenance and growth of seagrass beds in Tampa Ba y. Therefore, TBEP bases its compliance assessment on the comparison of both observed chlor ophyll a concentrations and seagrass extent to the goals that have been established.

The TBEP and TBNMC have been utilizing an annual assessment strate gy to track conditions in Tampa Bay with respect to chlorophyll a (Janicki et al., 2000). The strategy utilizes data collected by the Environmental Protection Commission of Hillsborough Coun ty (EPCHC) at numerous stations within the bay on a monthly basis. Conditions are a ssessed on an annual basis with respect to the FDEP-approved chlorophyll a thresholds in the four mainstem segments of the bay. To maintain consistency with these assessments, it is recommended that a similar approach be undertaken for tidal creeks when criteria are ultimately develop ed for these systems.

TBEP has dedicated funds to continue work in tidal tributa ries in Tampa Bay and will commit to
work with EPA to develop recommendations by September 2014.16 February 2011 19



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