HILLSBOROUGH RIVER BENTHIC FAUNA

A REVIEW OF 1991- 1992 DATA

WITH RESPECT TO FRESHWATER FLOW

Prepared For:

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT Building #2 2379 Broad Street Brooksville, Florida 34609-6899

Submitted By:

James K. Culter MOTE MARINE LABORATORY 1600 KEN THOMPSON PARKWAY Sarasota, Florida 34236

October 1997 Mote Marine Laboratory Technical Report Number #545

This document is printed on recycled paper.

Suggested reference Culter JK. 1997. Hillsborough river benthic fauna: a review of 1991-1992 data with respect to freshwater flow. Southwest Florida Water Management District. Mote Marine Laboratory Report no 545. 10 p. Available from: Mote Marine Laboratory Library. TABLE OF CONTENTS

Page No.

TABLE OF CONTENTS ...... i

LIST OF FIGURES ...... i

I. INTRODUCTION ...... 1

II. TIDAL RIVERS - GENERAL COMMENTS ...... 1

III. HILLSBOROUGH RIVER BENTHOS ...... 2 III. 1. PHYSICAL FACTORS ...... 2 III.2 HILLSBOROUGH RIVER SALINITY ...... 4 III.3 HILUBOROUGH RIVER DISSOLVED OXYGEN ...... 5 III.4 RECOMMENDATIONS FOR HYDROLOGIC ALTERATIONS ...... 6

IV. DISCUSSION...... 9

V. LITERATURE CITED ...... 10

LIST OF FIGURES

Figure 1. Location of sampling stations on Hillsborough River and Tampa Bypass Canal (Source: Water and Air Research, Inc. 1995) ...... 3

Figure 2. Location of salinity isohales based on a Hillsborough River Dam release of 60 cfs for three tidal scenarios ...... 7

Figure 3. Location of salinity isohales based on a release of 20 cfs from the Hillsborough River dam and 20 cfs from Sulfur Springs ...... 8

i I. INTRODUCTION

This report is a brief review of benthic macroinfaunal data collected from the Hillsborough River from October 1991 through July 1993. The data were collected, at quarterly intervals, as part of a three year comprehensive survey of the Hillsborough River and the Tampa Bypass Canal conducted for the West Coast Regional Water Supply Authority, Clearwater, and the City of Tampa, Florida. Components of the study included; a review of previous studies, a stormwater input survey, shoreline habitat inventory for the lower Hillsborough River and Palm River, water quality monitoring, biological collections of phytoplankton, benthic macroinfauna, ichthyoplankton, juvenile fish, and development of a hydrologic model, and evaluation of a withdraw and augmentation scenario.

The objectives of this review were:

to determine the apparent health of the benthos with respect to salinity, dissolved oxygen and river flow.

to render an opinion of the potential effects of flow (salinity) alterations on the composition and distribution of the benthos and to recommend a scenario that would contribute to the maintenance of a more natural system.

II. TIDAL RIVERS - GENERAL COMMENTS

Tidal rivers on the Florida peninsula contain most of the coastal oligohaline or low salinity waters, meaning that all of the wetlands, submerged aquatic vegetation, reefs, unconsolidated sediments, creeks, and other habitat features in the tidal river experience varying periods of freshwater, brackish water, and waters of higher salinity (Browder, 1991). Taken as a whole, this mosaic of habitats influenced by oligohaline waters comprises an important environment for the larval and juvenile developmental stages of many invertebrates and fishes of commercial, economic, or ecological importance (Edwards, 1991; Estevez et al. , 1991c; Peebles et al., 1991, Peebles and Flannery 1992).

The ecological importance of the low salinity reaches of estuaries has been amply documented, and their freshwater needs currently are used as guidelines for river flow regulation (Longley, 1994). Most recently, Jassby et al. (1995) have demonstrated that the 2 part per thousand (ppt) salinity position in the San Francisco Bay and Sacramento - San Joaquin Delta Estuary has simple and significant relationships with phytoplankton, plankton-based detritus, molluscs, mysids and shrimps, larval fish survival, and the abundance of several fish trophic guilds.

1 III. HILLSBOROUGH RIVER BENTHOS

The Hillsborough River study area was bounded upstream by the dam and Hillsborough Bay, downstream. Within this area four benthic stations numbered 3,5,7, and 9 were sampled, Figure 1. All stations were sampled at approximately the same depth (3.3, 3.3, 2.9 and 3.2 mean depth in meters).

III.1. PHYSICALFACTORS

The structure of the benthos is complex and dependent on many interacting factors. However, three physical factors are of primary importance in regulating species composition and abundance of the benthos: salinity, dissolved oxygen, and substratum composition. Substratum parameters such as grain size composition and organic content were not measured for the studies. Other factors can also be important or regulating in nature, such as circulation, temperature, levels of toxic contaminants, predation, etc. Circulation is important for reducing water stratification, transport of nutrients and food items, and dispersion and recruitment of fauna. Temperature maxima can be limiting under certain circumstances, such as addition of thermal wastewater, solar heating in stagnant waters, and solar heating of tidepools and shallow coastal areas, particularly in the tropics. In this area of Florida, temperature minima rarely affect benthic organisms, although intertidal communities are occasionally impacted by winter freezes during low tides. Toxic contamination, associated with heavy industry, can also be a problem, as many benthic organisms are sensitive to the presence of metals and organic chemical pollutants. However, this type of pollution is not a severe problem in the Hillsborough River, although the presence of contaminants from urban runoff have been identified by past studies.

Salinity, dissolved oxygen and substratum all exhibit variation within “normal” ranges that may be stressful or lethal to benthic invertebrate species. Estuarine zonation of fauna is defined by the relative dilution of seawater by freshwater. In this respect freshwater is toxic to marine species, and conversely saltwater is toxic to freshwater species. The toxicity results from the physiologic limitations of animals to regulate the ionic balances of the body fluids. Changes in the dissolved ion concentrations (salts) present in water create physiologic stress on aquatic animals. Most species can survive within a range of salinities, which may vary, depending on the presence of other stress factors. Species which are tolerant of the mixing zone, the estuary are labeled euryhaline. Euryhaline species generally are characteristic of the upper estuary and are generally derived from the marine fauna. Freshwater fauna generally do not exhibit a wide tolerance of salinity increase.

For the benthos the transitional area between estuary and freshwater is generally species poor. Florida tidal rivers can exhibit marked seasonal movement in the position of the estuary freshwater interface due to heavy seasonal rains.

2 Figure 1. Location of sampling stations on Hillsborough River and Tampa Bypass Canal (Source: Water and Air Research, Inc. 1995).

3 III.2 HILLSBOROUGH RIVER SALINITY

Individual salinity measurements for the benthic stations ranged from 0.0 ppt to 26.5 ppt. The upper three stations, 3, 5, and 7 all exhibited minimum bottom salinities as low as 0.0 ppt. For station 9 a minimum salinity of 10.0 ppt was reported. These low salinities exert considerable influence on the benthic fauna. The interstitial water of the substratum offers some buffering capacity for changes in salt concentrations since porewater exchange occurs primarily through diffusion and the burrowing activity of the fauna1 community. Therefore, the duration of water column changes in salinity is nearly as important as the magnitude of the change. The duration of the 0.0 ppt salinity events at stations 3, 5, and 7 were sufficient to reduce the species and abundance of the estuarine fauna. The duration of 0.0 ppt salinity events, below station 3, varied over the three year study period and occurred over two broad time periods, July - October, and February - May. The duration of the 0.0 ppt salinity events was approximately 2 months for Year 1, 4 months for Year 2, and three months for Year 3 (WAR, 1995; volume I, page 5-6).

Salinity drops within the river were dependent on the rainy season releases from the reservoir dam. During the releases there is the potential for a freshwater fauna to develop below the dam. This would depend on availability of adults to deposit eggs, for the aquatic insect larvae, and recruitment of larvae and some adults through downstream drift. Interstitial freshwater fauna such as the molluscs and annelids are less likely to be washed past the dam. Seasonal freshwater releases have the greatest effect on bottom salinities near the dam. The flow becomes largely stratified down river. For example; for Year 1, the difference in surface and bottom salinity for Station 2 was generally less than 2 ppt with many readings equal, however, for Station 10 the surface and bottom salinity became increasingly stratified with increased flow, with a surface to bottom difference of 19 ppt for September 1992.

With the cessation of rainy season flows the dam serves as salinity barrier, collapsing the transition zone between fresh and saltwater. Surface salinity at the station nearest the dam was greater than 5 ppt for 7 months during Year 1, was near 5 ppt for four months of Year 2, and greater than 5 ppt approximately 2 months for Year 3.

These changes in the salinity regime are reflected in the fauna. For October 1991 the only benthic invertebrate present at station 3 was a Chironomid group. It is likely this was at the end of the previous rainy season. Three months later (January 1992) the salinity was up to 12.0 ppt and the euryhaline polychaete Nereidae sp. A was found in high numbers at station 3. Salinity decreased for each successive benthic sampling and Nereidae sp. A also declined as a result. At the same time the Chironomid group increased to maximum densities by October 1992, after three months of 0.0 ppt salinity. Also in October the oligochaete Tubificidae immature sp. A appeared as a dominant taxa. A similar trend was observed at station 5, with Nereidae sp. A being abundant in January, and freshwater insect fauna becoming prevalent by October. The increasing bottom salinity at station 5

4 is also manifest with another euryhaline polychaete Paraprionospio pinnata, which occurred in high numbers in January and April. The abundance and diversity of species increased downstream as bottom conditions became more estuarine.

Confounding factors that may affect the interpretation of salinity induced changes to benthic community distributions include sediment type, bottom vegetation, and water column depth. Additional sources of variation (covariates) include variables such as temperature, dissolved oxygen, and current velocity.

III.3 HILLSBOROUGH RIVER DISSOLVED OXYGEN

Dissolved oxygen levels can vary widely within natural systems. While many benthic animals are tolerant of low dissolved oxygen levels, and a few can tolerate brief exposures to anaerobic conditions, general hypoxic conditions tend to reduce species diversity and abundance. Prolonged anaerobic conditions will defaunate the benthos.

Sediment organics, water turbulence, phytoplankton and macrophytes, and salinity stratification can contribute to oxygen increases or decreases within the water column. Dissolved oxygen levels within the substratum are dependent on diffusion and organism mediated circulation through tubes and burrows. Very few studies have quantitatively evaluated the relationship between water column and substratum dissolved oxygen levels. However, interstitial dissolved oxygen levels are usually substantially lower than water column levels. The redox layer (RPD layer) is the location within the substratum beneath which anaerobic conditions exist. The location of the RPD layer varies, depending on substratum composition and water column dissolved oxygen, but is often found within 15 centimeters of the sediment water interface.

Freshwater inflows also have an important effect on water column dissolved oxygen. Hypoxic events are the subject of ongoing study within Charlotte Harbor and are correlated with freshwater inflows. When more dense salt water becomes trapped with a freshwater cover (stratification) oxygen exchange can be impaired.

Hillsborough River dissolved oxygen levels have a profound influence on the abundance and diversity of the benthic fauna. Comparisons of shallow water versus deeper water fauna at the same station indicate that the productivity of the benthos could be substantially improved if the dissolved oxygen levels were increased. The shallow water fauna included a greater number of burrowing fauna as opposed to surface crawlers The reported data illustrated that dissolved oxygen levels were correlated with river flow and for both winter (January) and summer (June) dissolved oxygen levels were 10 percent of saturation or less for well over half the study area. A plot of June 1993 dissolved oxygen values shows a severe depression of dissolved oxygen (less than 2.0 mg/l) from 1.5 to 9 kilometers downstream of the dam. With daytime water column dissolved oxygen levels below 2.0 mg/l it is likely that the substratum is anoxic, particularly at night when water column dissolved oxygen drops.

5 III.4 RECOMMENDATIONS FOR HYDROLOGIC ALTERATIONS

Benthic fauna are excellent indicators of environmental health. At the same time it is relatively difficult to totally eliminate macroinfauna. Extremely low diversity of species and abundance are almost always indicative of a serious environmental alteration. Where low diversity and abundance occur naturally it is usually due to marginal, stressful habitat conditions or wide range changes of critical parameters.

The size of the habitat must also be taken into consideration. If the goal were to simply increase the diversity and abundance of invertebrates downstream of the Hillsborough River dam the recommendation could be to remove all fresh water and allow the higher diversity estuarine fauna to migrate upstream, providing that adequate dissolved oxygen levels could be maintained. However, this scenario would actually result in a decrease of riverine habitat diversity. By going from fresh (0.0 ppt salinity) upstream of the dam to estuarine (ca. + 15 ppt) downstream of the dam the transitional habitat would be lost. This is, in reality a coarse approximation of the existing situation with the estuarine fauna moving up to the dam during periods of no release.

With the goal of increasing the lower river system diversity a number of modeled flow scenarios were examined based on various levels of release from the dam in combination with upriver diversion of portions of the flow from Sulfur Springs. Increasing the system diversity depends on creating a 0.0 salinity region downstream of the dam. This would create a permanent freshwater community with a year-round connection to the estuary. Most freshwater fauna are not tolerant of even low salinity levels. The fauna of station 3 was sparse in long term freshwater infauna such as annelids and molluscs, due to the salinity fluctuations.

Another goal should be to reduce or eliminate the salinity stratification in one or both of the upriver deep areas (kilometer 1.5 to 3 and 3.75 to 4.75) which would enhance dissolved oxygen levels and provide deep freshwater habitat, again increasing the complexity of the river.

The modeled salinity distributions, based on flow data, indicate that the Sulfur Springs flow has a relatively small surface effect on salinity. Diversion of a portion of the flow for release at the dam should be considered which will result in a greater contribution to the maintenance of a freshwater zone. At the same time consideration of methods to enhance oxygenation of the discharge waters should be examined. Sulfur Springs may contain levels of hydrogen sulfide adequate to contribute to a reduction in dissolved oxygen within the water column.

The modeled flows show that a relatively large dam discharge, 60 cfs, is required to push saltwater below the 3 kilometer mark, Figure 2 (with an existing Sulfur Springs release of 40 cfs). The feasibility of achieving this effect through a lesser discharge or the combination of dam release and Sulfur Springs re-routing or the use of salinity control structures should be considered.

6 Figure 2. Location of salinity isohales based on a Hillsborough River Dam release of 60 cfs for three tidal scenarios. 7 Figure 3. Location of salinity isohales based on a release of 20 cfs from the Hillsborough River dam and 20 cfs from Sulfur Springs. 8 A minimum flow required to maintain a freshwater zone below the dam can be obtained by a combined flow of 20 cfs from the dam and 20 cfs diverted to the dam from Sulfur Springs, Figure 3. This flow would maintain approximately 0.75 kilometer of freshwater habitat below the dam at high tide. The effect of tide was shown to be relatively small, with a 0.25 kilometer increase in freshwater area for the same release at low tide. Again the feasibility of augmenting the release effect through the use of salinity control structures should be considered.

IV. DISCUSSION

Salinity zonation is a natural occurrence in Florida’s tidal rivers. with the exception of spring fed rivers peak flows coincide with the wet seasons and flow minima with the dry seasons. No flow scenarios are rare for natural Florida rivers. Natural flows are dampened through the buffering action of vegetation and groundwater percolation. Natural cycle flow peaks valleys occur over longer periods than the releases from regulated rivers and fauna have a greater time period to adapt, migrate or colonize moving salinity zones.

There is considerable evidence that motile crustaceans such as amphipods, tanaids, mysids and some isopods migrate along salinity fronts. They may be reacting to plankton blooms as nutrients are washed downstream, precipitation of dissolved organic matter, or the flow of small organic detritus along a water density gradient. These species can occur in high densities in the upper estuary. The author has observed high densities of motile crustaceans in the Myakka River, the Withlacoochee

9 v. LITERATURE CITED

Browder, J.A. 1991. Watershed management and the importance of freshwater flow to estuaries. pp. 7-22.In: S.F. Treat and P.A. Clark (ed.s), Proceedings, Tampa Bay Scientific Information Symposium 2. Tampa Bay Regional Planning Council, St. Petersburg, FL.

Edwards, R.E. 1991. Nursery habitats of important early-juvenile fishes in the Manatee River Estuary system of Tampa Bay. pp. 237-251. In: S.F. Treat and P.A. Clark (ed.s), Proceedings, Tampa Bay Scientific Information Symposium 2. Tampa Bay Regional Planning Council, St. Petersburg, FL.

Estevez, E.D. and M. J. Marshall. 1993. Sebastian River Salinity Regime. Final report to St. Johns River Water Management District. MML Technical Report No. 308. 171 p.

Estevez, E.D. and M. J. Marshall. 1994. Ecological Impact of Freshwater Flow Variations in the Manatee River, part II of Tampa Bay National Estuary Program Technical Publication #09-94. var. pag.

Estevez, E.D. and M. J. Marshall. (in press). A landscape model assessing estuarine impacts of freshwater inflow alterations, In: S .F. Treat (ed.), Proceedings, Tampa Bay Scientific Information Symposium 3. Tampa Bay Regional Planning Council, St. Petersburg, FL.

Jassby, A.D. ,W.J. Kimmerer, S.G. Monismith, C. Armor, J.E. Cloern, T.M. Powell, J.R. Schubel and T.J. Vendlinski. 1995. Isohaline position as a habitat indicator for estuarine populations. Ecological Applications, 5(1): 272-289.

Longley, W.L., ed. 1994. Freshwater inflows into Texas bays and estuaries: Ecological relationships and methods for determination of needs. Texas Water Development Board and Texas Parks and Wildlife Department, Austin, Texas, 386 pp.

Peebles E.B., M.S. Flannery, R.E. Matheson, Jr. and J.P. Rast. 1991. Fish nursery utilization of the Little Manatee River Estuary (Florida): Relationships to physiochemical gradients and the distribution of food resources. In: Trear, S .F. and P. A. Clark, eds. Proceedings, Tampa Bay Area Scientific Information Symposium 2, February 27 - March 1, 1991. Tampa Bay Regional Planning Council, Tampa, Florida. 528 pp.

Peebles, E.B. and M.S. Flannery. 1992. Fish nursery use of the Little Manatee River estuary (Florida): Relationships with freshwater discharge. final report submitted to the Southwest Florida Water Management District, December 1992.

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