Smithsonian Marine Station at Fort Pierce

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Introduction

Filter-feeding oyster are important members of the Indian River Lagoon because they reduce turbidity and improve water quality by removing suspended organics, phytoplankton, and detritus (Peterson et al. 2003). Since the 1940s, over 250 acres of oyster habitat has been lost in the Indian River Lagoon (Wilson et al. 2005). This is evident in aerial photos dating as far back as 1943 (Grizzle et al. 2002). The extent to which natural as well as anthropogenic effects including human-induced salinity fluctuations, muck removal projects, and biotoxin accumulation has impacted the Indian River Lagoon oyster reefs is not well understood. Reports such as the Independent Scientific Review of the Indian River Lagoon - South Project Implementation Report (Bartell et al.) commissioned by the U. S. Army Corps of Engineers Jacksonville District, the Indian River Lagoon Biotoxin and Aquatic Animal Health History and Background Report (Provancha and Van den Ende 2006), and the Indian River Lagoon Comprehensive Conservation and Management Plan Update 2008 suggest that heavy boating, poor management of sewage discharge, agricultural runoff, freshwater flooding from flood control canals, muck removal, and biotoxins are having negative impacts on the plants and animals that live in the Indian River Lagoon.

Boating Impacts

The most pronounced and documented changes to the oyster reefs are the dead margins appearing in areas of high boat use (Grizzle et al. 2002). The dead margins consist primarily of piles of dead oyster shells that occur when boat propellers drag along the bottom in shallow areas displacing living and dead oyster shells. Considerable damage is also caused by boat wakes which eroding both shorelines and oyster reefs and stir up bottom sediments causing the water to cloud (Grizzle et al. 2002). Wave action caused by boats results in sediment build-up on the oyster beds as the disturbed sediment moves out of the water column interfering with filter feeding and larval settlement. Although dead margins are sometimes considered beneficial because they may create a buffer back reef against wave-motion, current research suggests that oyster reefs with dead margins have reduced habitat complexity as well as reduced diversity of species (Stiner and Walters 2008).

Salinity Fluctuations

Salinity fluctuations in the Indian River Lagoon are a major concern. They can be caused by the input of freshwater from sewage discharge, agricultural runoff, and freshwater from flood control canals. Algal blooms occur when environmental conditions are optimal for growth. Discharge of nutrients from agricultural, ranch and urban areas cause an increase in nutrients in the sediments of the lakes of Upper St. John River Basin (Bremmer et al. 1999). Freshwater runoff that causes nutrient enrichment in the estuarine waters can lead to blooms of some species but the data is not clear that this is true for all species (Burkholder 1998). Blooms of some species occur during El Nino events when the water temperatures are increased (Burkholder 1998). Periods of reduced salinities that lead to oyster death see subsequent algal blooms. Point and non-point runoff in the Indian River Lagoon do have impacts but it is unclear how much impact relative to the dominant flood-control influences (WS Arnold, personal communication).

A decrease in salinity to < 10‰ appears to shorten the reproductive and settlement season of the oyster Crassostrea virginica (Bartell et al.), a major reef builder in the estuaries of the Indian River Lagoon. The C-44 canal is a major drainage canal that connects Lake Okeechobee with the South Fork of the St. Lucie estuary. The fresh water flows from this flood canal appear to result in low salinities for several weeks at a time every year potentially interfering with the reproductive season of C. virginica (Bartell et al., Wilson et al. 2005). Recent unpublished data show that fresh water from flood control canals in the St. Lucie and Sebastian estuaries combined with effects of the hurricanes of 2004 and 2005 almost completely eliminated live oysters from these areas. While live oysters did return within about one year, the chronic and long-term release of fresh water particularly into the St. Lucie system appears to have reduced the areal coverage of oysters in that estuary (W. S. Arnold, personal communication). Drainage of fresh water into the St. Lucie Estuary and its tributaries has also recently been shown to cause a shift in species composition (Wilson et al. 2005). Human management that results in salinities outside of the suitable ranges for survival and reproductive success of reef building oysters will negatively impact the reefs (W. S. Arnold, personal communication). In addition to the low salinity effects, large, poorly timed, and poorly located freshwater discharges into the Indian River Lagoon has resulted in the accumulation of large amounts of muck on the floor of the lagoon, clouding of the water column, and the destruction of oyster and sea grass beds (Bartell et al.)

Dredging and Muck Removal

Dredging and sand replenishment is a process used to replenish sand on eroded beaches in the estuaries. Near shore hard bottom habitats can be buried during this process or waters can become silty blocking light and interfering with the filter feeding of oysters. In areas where dredging or burial of oyster reefs occur, there is a significant of juvenile fish. This disturbs ecologically and commercially important species. Although long term effects have not been measured, it is likely that these events will result in permanent displacement of fish populations. Natural recovery from wave action is unlikely in areas where oyster reefs are buried (Lindeman and Snyder 1998).

Muck removal, primarily by dredging, is occurring in areas where the accumulation of sediment in the Indian River Lagoon has resulted in the destruction of seagrass beds and oyster reef habitats. The purpose of these projects is to create new space for oyster recruitment by removing dead habitat. However, while appearing beneficial, several problems have arose including clouding of the water, algal blooms that quickly take over, and the re-accumulation of sediment from other anthropogenic sources. A potential solution for these new problems is to set sediment traps in affected areas (Bartell et al.).

Biotoxins

Biotoxins are an important health threat to humans consuming shellfish from the Indian River Lagoon. In addition, there are several reported cases of marine mammals and fish becoming ill and/or dying after coming in contact with their microscopic source of biotoxins (Burkholder 1998, Steidinger et al. 1998). The impact of biotoxins from harmful algal blooms on the health of oysters and other invertebrate species is less well documented. Harmful algal blooms usually refer to expansive growths of cytotoxic bluegreen algae and phytoplankton. The Indian River Lagoon is reported to host 21 potentially toxic microalgal species (Provancha and Van den End 2006).

The most commonly encountered neurotoxins are saxitoxin, tetrodotoxin, and domoic acid. In the Indian River Lagoon, saxitoxin is produced by Pyrodinium var. bahamense, Alexandrium, and Gymnodinium and causes paralytic shellfish poisoning. In 1996, a bloom of G. pulchellum was reported to cause fish (species: Centropomus undecimalis, Mugil cephalus, Arius felis, Sciaenops ocellatus, Archosargus probatocephalus, and Pogonias cromis) and invertebrate kills (species: Callinectes sapidus, and Panaeus spp.) in the Indian River Lagoon (Steidinger et al. 1998). Tetrodotoxin is found in puffer fish and causes puffer fish poisoning. There is evidence suggesting that tetrodotoxin is produced by one or more marine bacteria. Domoic acid is produced by Psedo-nitzschia and causes amnesic shellfish poisoning. The dinoflagellates or diatoms that produce many of the biotoxins are always present and are not usually toxic except in times when large blooms of these micro-algae occur. During a bloom filter feeding invertebrates such as clams, mussels, and oysters ingest the microalgae and accumulate the biotoxins in their tissues which are then passed on to higher level consumers (e.g. carnivorous gastropods and crustaceans) that will also accumulate the biotoxins in their flesh (Burkholder 1998, Provancha and Van den End 2006). Some bivalves exhibit symptoms of narcosis and altered feeding behavior in the presence of Gymnodium and Alexandrium spp. (Burkholder 1998).

References:

Bartell SM, Burns JJ, Fontane DJ, McAnally WH, Motz LH, and RR Twilley. Independent Scientific Review of the Indian River lagoon - South Project Implementation Report. PDF document available online.

Brenner M, Keenan LW, Miller SJ and CL Schelske. 1999. Spatial and temporal patterns of sediment and nutrient accumulation in shallow lakes of the Upper River Basin, Florida. Wetlands Ecology and Management 6:221-240.

Burkholder JM. 1998. Implications of harmful microalgae and heterotrophic dinoflagellates in management of sustainable marine fisheries. Ecological Applications 8:S37-S62.

Grizzle RE, Adams JR, and LJ Walters. 2002. Historical changes in intertidal oyster (Crassostrea virginica) reefs in a Florida lagoon potentially related to boating activities. Journal of Shellfish Research 21:749-756.

Indian River Lagoon Comprehensive Conservation and Management Plan Update 2008. Indian River Lagoon National Estuary Program, Palm Bay, FL. PDF document available online.

Lindeman KC and DB Snyder. 1998. Nearshore hardbottom fishes of southeast Florida and effects of habitat burial caused by dredging. Fisheries Bulletin 97:508-525.

Peterson CH, Grabowski JH, and SP Powers. 2003. Estimated enhancement of fish production resulting from restoring oyster reef habitat: quantitative variation. Marine Ecology Progress Series 264:249-264.

Provancha JA and O Van den Ende. 2006 (draft). Indian River Lagoon Biotoxin and Aquatic Animal Health History and Background ReportTechnical Services Project Contract SJ432AA. Indian River Lagoon National Estuary Program and St. Johns Water Management District.

Steidinger, K. A., J. H. Landsburg, E. W. Truby, and B. S. Roberts. 1998. First report of Gymnodium pulchellum (Dinophyceae) in North America and associated fish kills in the Indian River, Florida. Journal of Phycology 34:431-437.

Stiner, J. L. and L. J. Walters. 2008. Effects of recreational boating on oyster reef architecture and species interactions. Florida Scientist 71:31-44.

Wilson, C., L. Scotto, J. Scarpa. A. Volety, S. Laramore, and D. Haunert. 2005. Survey of water quality, oyster reproduction and oyster health status in the St. Lucie Estuary. Journal of Shellfish Research 24:157-165.

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