Effect of Oyster Reef Loss on Marine Food Webs in Southwest Florida

Effects of Oyster Reef Loss on Marine Food Webs in Southwest Florida

By Jolie Alberty

Crassostrea virginica oyster reef exposed during low tide. Photo found on Sanibel Sea School webpage (Lucas)

Brief Introduction

Crassostrea virginica are the bivalves which glue together and create oyster reefs. These oysters are filter feeders that remove organic material in the water column, increasing water quality in the waters that they inhabit (Luckenbach et al., 2005).

Oyster reefs are unique biogenic structures (i.e., structures made by living organisms) that provide valuable ecosystem services for humans and the surrounding organisms that depend on them. Ecosystem services are defined as any service that the environment naturally provides which are financially beneficial to humans. Native to the West Atlantic Ocean, eastern oysters are commercially and ecologically important because they are so ubiquitous and available to provide food as well as livelihood for humans (Pinel, 2021).

Crassostrea virginica are the bivalves which glue together and create oyster reefs. The reefs depend on the incoming spat to grow and develop on dead oyster shells. So, reefs are sustained when accretion is greater than erosion.

Reefs provide several ecosystem services which benefit the environment and humans:

• Revenue from ecotourism as waters are clearer because of filtration (Luckenback et al., 2005)
• Protection of shorelines from erosion, storms, waves, etc. (Grabowski et al., 2012)
• Shelter and nurseries for fish and invertebrates that inhabit the area (Pinel, 2021)

Oyster Reef Loss

The main anthropogenic causes of oyster reef loss are overharvesting of oysters and destruction from irresponsible boating (Beck et al., 2011). Poor water quality can also leave oysters more susceptible to disease or infection (as shown below), which exacerbates the reef loss from erosion (Beck et al., 2011).Specifically in Southwest Florida, the main perpetrators for reef loss are reconstruction of watersheds and freshwater sources which increase nutrient and sediment inputs as well as dredging to make room for roadways (Volety et al., 2014).

C. virginica from Estero Bay with infection (such as Dermo or MSX) (Photograph by Jolie Alberty)

Effects of Loss on the Marine Food Web

The organisms directly affected by the decline in reef ecosystems in terms of abundance and diversity include marine invertebrates and smaller fish species which in turn, affect commercial fish species and terrestrial species such as birds. Pinel 2021 describes the value of oyster reefs to several bird populations (including wading, sea, shore, and scavenging birds) as reefs provide suitable foraging hotspots. Thus, the loss of reefs affects the abundance and diversity of birds in an area; with no food provided by the reefs, birds are forced to fly and forage elsewhere. 

Besides birds, other higher trophic level species will also suffer from the consequences of reef loss. Predatory commercial fish (such as striped bass, seatrout, etc.) that use the oyster reefs as hunting grounds. Wasno 2014 uses trophic transfer in Estero Bay to describe how benthic invertebrates and lower level trophic fishes' biomass transports energy to the higher trophic level fishes and thus correlates to fishery stocks (i.e., oyster reef production and fisheries production are positively correlated). The author details how upper trophic level fishes in the pelagic ecosystems are affected and supported by the stocks in the oyster reefs; therefore, any change in those ecosystems will cause a change in the surrounding pelagic ecosystem species. Food sources would be low with declining oyster reefs, and these predatory fishes would be forced to find prey elsewhere, altering their diet as well as the food web.

Success Stories

There have been multiple strategies offered for construction and restoration efforts for reefs. Restoration that integrates ecological values are coined "living shorelines", and the main objective is to maintain pre-existing reef structures (Scyphers et al., 2011).

Luckenbach et al., 2005 describes the study between two separate reef sites--one tributary in Virginia and another in South Carolina. Both sites show results of successful reef restoration efforts; oysters were of considerable size (on the mature reef in South Carolina) and both reefs housed diverse species. While the monitoring is still ongoing, it portrays how a successful restoration effort can be made.

Photo taken from Tampa Bay Watch webpage ("Oyster Reef Balls")

Tampa Bay, FL has begun reef reconstruction based on "oyster reef balls", as part of their Community Oyster Reef Enhancement program (CORE). These balls mimic the ecosystem functions that oyster reefs provide (protection from coastal erosion; shelter for many species of fish, marine invertebrates, and birds; and providing stable substrate for oyster spat). The balls are made of concrete incorporated with silica which are directly placed along shorelines by mangrove forests and other protected habitats ("Oyster Reef Balls"). 

Structures to help reef restoration from Naples government website ("Restoring Oyster Reefs")

Another story would be that of Naples Bay, which have deployed suitable substrate near reef sites in order to facilitate reef restoration through spat settling. The structures are made from various substances that are suitable for oysters to grow and develop--concrete, limestone, shell, etc. ("Restoring Oyster Reefs"). Selected reef sites in Naples Bay have been monitored through a program by Florida Gulf Coast University scientists Drs. James Douglass and Serge Thomas in which the success of the restoration project was evaluated based mainly on the amount of oysters living on the reef but also the surrounding environmental parameters (like low salinity from freshwater inflow) as well as metal loading pollution (Pinel, 2021).

Final Words

Recommendations to restore reefs and prevent further reef loss are developing and enforcing sustainable fishing practices, conserving estuaries in which the reefs are located along with their ecosystem services that they provide (Beck et al., 2011) as well as more specifically harvesting oysters in a sustainable way, upkeeping water quality near the reefs, and conserving the structural integrity of the reefs for habitat uses (Breitburg et al., 2000). Through student and professor-led projects, universities can help restoration and reconstruction efforts, which would otherwise take a lot of time, effort, and money through governmental means alone. It sets up a great opportunity for students to learn and can make the schools environmentally conscious. Restoring and construction reef structures will not only increase the biodiversity (as it did in Luckenbach et al., 2005), but also stabilize the reef food web.

Literature Cited

Beck, M.W., Brumbaugh, R.D., Airoldi, L., Carranza, A., Coen, L.D., Crawford, C., Defeo, O., Edgar,

G.H., Hancock, B., Kay, M.C., Lenihan, H.S., Luckenbach, M.W., Toropova, C.L., Zhang, G., and Guo, X. (2011). Oyster Reefs at Risk and Recommendations for Conservation, Restoration, and Management. Bioscience, 61: 107-116.

Breitburg, D.L., Coen, L.D., Luckenbach, M., Mann, R.L., Posey, M., and Wesson, J.A. (2000). Oyster Reef Restoration: Convergence Of Harvest and Conservation Strategies. Journal of Shellfish Research, 19(1): 371-377.

Grabowski, J.H., Powers, S.P., and Smyth, A.R. (2012). Economic Valuation of Ecosystem Services Provided by Oyster Reefs. Bioscience, 62: 900-909.  

Lucas, Sam. "Oysters in Southwest Florida." Sanibel Sea School, sanibelseaschool.org/experience-

blog/2020/10/13/oysters-in-southwest-florida.

Luckenbach, M.W., Coen, L.D., Ross, P.G., and Stephen, J.A. (2005). Oyster Reef Habitat Restoration: 

Relationships Between Oyster Abundance and Community Development based on Two Studies in Virginia and South Carolina. Journal of Coastal Research, 40: 64-78.

"Oyster Reef Balls." Tampa Bay Watch, tampabaywatch.org/restoration/oyster-communities/oyster-reef-

balls/.

Pinel, R.E. (2021). Impacts of water quality and reef restoration on oysters in Naples Bay, Florida, with a 

focus on copper pollution. [Master's thesis, Florida Gulf Coast University].

"Restoring Oyster Reefs in Naples Bay." City of Naples Florida, 

naplesgov.com/naturalresources/page/restoring-oyster-reefs-naples-bay

Scyphers, S.B., Powers, S.P., Heck, K.L. Jr, and Byron, D. (2011). Oyster Reefs as Natural Breakwaters 

Mitigate Shoreline Loss and Facilitate Fisheries. PLoS ONE, 6(8).

Wasno, R.M. (2014). Investigation of Trophic Transfer from Oyster Reefs to Predatory Fishes in 

Southwest Florida. [Master's thesis, Florida Gulf Coast University].