Characterization of anthropogenic and natural disturbance on vegetated and unvegetated bay bottom habitats in the Corpus Christi Bay National Estuary Program study area

Abstract

Benthic habitats can be broadly classified as either vegetated or unvegetated. Seagrasses are the dominant form of submerged vegetation. Vegetated bay bottoms are generally more productive than unvegetated bottoms. Over 85% of seagrasses in Texas occur in estuaries of the Corpus Christi Bay National Estuary Program (CCBNEP) study area. Seagrass ecosystems are noted for high commercial and sport fishery landings. This correlation is often attributed to the high primary and secondary productivity associated with large seagrass beds in the CCBNEP study area. Disturbances, operating at many different scales, however, can profoundly affect benthic processes in estuaries and threaten fishery resources. For example, upper Laguna Madre appears to be undergoing a dynamic transition from a seagrass-dominated to a phytoplankton-dominated system as a result of a brown tide algal bloom. This large scale disturbance has resulted in pronounced changes in the trophic dynamics and relative importance among key plant and animal species within the system. What is not known is if the disturbance in Upper Laguna Madre is natural or influenced by man's activities, or if there are interactive effects due to natural fluctuations in the environment and anthropogenic input. Therefore, there is a need to identify natural and anthropogenic disturbances and characterize effects on benthic habitats. The four most important benthic habitats in the CCBNEP study area are: seagrass beds, oyster reefs, open bay muddy bottoms and shoreline sandy bottoms. Disturbances to benthic habitats are well known. Seagrasses are stressed by nutrient enrichment, propeller scarring, and especially light reduction caused by brown tide, turbidity, and dredging. Oyster reefs are stressed by reduced freshwater inflows and concomitant higher salinities. Bay bottom habitats are a victim of their own bounty. The large economic benefit the CCBNEP region derives from shrimping and sport fishing, which is supported by bay bottom habitats, is at risk due to mechanized harvest. Trawling continually turns over sediment, keeping the benthos in a continual state of low abundance and biodiversity. Bycatch-removes potential food for recreationally and commercially important fish species. Numerous disturbances resulting from human activities and natural events that affect physical, chemical, and biological characteristics of estuarine benthic habitats have been identified within the CCBNEP study area. Examples of human activities include: shrimp harvesting with bottom trawls, commercial tug and barge operations, recreational boating, dredging, altered currents, altered inflow, nutrient and contaminant input, and hydrocarbon exploration and production. Each activity has variable effects on the different types of bay bottom habitats. Examples of natural disturbances include: wind-generated resuspension and deposition of fine sediments, abrupt salinity changes due to direct rainfall and runoff, erosion due to storms, fish mortality due to freezes, long-term climatic changes, and harmful algal blooms.