Phytoplankton bloom effects on a Gulf estuary: Water quality changes and biological response

摘要

A long-term (16-year) study addressed the relationship of the trophic organization of a river-dominated Gulf of Mexico estuary (Perdido River-Bay system) with interannual trends of freshwater input, nutrient loading, and biological controlling features. The purpose of this study was to determine the long-term consequences of anthropogenic nutrient loading on a river estuarine system with an emphasis on the origin and impacts of a series of plankton blooms that were directly linked with nutrient loading to the Perdido drainage system. A multidisciplinary database was used to determine responses of plankton assemblages and associated coastal food webs to natural and anthropogenic nutrient loading. Orthophosphate and ammonia loading from a pulp mill into Elevenmile Creek and the bay generated a series of plankton blooms dominated by diatoms, raphidophytes, dinoflagellates, and cyanobacteria. Blooms followed species-specific seasonal patterns. Interannual phytoplankton trends were associated with nutrient loading and rainfall cycles. Blue-green algae blooms (Merismopedia tenuissima) in the receiving Elevenmile Creek were enhanced by drought conditions. There were statistical associations of reduced biological activity with the Merismopedia blooms. In the bay, initial diatom blooms were replaced by raphidophytes and dinoflagellates in time. Bloom effects on phytoplankton assemblages included initial reductions of species richness/diversity and direct/indirect replacement processes. Long-term changes of plankton blooms indicated complex adaptive processes that led to nonlinear responses to nutrient loading and time-based alterations of interspecific relationships. Blooms and related changes in phytoplankton composition were accompanied by loss of fish and invertebrate populations and altered food web processes. Raphidophyte (Heterosigma akashiwo) and dinoflagellate (Prorocentrum cordatum) blooms in the bay were statistically associated with reduced invertebrate and fish populations and disruptions of bay food webs. Reduced nutrient loading by the pulp mill resulted in diminished bay plankton blooms and partial recovery of infaunal macroinvertebrates. However, nutrient loading from a second source in the upper bay during later years was associated with increased bloom activity. This was added to reduced benthic invertebrate and fish assemblages in deeper bay areas affected by hypoxia caused by salinity stratification. Lower Perdido Bay was adversely affected by nonpoint source runoff from agricultural and urban sources with diminished water quality and loss of sediment organisms. Cumulative impacts due to anthropogenic nutrient loading from multiple sources and associated plankton blooms, along with altered physicochemical conditions due to a dredged opening at the bay terminus, resulted in widespread reductions of secondary productivity in Perdido Bay. Long-term trends of bay productivity are the result of interactions of natural climatological cycles and anthropogenic activities.