Eutrophication -- 10.1 Introduction

As we enter the third millenium, more than 70% of the world's human population resides within 100 km of the coast [484]. Large increases in pollutant discharge have accompanied agricultural and urban development in coastal watersheds. Deterioration of coastal ecosystems as a result of increasing urbanization of their watersheds is accelerating, yet there is a paucity of information on how primary producer and higher-ranked consumer communities are being altered by these perturbations. Understanding how human-induced ecological change (e.g. decreasing biodiversity, water quality, and fisheries) affects ecosystem composition and functioning is a major research challenge worldwide. For estuarine and coastal systems (jointly termed coastal here) that receive and process a bulk of human pollutants, two troubling changes have been nutrient enrichment [342] and a decrease in grazer and higher consumer populations (e.g. shellfish, finfish) [246]. The combined effect of increased nutrient loading and the reduction in grazers has dramatically increased coastal phytoplankton stocks. Increases in phytoplankton have been linked to significant changes in nutrient (C, N, P, Si) cycling, water quality and ecosystem health [51][365]. Decreases in water clarity, expanding zones of low-oxygen water [381], increases in harmful algal blooms [360][390] may result from increased nutrient input to coastal waters.

paerlFig1

Figure: Pamlico Sound Estuarine System as adjacent Atlantic Ocean coastal waters of eastern North Carolina, USA, as observed by the true ocean color satellite remote sensing system SeaWiFS (photos courtesy NASA). Left frame: 11 September, 1998, exactly one year before hurricane Floyd, under quiescent conditions. Insert: 16 September, 1999, during landfall of hurricane Floyd. Right frame: 23 September, 1999, approximately one week after Floyd. Note the brown-stained floodwaters discharging into Pamlico Sound and overflowing into the coastal Atalntic Ocean. Some of the turbid, sediment-laden water is being carried out to sea by the Gulf Stream which passes closely by the North Carolina coastline (from south to north).

Coastal ecosystems are also under the influence of natural perturbations such as droughts, hurricanes and flooding, the intensity and frequency of which appear to be increasing [178]. During Fall 1999, Hurricanes Dennis, Floyd and Irene inundated coastal North Carolina with up to 1 m of rainfall, causing a 100 year flood in the watershed of the Pamlico Sound, the USA�s second largest estuary and key fisheries nursery for the mid-and southeast Atlantic regions. Sediment and nutrient-laden floodwaters displaced over 80% of the Sound�s volume, depressed salinity by 70%, and accounted for half the annual nitrogen (N) load to this N-sensitive system [363] (see Figure).

paerlFig2

Figure: Total nitrogen (dissolved and particulate inorganic and organic) loading recoded at the entrance to the Neuse River Estuary (Streets Ferry Bridge) during non-hurricane and hurricane years. Typically, a large percentage of annual N loading in non-hurricane years occurs during the rainy late-winter early-spring period from January through early-May. This is shown for a relatively dry year (1994), as well as more `normal' years (1996 and 1999). Note that during drought conditions (early summer of 1999), very little N loading takes place. Also shown are N inputs due to hurricanes that impacted the Neuse River Estuary watershed with heavy rainfall. During 1996, Hurricanes Bertha (minimal hurricane) and Fran (category 3 hurricane) impacted the watershed, while in the summer-fall of 1999, Dennis (category 2), Floyd (category 3) and Irene (category 2) sequentially impacted the watershed within a 6 week period. Note that the amounts of N loading due to large hurricanes (Fran and Floyd) accounted for large percentages of annual N loading to this estuary.

Biogeochemical and ecological effects included hypoxic (<4 mg O₂ L^-1) bottom waters, major changes in nutrient cycling, a 3-fold increase in algal biomass, altered fish distributions, catches, and an increase in fish disease [363]. Predicted elevated hurricane activity may cause long-term biogeochemical and trophic change in this and other coastal fisheries nursery habitats. Distinguishing and integrating the impacts of these natural events from anthropogenic impacts are difficult but essential to understanding and managing coastal biotic resources.

The effects and manifestations of human and natural perturbations are usually most readily detectable and consequential at the microbial level, where a bulk of ecosystem energy and nutrient flow is mediated. Microbes have fast growth rates, and respond to low levels of pollutants and environmental perturbations. These features make them sensitive, meaningful, and useful indicators of ecological change. Here, we will examine how the dominant primary producers, phytoplankton, respond and adapt to nutrient and climatic perturbations. We will consider how anthropogenic stressors interact with natural forcing features to determine the composition, distribution and activities of phytoplankton communities in North Carolina�s Pamlico Sound System, which has been heavily impacted by both human (nutrient enrichment) and climatic (hurricanes) perturbations over the past few decades.