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We examined the spatiotemporal phytoplankton community patterns and identified the nature of the underlying causal mechanisms in a freshwater–saltwater continuum, the Neuse River Estuary (North Carolina, USA). We used a Bayesian structural equation modeling (SEM) approach that considers the regulatory role of the physical environment (flow, salinity, and light availability), nitrogen (dissolved oxidized inorganic nitrogen and total dissolved inorganic nitrogen), phosphorus, and temperature on total phytoplankton biomass and phytoplankton community composition. Hydrologic forcing (mainly the river flow fluctuations) dominates the up-estuary processes and loosens the coupling between nutrients and phytoplankton. The switch from an upstream negative to a downstream positive phytoplankton–physical environment relationship suggests that the elevated advective transport from the upper reaches of the estuary leads to a phytoplankton biomass accumulation in the mid- and down-estuary segments. The positive influence of the physical environment on the phytoplankton community response was more evident on diatom, chlorophyte and cryptophyte dynamics, which also highlights the opportunistic behavior of these taxa (faster nutrient uptake and growth rates, tolerance on low salinity conditions) that allows them to dominate the phytoplankton community during high freshwater conditions. Model results highlight the stronger association between phosphorus and total phytoplankton dynamics at the upstream freshwater locations; both nitrogen and phosphorus played a significant role in the middle section of the estuary, while the nitrogen–phytoplankton relationship was stronger in the downstream meso-polyhaline zone. Finally, our analysis provided evidence of a protracted favorable environment (e.g., longer residence times, low DIN concentrations and relaxation of the phosphorus limitation) for cyanobacteria dominance as we move to the down-estuary area, resulting in structural shifts on the phytoplankton community temporal patterns.

Nutrient enrichment and eutrophication of estuarine and coastal waters are accelerating, and there is a need to develop rapidly detectable and quantifiable indicators of these changes. Coastal systems are also impacted by climatic perturbations, including droughts, storms, and floods, the frequencies of which may be increasing. Phytoplankton are excellent indicators of ecological change. They are relatively easy to detect, identify, and quantify; they conduct a large share of primary production; and they are sensitive to diverse environmental stressors. In this study, phytoplankton total biomass, as chlorophyll a, and group-specific chemotaxonomic indicators (including chlorophylls and carotenoids) were used to characterize community responses to human (nutrient) and climatic (hydrologic) perturbations in the Neuse River Estuary-Pamlico Sound, North Carolina, USA. This estuarine-coastal continuum is experiencing anthropogenic nutrient enrichment and, since 1996, a rise in hurricane frequency. Freshwater input and flushing strongly interacted with supplies of the limiting nutrient nitrogen (N) to determine the location, magnitude, and composition of phytoplankton biomass along this continuum. Elevated flow (high flushing) following hurricanes favored dominance by the fast-growing chlorophytes and cryptophytes. Diatoms tended to dominate under moderate flow, while dinoflagellates and cyanobacteria increased in dominance when low flow prevailed in winter/spring and summer/fall, respectively. Depending on seasonal hydrologic cycles and episodic (hurricane) conditions, phytoplankton community structure differed substantially. These changes impact eutrophi- cation, food web, biogeochemical (e.g., hypoxia), and habitat conditions in this and other coastal ecosystems currently experiencing changes in nutrient inputs and climatic events. Phytoplankton-based indicators are adaptable to unattended monitoring platforms (e.g., ferries) that can be coupled to remote sensing and modeling efforts, in order to evaluate and help manage ecological change at ecosystem and regional scales.

We introduce a Bayesian structural equation modeling framework to explore the spatiotemporal phytoplankton community patterns in the Neuse River Estuary (study period 1995e2001). The initial hypothesized model considered the influence of the physical environment (flow, salinity, and light availability), nitrogen (dissolved oxidized inorganic nitrogen, and total dissolved inorganic nitrogen), and temperature on total phytoplankton biomass and phytoplankton community structure. Generally, the model gave plausible results and enabled the identification of the longitudinal role of the abiotic factors on the observed phytoplankton dynamics. River flow fluctuations and the resulting salinity and light availability changes (physical environment) dominate the up-estuary processes and loosen the coupling between nitrogen and phytoplankton. Further insights into the phytoplankton community response were provided by the positive path coefficients between the physical environment and diatoms, chlorophytes, and cryptophytes in the down-estuary sections. The latter finding supports an earlier hypothesis that these three groups dominate the phytoplankton community during high freshwater conditions as a result of their faster nutrient uptake and growth rates and their tolerance on low salinity conditions. The relationship between dissolved inorganic nitrogen concentrations and phytoplankton community becomes more apparent as we move to the down-estuary sections. A categorization of the phytoplankton community into cyanobacteria, dinoflagellates and an assemblage that consists of diatoms, chlorophytes, and cryptophytes provided the best results in the upper and middle segments of the estuary. Finally, the optimal down-estuary grouping aggregates diatoms and chlorophytes, lumps together dinoflagellates with cryptophytes, while cyanobacteria are treated separately. These structural shifts in the temporal phytoplankton community patterns probably result from combined bottom-up and top-down control effects. 2006 Elsevier Ltd. All rights reserved.

Since the mid 1990s, the Atlantic and Gulf Coast regions have experienced a dramatic increase in the number of hurricane landfalls. In eastern North Carolina alone, eight hurricanes have affected the coast in the past 9 years. These storms have exhibited individualistic hydrologic, nutrient, and sediment loading effects and represent a formidable challenge to nutrient management aimed at reducing eutrophication in the Pamlico Sound and its estuarine tributaries. Different rainfall amounts among hurricanes lead to variable freshwater and nutrient discharge and variable nutrient, organic matter, and sediment enrichment. These enrichments differentially affected physical and chemical properties (salinity, water residence time, transparency, stratification, dissolved oxygen), phytoplankton primary production, and phytoplankton community composition. Contrasting ecological responses were accompanied by changes in nutrient and oxygen cycling, habitat, and higher trophic levels, including different direct effects on fish populations. Floodwaters from the two largest hurricanes, Fran (1996) and Floyd (1999), exerted multi-month to multi-annual effects on hydrology, nutrient loads, productivity, and biotic composition. Relatively low rainfall coastal hurricanes like Isabel (2003) and Ophelia (2005) caused strong vertical mixing and storm surges, but relatively minor hydrologic and nutrient effects. Both hydrologic loading and wind forcing are important drivers and must be integrated with nutrient loading in assessing short-term and long-term ecological effects of these storms. These climatic forcings cannot be managed but should be considered in the development of water quality management strategies for these and other large estuarine ecosystems faced with increasing frequencies and intensities of hurricane activity.

Eutrophication is a serious water quality problem in estuaries receiving increasing anthropogenic nutrient loads. Managers undertaking nutrient-reduction strategies aimed at controlling estuarine eutrophication are faced with the challenge that upstream freshwater segments often are phosphorus (P)-limited, whereas more saline downstream segments are nitrogen (N)-limited. Management also must consider climatic (hydrologic) variability, which affects nutrient delivery and processing. The interactive effects of selective nutrient input reductions and climatic perturbations were examined in the Neuse River Estuary (NRE), North Carolina, a shallow estuary with more than a 30-year history of accelerated nutrient loading and water quality decline. The NRE also has experienced a recent increase in Atlantic hurricanes and record flooding, which has affected hydrology and nutrient loadings. The authors examined the water quality consequences of selective nutrient (P but not N) reductions in the 1980s, followed by N reductions in the 1990s and an increase in hurricane frequency since the mid-1990s. Selective P reductions decreased upstream phytoplankton blooms, but increased downstream phytoplankton biomass. Storms modified these trends. In particular, upstream annual N and P concentrations have decreased during the elevated hurricane period. Increased flushing and scouring from storms and flooding appear to have enhanced nutrient retention capabilities of the NRE watershed. From a management perspective, one cannot rely on largely unpredictable changes in storm frequency and intensity to negate anthropogenic nutrient enrichment and eutrophication. To control eutrophication along the hydrologically variable freshwater-marine continuum, N and P reductions should be applied adaptively to reflect point-source-dominated drought and non-point-source-dominated flood conditions.

We examined the effects of anthropogenic and climatic perturbations on nutrient-phytoplankton interactions and eutrophication in the waters of the largest estuarine systems in the U.S.A., the Chesapeake Bay (CB), Maryland/ Virginia, and the Neuse River Estuary/Pamlico Sound (NRE/PS) system, North Carolina. Both systems have experienced large post-World War II increases in nitrogen (N) and phosphorus (P) loading, and nutrient reductions have been initiated to alleviate symptoms of eutrophication. However, ecosystem-level effects of these nutrient reductions are strongly affected by hydrologic variability, including severe droughts and a recent increase in Atlantic hurricane activity. Phytoplankton community responses to these hydrologic perturbations, including storm surges and floods, were examined and when possible, compared for these systems. In both systems, the resulting variability in water residence time strongly influenced seasonal and longer-term patterns of phytoplankton biomass and community composition. Fast-growing diatoms were favored during years of high discharge and short residence time in CB, whereas this effect was not observed during high discharge conditions in the longer residence time NRE/ PS. In the NRE/PS, all phytoplankton groups except summer cyanobacterial populations showed decreased abundance during elevated flow years when compared to low flow years. Although hurricanes affected the CB less frequently than the NRE/PS, they nonetheless influenced floral composition in both systems. Seasonally, hydrologic perturbations, including droughts, floods, and storm-related deep mixing events, overwhelmed nutrient controls on floral composition. This underscores the difficulty in predicting seasonal and longer-term phytoplankton production and compositional responses to nutrient input reductions aimed at controlling eutrophication of large estuarine ecosystems.

In estuaries, phosphorus (P) and nitrogen (N) inputs generally control freshwater and saltwater primary production, respectively. Improved wastewater P removal and a P-detergent ban in the late 1980s decreased P loading to the nutrient over-enriched Neuse River Estuary, NC, without a contemporaneous reduction in N loading. This led to a decrease in upstream freshwater phytoplankton production and a reduction in nuisance algal blooms. While this nutrient management approach appeared to be effective in reducing the symptoms of freshwater eutrophication, it may have also diminished the upstream algal N filter, promoting N enrichment, relative to P enrichment, and eutrophication of the more saline downstream N-limited waters. Recent N controls implemented by the State of North Carolina should help address the problem. These findings underscore the need for watershed- and basin-scale, dual nutrient (N and P) reduction strategies that consider the entire freshwater--marine continuum as well as hydrologic variability (e.g., hurricanes, floods, droughts) when formulating long-term controls of estuarine eutrophication.

We conducted a study to determine the seasonal relationship between iron, sulfur, and phosphorus in the upper sediments and pore waters of a shallow intercoastal bay. From April 1999 to September 2000, sediment cores were collected from Rehoboth Bay, Delaware. Analyses of the sediments in the upper 4 cm revealed that redox conditions controlled Fe-S-P concentrations in the sediments, pore waters, and overlying water. Monthly sampling showed a marked decrease in the reactive solid phase P pool (ascorbate leachable fraction, ASC-P) and sharp increases in soluble P (measured as PO ) in pore waters and overlying waters, as the conditions became more reducing through32 4 out the summer months. These changes were paralleled by decreases in the amorphous Fe(III) (ascorbate leachable fraction, ASC-Fe) and total Fe(III)oxyhydroxide pools [dithionite extracted fraction, Fe(III) oxide] and increases in solid FeS/FeS2. The release of soluble P from sulfidic sediments to oxygenated overlying waters only occurred during periods of solid FeS/FeS2 production, which indicates that Fe(III) oxides act as a barrier to diffusive P flux. During these anoxic conditions, the regenerative P appears to induce secondary benthic algal blooms and promotes eutrophication in these inland bays through late summer. By the late fall and into early spring, sulfide production diminished and oxic conditions were reestablished as indicated by increases in solid amorphous and crystalline Fe(III) oxides and decreases in FeS/FeS2 concentrations. During this period, increasing ASC-Fe concentrations correlated with increases in ASC-P concentrations and decreases in pore-water PO . The seasonal correlations 32 4 between Fe-S-P indicate that Fe redox chemistry controls sediment P flux to the overlying water column.

Human encroachment on aquatic ecosystems is increasing at an unprecedented rate. The impacts of human pollution and habitat alteration are most evident and of greatest concern at the microbial level, where a bulk of production and nutrient cycling takes place. Aquatic ecosystems are additionally affected by natural perturbations, including droughts, storms, and floods, the frequency and extent of which may be increasing. Distinguishing and integrating the impacts of natural and human stressors is essential for understanding environmentally driven change of microbial diversity and function. Microbial bioindicators play a major role in detecting and characterizing these changes. Complementary use of analytical and molecular indicator tools shows great promise in helping us clarify the processes underlying microbial population, community, and ecosystem change in response to environmental perturbations. This is illustrated in phytoplankton (microalgal and cyanobacterial) and bacterial community changes in a range of US estuarine and coastal ecosystems experiencing increasing development in their water- and airsheds as well as climatic changes (e.g., increasing hurricane frequency). Microbial indicators can be adapted to a range of monitoring programs, including ferries, moored instrumentation, and remote sensing, in order to evaluate environmental controls on microbial community structure and function over ecosystem to global scales.

Human development of coastal watersheds has greatly increased nutrient loading and accelerated estuarine and coastal eutrophication. These waters are also affected by climatic perturbations (e.g., droughts, hurricanes, floods), which may be increasing. The ecological effects of these stressors are often most evident at the microbial level, where the bulk of primary production and biogeochemical cycling occurs. Phytoplankton dominate coastal primary production and thus may be indicative of eutrophication and other major perturbations underlying coastal ecosystem change. Using photopigments that are diagnostic for phytoplankton functional groups (chlorophytes, cryptophytes, cyanobacteria, diatoms, and dinoflagellates), we examined the relative responses of these taxonomic groups to nutrient and hydrologic alterations and evaluated their use as indicators of ecological change in the Neuse River Estuary, North Carolina, and Galveston Bay, Texas. Photopigment indicators can be routinely incorporated in water-quality monitoring programs to assess environmental controls on ecosystem structure and function over varying spatial and temporal scales.