Eutrophication -- 10.2 Anthropogenic stressors

Nitrogen availability most frequently controls microbial and higher plant primary production in estuarine and coastal waters [402][342]. Concentrations and loading rates of this nutrient also most directly reflects human population density and activity in coastal water- and airsheds [368] (Figure). N inputs appear to control rates of primary production in most coastal waters worldwide (Figure 4). Not surprisingly, excessive N loading is a key causative agent for accelerating primary production, or eutrophication [342][365]. Symptoms include phytoplankton blooms, which may accumulate as ungrazed organic detritus in the sediments, providing the `fuel' for large-scale oxygen consumption and depletion in bottom waters and sediments. This chain of events is particularly problematic in salinity or temperature-stratified (no-mixed) waters, where oxygen cannot be replenished from the atmosphere (Figure). Under these conditions, persistent low oxygen or `hypoxic' conditions can alter nutrient (N, P, trace metals) cycling and promote fish disease and mortality [131][366].

Left frame -- Direct relationship between dissolved inorganic N input and phytoplankton biomass, as mean annual chlorophyll a content of several Western Australian estuarine systems. Figure adapted from Twomey et al., 2000. Right frame -- Direct relationships between dissolved inorganic N input and primary production in a various North American and European estuarine and coastal ecosystems. Figure adapted from Nixon et al., 1995. Details of systems: The open circles are for large (13 m³, 5 m deep) well-mixed mesocosm tanks at the Marine Ecosystems Research Laboratory (MERL) during a multi-year fertilization experiment (Nixon et al. 1986; Nixon 1992). Natural systems (solid circles) include (1) Scotian shelf � DIN from Houghton et al. (1978), production from Mills & Fournier (1979), (2) Sargasso Sea -- DIN from Jenkins (1988), production from Lohrenz et al. (1992) mean of 1989 and 1990 values of 110 and 144 g C m^-2 yr^-1, (3) North Sea -- DIN from Laane et al. (1993) assuming that the ratio of DIN/TN in the input from the Atlantic equals that in the Channel, production from Seitzinger & Giblin (this volume), (4) the Baltic Sea -- DIN and production from Ronner (1985), including DIN flux across the halocline, (5) North Central Pacific -- DIN from Platt et al. (1984), production from Tupas et al. (1993, 1994) mean of 1992 and 1993 values of 150 and 185 g C m^-2 yr^-1, (6) Tomales Bay, CA, DIN and production form Smith (1991), (7) Continental shelf off New York -- DIN and production from Walsh et al. (1987), (8) Outer continental shelf off southeastern U.S., DIN and production from Verity et al. (1993), (9) Peru upwelling -- DIN calculated from annual mean upwelling rate of 0.77 m d^-1 (Guillen & Calienes 1981) and an initial 20 M concentation of NO₃ in upwelled water (Walsh et al. 1980), production off Chimbote from Guillen & Calienes (1981), (10) Georges Bank -- DIN from Walsh et al. (1987), production from O'Reilly et al. (1987). The equation is a functional regression.

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Figure: Examples of phytoplankton blooms resulting from excessive nutrient loading is estuarine and coastal waters. Upper left: Cyanobacterial (blue-green algal) bloom in the Gulf of Finland region of the Baltic Sea. Upper right: Dinoflagellate red tide bloom near the Japanese Coastline (Sea of Japan). Lower left: Cyanobacterial bloom in the St Johns River Estuary, near Jacksonville, Florida. Lower right: Mixed cyanobacterial-chlorophyte bloom in a coastal lagoon, North Island, New Zealand.

A key factor promoting these negative impacts on coastal water quality is freshwater discharge (runoff). Discharge delivers nutrients to the coastal zone and determines hydrologic properties (flushing or residence time), vertical stratification, turbidity and color of the water column, all of which mediate productivity, nutrient cycling, dissolved oxygen dynamics and habitability in an interactive manner of coastal waters (Figure). For example, the rate of water discharge to estuaries, embayments and fjords controls their hydraulic residence time. Residence time, in turn, plays a critical role in determining the availability and rate of use of nutrients by phytoplankton and higher plants. Because discharge controls transport of phytoplankton through of these systems, it plays an interactive role with nutrient supply in controlling growth, competition and succession among members of the phytoplankton community. For example, high rates of freshwater discharge reduce the salinity and residence time. These conditions favor fast-growing oligohaline phytoplankton, such as chlorophytes (green algae). In contrast, low discharge conditions promote long water residence, high salinity conditions, which favor slower growing, halophyllic taxa, such as dinoflagellates and certain cyanobacteria. The differential impacts of discharge on phytoplankton community composition can be seen in the Neuse River Estuary, NC and Chesapeake Bay, MD/VA.

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Figure: Vertical section of the eutrophic Neuse River Estuary, NC, ranging from the freshwater head of the estuary (left hand side) to the mesohaline entrance to Pamlico Sound. Data on which these graphs were plotted were obtained from the biweekly Neuse River Modeling and Monitoring Program, ModMon The upper frame illustrates the strong vertical salinity stratification that results from light freshwater inflow `sandwiched' over denser salt water entering from Pamlico Sound. Stratifiaction persists during the summer months. The lower frame shows hypoxia that characterizes the bottom water as a result of persistent vertical stratification. This causes the bottom water to become isolated from the well-mixed, aerated, highly-productive surface water. Phytoplankton biomass formed in the surface waters sinks into the bottom, thus serving as the fuel for decomposition and oxygen consumption. Nutrient-stimulated phytoplankton blooms are a large source of oxidizable organic matter, which can exacerbate bottom water oxygen consumption and hypoxia. See the ModMon website for more extensive data sets that show this process on seasonally and interannually.

Using diagnostic chlorophyll and carotenoid photopigments as indicators of major phytoplankton functional groups (i.e., diatoms, dinoflagellates, chlorophytes, cyanobacteria, cryptomonads), we have examined the interactive effects of nutrient and hydrologically-driven changes of phytoplankton community composition and activity in the Neuse River Estuary (NRE), Pamlico Sound (PS) and Chesapeake Bay (CB). High performance liquid chromatography (HPLC), coupled to photodiode array spectrophotometry (PDAS) can be used to determine phytoplankton group composition based on the diagnostic photopigments. Pigments include specific chlorophylls (a, b, c), carotenoids and phycobilins. A statistical procedure, ChemTax [299] partitions chlorophyll a (i.e., total microalgal biomass) into the major algal groups, to determine the relative and absolute contributions of each group. In the NRE, key photopigment markers include Chl b and lutein (chlorophytes), zeaxanthin, myxoxanthophyll and echinenone (cyanobacteria), fucoxanthin (diatoms), peridinin (dinoflagellates) and alloxanthin (cryptomonads).

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Figure: Conceptual figure, showing the various watershed and airshed anthropogenic nutrient sources, their input to estuarine and coastal waters via freshwater discharge, the establishment of hypoxia due to freshwater overlaying denser saltwater and the stimulation of primary production (eutrophication) and algal booms due to coastal nutrient enrichment. Note the linkage between nutrient enriched primary production and hypoxia as phytoplankton sink into stratified bottom water. Also, note potential negative impacts of hypoxia on bottom dwelling finfish and shellfish as well as submersed aquatic vegetation communities, all of which require oxic conditions.

HPLC pigment analyses can be adapted to routine monitoring programs [373]. In addition, HPLC measurements can be used to calibrate remotely-sensed (aircraft, satellite) phytoplankton distributions on the ecosystem and regional scale. Using data from ongoing studies in the NRE (1994-present), PS (1999-present) and Chesapeake Bay (1993-present), it can be seen that these estuarine systems have experienced the combined stresses of anthropogenic nutrient enrichment, droughts (reduced flushing combined with minimal nutrient inputs), and in the NRE/PS since 1996, elevated hurricane activity (high flushing accompanied by elevated nutrient inputs). These distinct perturbations have allowed us to examine impacts of both anthropogenic and natural stressors on phytoplankton community structure. Seasonal and/or hurricane induced variations in river discharge, and the resulting changes in flushing rates and hence, estuarine residence times, have differentially affected phytoplankton taxonomic groups as a function of their contrasting growth characteristics. For instance, the relative contribution of chlorophytes, cryptophytes, and diatoms to the total chl a pool appeared strongly controlled by periods of elevated river flow in the NRE. It is hypothesized that these effects are due to the efficient growth rates and enhanced nutrient uptake rates of these groups. Cyanobacteria, on the other hand, showed greater relative biomass when flushing was minimal (i.e., longer residence times) during the summer..

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Figure: Concentrations of chlorophyll a (mg Chl a L^-1) contributed by chlorophytes, cyanobacteria and dinoflagellates. Values were derived using ChemTax for surface water at a mesohaline location (Station 120, see Figure of the Neuse R.) Estuary during 1994-2000. Data were collected bi-weekly and were temporally extrapolated. White areas indicate instances where data were not collected. ChemTax data were plotted along with freshwater discharge at the head of the estuary. The dates of landfall of the four major hurricanes that have significantly affected flow since mid-1996 are shown.

Further evidence that hydrologic changes have altered phytoplankton community structure is provided by the observed historical trends in dinoflagellate and chlorophyte abundance in the NRE. Both decreases in the occurrence of winter-spring dinoflagellate blooms and increases in the abundance of chlorophytes coincided with the increased frequency and magnitude of tropical storms and hurricanes since 1996. The relatively slow growth rates of dinoflagellates may have led to their reduced abundance during these high river discharge events. These results indicate that phytoplankton composition has been altered since 1994 in conjunction with major hydrologic changes, specifically floods following hurricanes. These phytoplankton community changes could have potentially altered trophodynamics and nutrient cycling in the NRE during these years.

The reconstructed taxonomic composition for Chesapeake Bay also shows strong contrasting responses between dominant phytoplankton groups during spring and summer due to the variability of freshwater flow and nutrient loading. This pattern is strongest in the spring - early summer wherein high flow alleviates N limitation of the mid to lower estuary and supports diatom blooms in the spring, and sometimes in the summer. Low flow produces improved photic conditions but causes an expanded zone of N limitation in the main stem of the bay during the summer, thereby changing phytoplankton dominance to those groups that can grow efficiently under these conditions.

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Figure: Regional means s.e. (1995 - 2000) for chl-a (mg m^-3) and the relative abundance (fraction chl-a^taxa) of phytoplankton groups determined by ChemTax for Chesapeake Bay (mid-Bay location). One approach to developing indicators from measurements of phytoplankton biomass and composition is to define the `average' conditions, as shown above, and then conduct analyses of deviations (seasonal, regionally, inter-annually) in relation to differences in environmental forcing functions and patterns of primary production. Data courtesy L. Harding, Univ. of Maryland Horn Point Laboratory, Cambridge, Maryland.

Depending on its source, path of travel and fate, runoff contains distinct amounts and types of nutrients. Much depends on how the watershed has been modified and impacted by human activities, including agriculture, urbanization, and industry. Therefore, when and where storms impact the watershed, their rain content and intensities significantly affect nutrient makeup and amounts discharged to coastal waters.

Freshwater discharge, in addition to its nutrient content, strongly controls vertical stratification of the water column, which in turn determines hypoxia potentials, internal nutrient regeneration and availability (Figures 4, 5, 6, 7), which in turn controls phytoplankton growth and bloom potentials. In turn, the magnitudes and aerial extent of blooms affect hypoxia potentials by controlling the formation and downward flux of organic matter providing the fuel for bottom water oxygen consumption. Therefore, the interacting and feedback effects of physical structuring with nutrient availability are key variables determining eutrophication and hypoxia dynamics.

In most instances, it is very difficult to control water quality and eutrophication by manipulating freshwater discharge. Exceptions would include aquaculture and retention ponds, small lagoons and reservoirs, where water input and flow can be tightly controlled. However, in large coastal watersheds, flow controls are neither technically nor economically feasible and realistic because large unpredictable weather events such as hurricanes as well as droughts tend to dominate hydrologic characteristics of these systems. This leaves nutrient and sediment inputs as the chief controllable variables.

10.2 Anthropogenic nutrient stressors: Their interaction with hydrology