Evidence and outlook for the northern Gulf of Mexico

Journal of Sea Research 54 (2005) 25 – 35 www.elsevier.com/locate/seares

Coupling between climate variability and coastal eutrophication: Evidence and outlook for the northern Gulf of Mexico Dubravko Justicá,T, Nancy N. Rabalaisb, R. Eugene Turnera a

Coastal Ecology Institute and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA b Louisiana Universities Marine Consortium, 8124 Hwy. 56, Chauvin, LA 70344, USA Received 8 October 2004; accepted 3 February 2005 Available online 27 April 2005

Abstract It is generally believed that coastal eutrophication is primarily controlled by the magnitude of anthropogenic nutrient loading and this cause-effect relationship is often used as a common explanation for the widespread eutrophication observed during the second half of the 20th century. This paper examines the coupling between climate variability and coastal eutrophication, and discusses how future changes in climate may affect nutrient fluxes to the coastal zone, nutrient ratios, phytoplankton production and the severity of hypoxia. We focus on the northern Gulf of Mexico, a coastal ecosystem dominated by inflow of the Mississippi River, where recorded decadal and interannual variations in the size of a large hypoxic zone ( N 2 104 km2) provide examples of anthropogenic and climatic controls on eutrophication. Using a mathematical model, four hypothetical future climate scenarios were examined. The scenarios were based on projected changes in the Mississippi River discharge, nitrate flux, and ambient water temperatures, and the simulation results were compared to the standard model. The forcing functions in the standard model included the observed time-series of temperature, riverine freshwater discharge and nitrate flux over the 45-y period 1955–2000. In all four model scenarios, simulated frequency of hypoxia differed significantly from the standard model, ranging from a 58% decrease to a 63% increase. The Gulf of Mexico responses to climate-driven variations in freshwater inflow may not be representative for other coastal ecosystems. A comparison of the northern Gulf of Mexico and the Hudson River estuary revealed that the increased riverine freshwater inflow, which causes eutrophication in the northern Gulf of Mexico, improves trophic conditions in the Hudson River estuary. Hence, the degree to which coastal eutrophication will be affected by future climate variability will vary from one system to another, depending on the characteristics of the physical environment and the current eutrophication status. D 2005 Elsevier B.V. All rights reserved. Keywords: Climate variability; Coastal eutrophication; Nutrient flux; Hypoxia; Mississippi River; Gulf of Mexico

1. Introduction T Corresponding author. E-mail address: [email protected] (D. Justic´). 1385-1101/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.seares.2005.02.008

Eutrophication is the manifestation of nutrientenhanced aquatic primary productivity and is often

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D. Justic´ et al. / Journal of Sea Research 54 (2005) 25–35

indicated by the presence of noxious algal blooms and bottom-water hypoxia. Eutrophication has been reported from a wide variety of coastal and estuarine ecosystems (Officer et al., 1984; Rosenberg, 1985; Justic´ et al., 1987; Diaz and Rosenberg, 1995; Nixon, 1995; Cloern, 2001; Rabalais and Turner, 2001). The extent and severity of these phenomena have increased during the late 20th century (Benovic´ et al., 1987; Andersson and Rydberg, 1988; Cooper and Brush, 1991; Hickel et al., 1993; Turner and Rabalais, 1994), coincidentally with increased use of fertiliser in the watersheds and higher nitrogen and phosphorus concentrations in freshwaters (Turner and Rabalais, 1991; Howarth et al., 1996). The temporal association between the increased use of nutrients in the watersheds and outbreaks of coastal eutrophication points to the anthropogenic nature of the phenomenon. Nevertheless, superimposed on this late 20th century eutrophication trend we find strong climatic signals that have impacted estuarine salinity, stratification, nutrient budgets, primary productivity and the magnitude of seasonal oxygen depletion. Also, geological records allow us to infer that climate variability has Anthropogenic Nutrient Inputs

influenced coastal and estuarine ecosystems over time-scales much greater than the instrumented records. Quantifying the links between climate variability and coastal eutrophication is important given predictions that the earth’s climate may become more variable over the next 100 y (IPCC, 2001). Precipitation, evapo-transpiration and runoff are all expected to increase globally (Miller and Russell, 1992), and hydrologic extremes such as floods and droughts may become more common and more intense (Easterling et al., 2000). Changes in global temperatures and the hydrologic cycle may influence estuarine and coastal eutrophication in two main ways (Fig. 1). First, the magnitude and seasonal patterns of freshwater and nutrient inputs would alter and influence nutrientcontrolled coastal productivity. Second, the characteristics of the physical environment may change, thereby affecting the susceptibility of coastal and estuarine ecosystems to eutrophication. In this paper, we examine historical evidence which indicates a link between climate variability and coastal eutrophication, and discuss how future Climate Variability Climate Change

Enhanced Hydrologic Cycle

Riverine Nutrient Fluxes/Ratios

Increased Global Temperatures

Physical Environment •Temperature •Salinity •Residence Time •Stratification •Turbidity

Nutrient-Enhanced Productivity

Vertical Carbon Flux

Sedimentary Carbon And Nutrient Pools

Bottom Water Hypoxia

Fig. 1. Coupling between climate variability and coastal eutrophication. Broken arrows indicate feedback control.


changes in climate may affect the eutrophication process. We focus on the northern Gulf of Mexico, a coastal ecosystem dominated by inflow of the Mississippi River, where recorded decadal and interannual variations in the size of a large hypoxic zone provide examples of anthropogenic and climatic controls on eutrophication.

2. Evidence Changes in temperature and freshwater inflow are important mechanisms by which climate variability has influenced coastal and estuarine ecosystems over the past 10 000 y. Geological records for the southwestern US, for example, indicate that floods were frequent during transitions from cool to warm climate conditions (Ely et al., 1993). Apparently, moderate changes in climate were associated with large changes in the magnitude of floods, as evident from a 7000-yold record of floods for the tributaries of the upper Mississippi River (Knox, 1993). The record shows that an abrupt shift in flood behaviour occurred approximately 3300 y ago, with the onset of frequent floods of a magnitude that now recurs every 500 y or more. Recent evidence also points to significant decadal trends involving large variations in precipitation and runoff. Lettenmaier et al. (1994) and Lins and Michaels (1994), for example, found significant upward trends in precipitation and river discharge across most of the contiguous US over the past five decades. Karl and Knight (1998) found that the 8% increase in precipitation across the contiguous US since 1910 is primarily reflected in extreme daily precipitation events, i.e., the highest 10 percentiles of the precipitation distribution. The Mississippi River discharge, in particular, appears to be influenced by the Atlantic Multi-decadal Oscillation (AMO), a 65– 80 y climatic cycle with a 0.4 8C range in atmospheric temperature (Enfield et al., 2001). AMO warm phases occurred during the periods 1860–1880 and 1940– 1960, and cold phases between 1905–1925 and 1970– 1990 (Kerr, 2000). The total annual discharge of the Mississippi River varied by 10% between the warm and cold AMO phases (Enfield et al., 2001). The geological records provide extensive evidence of impacts of climate-driven variability in freshwater inflow on coastal and estuarine ecosystems. In the

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Chesapeake Bay, for example, reconstructed salinity patterns revealed that 14 dry and wet cycles have occurred during the last 1000 y (Cronin et al., 2000). The highest salinities occurred during the sixteenth and early seventeenth century and corresponded to large continent-wide droughts known from tree-ring records (Stahle et al., 1999). These oscillations had profound affects on benthic and planktonic communities. A notable impact was a tendency towards greater oxygen depletion during wet periods and high river discharges (Karlsen et al., 2000). Recent climatedriven variations in freshwater inflow to the Chesapeake Bay have also had a profound influence on aquatic primary productivity (Malone, 1991), the magnitude of seasonal oxygen depletion (Officer et al., 1984) and phytoplankton biomass and composition (Tyler, 1986; Harding and Perry, 1997). These contemporary climatic signals are superimposed on late 20th century trends in eutrophication and hypoxia that have emerged in response to anthropogenic nutrient enrichment (Cooper and Brush, 1991; Karlsen et al., 2000). In the northern Gulf of Mexico (Fig. 2), climatedriven variations in the Mississippi River fluxes of freshwater and nutrients strongly influence the areal extent and severity of hypoxia ( b 2 mg O2 l 1). Although the size of the summertime hypoxic zone has varied from near zero to N 2 104 km2 over the past 19 y, it was notably larger in wet years compared to dry years (Rabalais et al., 1996; Rabalais et al., 1999; Rabalais and Turner, 2001). During the drought of 1988 when there was a 52-y low discharge of the Mississippi River, bottom oxygen concentrations in the northern Gulf of Mexico were significantly higher than average, and a continuous hypoxic zone was absent (Fig. 2). During the flood of 1993 (62-y maximum discharge of the Mississippi River for August and September), however, the area of hypoxia doubled with respect to the 1985–1992 average (Rabalais et al., 1998). The 1993 hypoxia was associated with increased stability of the water column and enhanced primary productivity, as indicated by the greatly increased nutrient concentrations and phytoplankton biomass in the coastal waters influenced by the Mississippi River (Dortch, 1994; Rabalais et al., 1998). Both the drought of 1988 and the flood of 1993 were caused by anomalous precipitation patterns associated in part with the El


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94o

Q ( 104 m 3 s -1)

4 3 2 1 1985 0 4 3 2 1 0 4 3 2 1 0

93o

92o

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30o

N

100 km

29o C6

28o

1988

1993 0

2

4

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8

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Mo nth

fM lf o Gu

co exi

Fig. 2. The Mississippi River discharge (Q, left panels) and corresponding areas of hypoxia (right panels) in the northern Gulf of Mexico during July 1985, August 1988, and July 1993. The solid line represents the mean monthly discharge for a given year and the dashed line is the mean monthly discharge for the 1985–1993 period (modified from Justic´ et al., 1996). The shaded areas represent the distributions of bottom waters with dissolved oxygen concentration b 2 mg O2 l 1. Note that during 1988 hypoxia was observed only at one location off the Louisiana coast. The reference station C6 is indicated in the upper right panel. A more detailed description of the Gulf’s hypoxia monitoring program is given in Rabalais and Turner (2001).

Ninõ/Southern Oscillation (ENSO) (Trenberth and Guillemot, 1996). During 1988, a particularly strong, cold ENSO phase (La Ninã) in the tropical Pacific triggered a series of anomalous circulation events that are believed to be responsible for the drought. In contrast, the 1993 flood was partly the outcome of an extended, warm ENSO phase (El Ninõ) (Trenberth and Guillemot, 1996).

3. Changing climate There is increasing acceptance that the buildup of greenhouse gases in the atmosphere is warming the earth (IPCC, 2001). The last decade of the 20th century was the warmest on record, and climatic records indicate that recent warming has no counterpart in the last 1000 y (Crowley, 2000). The global Earth’s temperatures increased by almost 1 8C during the last 150 y (Jones et al., 1999), and general circulation models (GCMs) have projected further temperature increases of 1–6 8C over the next 100 y (IPCC, 2001). An increase in global temperatures of

such a magnitude is expected to produce a general intensification of the hydrologic cycle that would be manifested in increased global precipitation, evapotranspiration and runoff (Miller and Russell, 1992; IPCC, 2001). General circulation models (GCMs) are not consistent in their predictions of the effects of climate change on precipitation and temperature, which are two important drivers of freshwater inflow to estuaries. A GCM-based study that examined the impact of global warming on the annual runoff of the world’s 33 largest river basins (Miller and Russell, 1992), indicated that runoff is likely to increase for 25 of them. The model predicted higher annual runoffs for all river basins in high northern latitudes, with a maximum increase of 47%. At low latitudes, results were more variable, ranging from a increase of 96% to a decrease of 43%. In most cases, the increase in runoff was coincidental with increased rainfall within the drainage basins (Miller and Russell, 1992). Under this scenario, the average annual runoff of the Mississippi River Basin would


increase 20% if the concentration of atmospheric CO2 doubles. Nevertheless, other studies have shown that runoff estimates for the Mississippi River Basin differed greatly between the Canadian CGCM1 model and the Hadley HADCM2 model (Wolock and McCabe, 1999). Both models predict an increase in future extreme rainfall and runoff events, but they disagree in terms of both the magnitude and direction of changes in average annual runoff. The average annual runoff of the Mississippi River Basin, for example, was projected to decrease by 30% for the Canadian model, but increase by 40% for the Hadley model by the year 2099. Estimated changes of freshwater inflow into major US estuaries projected by the Hadley model by the year 2099 range from 40% to + 100%. Similar calculations based on the Canadian model indicate significantly reduced inflows for all coastal regions except the US Pacific coast (Wolock and McCabe, 1999). It is likely many coastal and estuarine ecosystems will experience changes in freshwater inflow. However, at present it is unclear in what manner these changes will occur.

4. Climatic influences on riverine nutrient fluxes The degree to which climate change will influence the delivery of riverine nutrients to coastal waters will depend on the magnitude of inputs of anthropogenic nutrients to the landscape. For watersheds with little anthropogenic impact, nitrogen export is positively correlated with discharge (Lewis et al., 1999). Assuming that the soil pool of nitrogen is in a steady state, nitrogen export from these watersheds should be related to nitrogen input resulting from nitrogen fixation and to atmospheric deposition of background levels of nitrogen. Incidentally, both nitrogen fixation and atmospheric deposition of nitrogen are positively correlated with precipitation and soil moisture (Cleveland et al., 1999; Holland et al., 1999). Riverine nitrogen fluxes should thus increase in response to increased precipitation and runoff. In watersheds with significant anthropogenic influence, riverine nitrogen flux appears to be strongly influenced by the magnitude of net anthropogenic inputs of nitrogen (Howarth et al.,

29

1996; Boyer et al., 2002). Such inputs, for example, account for N 70% of the variability in nitrogen export from the temperate regions of the North Atlantic Basin (Howarth et al., 1996), indicating that future climate change is unlikely to strongly impact riverine nitrogen flux in such watersheds. Nevertheless, although the long-term flux of nitrogen of the Mississippi River is well described by the net anthropogenic input of nitrogen (Howarth et al., 1996), year-to-year variations in nitrate flux are also strongly influenced by the variability in precipitation and runoff (McIsaac et al., 2001). During dry years nitrate accumulates in soils and underground waters. In wet years, nitrate is flushed into streams and enters the main river channels (Goolsby et al., 1999). Furthermore, in the upper part of the watershed, a reduced water residence times in canals, lakes, and small streams reduces nitrogen loss by denitrification (Howarth et al., 1996; Alexander et al., 2000). Wet years which follow dry years tend to produce the largest increases in nitrate flux (Goolsby et al., 1999; Justic´ et al., 2003). Thus, unless anthropogenic inputs of nitrogen to watersheds are reduced, higher and more variable precipitation (Wolock and McCabe, 1999; Easterling et al., 2000) would enhance the magnitude of delivery of nitrogen to the coast. It is difficult to predict how climate variability will influence riverine fluxes of phosphorus and silica. In watersheds with significant anthropogenic influence, riverine concentrations of phosphorus have generally varied with nitrogen concentrations (Marchetti et al., 1989; Turner and Rabalais, 1991; Howarth et al., 1996). Silica concentrations, however, have remained constant, or decreased in some rivers (Justic´ et al., 1995a,b). The Mississippi River concentrations of dissolved inorganic nitrogen and total phosphorus increased three-fold and two-fold, respectively, during the second half of the 20th century, but the concentration of silica decreased by 50%. One consequence of this has been a four-fold decrease in the riverine silica to nitrogen ratio (Turner and Rabalais, 1991). The Mississippi River concentrations of nitrate, reactive phosphorus and silica are strongly dependent on discharge, although their dynamics are uncoupled (Fig. 3). Peak concentrations of phos-


30

NO 3 + NO2 (µM)

200 160 120 80 40 0

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phorus and silica correspond to discharges of 1 104 m3 s 1 and 2 104 m3 s 1, respectively. It is clear that the proportions of nutrients are likely to change in response to climate-driven changes in fluxes of freshwater and nutrients. Nitrate concentration increases linearly with discharge up to ~1.3 104 m3 s 1, while at higher discharges it appears to be independent of discharge. As a result of these differences in the behaviour of individual nutrients, the molar N:P ratio increases five-fold over a range of observed nitrogen flux values, while the molar Si:N ratio decreases two-fold (Fig. 4).

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40 20

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-1

Q (10 m s ) Fig. 3. The relationships between the Mississippi River discharge (Q) at Tarbert Landing and riverine concentrations of nitrate (N), reactive phosphorus (P) and silica (Si) at St. Francisville for the period 1980–2002. The monitoring stations Tarbert Landing and St. Francisville are located 478 km and 430 km, respectively, upstream from the Mississippi River Delta. Solid line represents a third order least squares polynomial fit. Data sources are discussed in Goolsby et al. (1999).

0.0

5 6

-1

NO3 + N O2 Flux (10 kg d ) Fig. 4. Relationships between the nitrate flux in Mississippi River as measured at Tarbert Landing and the atomic ratios of nitrate (N), reactive phosphorus (P) and silica (Si) at St. Francisville for the period 1980–2002. Dashed horizontal lines indicates the Redfield ratio (N:P= 16, Si:N = 1, by atoms).


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5. Ecosystem responses to altered freshwater inflow and nutrient fluxes

Table 1 Contrasting responses of two river-dominated coastal ecosystems to climate-driven variations in freshwater inflow

Variability in freshwater inflow can influence many environmental factors of significance to coastal and estuarine ecosystems, including salinity, turbidity, residence times, stratification, nutrient concentrations, and nutrient ratios (Fig. 1). Increased freshwater inflow can influence turbidity, water column stability and residence time, which all can modify the response of phytoplankton to nutrient enrichment and influence the process of eutrophication (Cloern, 1999, 2001). Phytoplankton doubling rates rarely exceed 1–2 per day, so that estuaries with short residence times (hours to days) generally have a low phytoplankton biomass (Malone, 1977; Cloern et al., 1983). Eutrophication of the Hudson River estuary between the 1970s and 1990s, for example, was attributed to a decrease in freshwater inflow and increased water residence times in the estuary (Howarth et al., 2000). During the 1970s, the summer mean freshwater discharge was N 200 m3 s 1 and the residence time of surface water was b 1 d (Malone, 1977). However, during the 1990s, the residence time often exceeded 1 d, and this resulted in increased primary production (Howarth et al., 2000). Water residence times can also affect the eutrophication process by influencing nutrient budgets. There is a strong inverse relationship between estuarine residence times and nitrogen export to open coastal waters (Nixon et al., 1996; Dettmann, 2001). Thus, estuaries with short residence times (hours to days) are generally less susceptible to eutrophication than estuaries with long residence times (months to years). Increased freshwater inflow may influence water column stability in a number of ways (Table 1). The Hudson River estuary, for example, becomes less stratified as freshwater inflow increases (Howarth et al., 2000). The opposite is the case in the northern Gulf of Mexico, where vertical density gradients are correlated with the Mississippi River discharge (Justic´ et al., 1996). Increased stratification could stimulate development of phytoplankton blooms by increasing the residence time of phytoplankton in the euphotic zone and slowing the sinking rates of phytoplankton (Malone, 1977; Howarth et al., 2000). Increased stratification is also likely to diminish vertical oxygen transport and promote the development of hypoxia in bottom waters (Officer et al., 1984; Justic´ et al., 1996).

Northern Gulf of Mexico

Hudson River Estuary

Dry 1988

Dry 1990s

Condition Period Discharge Turbidity Stratification Residence time Nutrient loading Primary productivity Chlorophyll-a Hypoxia Eutrophic

Wet 1993 + + +

+ + + + + +

Wet 1970s +

+ + + ? + + ? +

?

?

The d+T and d T signs denote departures from average flow conditions. The trophic status was determined based on primary productivity, chlorophyll concentration, and the areal extent and severity of hypoxia. The compilation is based on the data from the northern Gulf of Mexico (Justic´ et al., 1996; Rabalais et al., 1998), and the Hudson River estuary (Howarth et al., 2000).

The concentrations of dissolved nitrogen, phosphorus, and silica in rivers are typically at least an order of magnitude higher than those in coastal waters (Justic´ et al., 1995a,b), and increased freshwater inflow may lead to increased phytoplankton production and/or biomass (Nixon et al., 1996; Boynton and Kemp, 2000). Changes in the riverine nutrient ratios (Fig. 4) can influence the development of harmful algal blooms. Under nutrient-replete conditions, the atomic ratio Si:N:P of marine diatoms is approximately 16:16:1 (Redfield et al., 1963; Brzezinski, 1985). Consistent with the hypothesis proposed by Officer and Ryther (1980), a decrease in the Si:N ratio below 1 may reduce the potential for diatom growth, in favour of flagellates. The evidence presented by Smayda (1990) indicates that nutrient ratios influence the incidence of noxious and harmful flagellate blooms in the coastal waters worldwide. Evidence from the northern Gulf of Mexico indicates that abundance of lightly silicified diatom Pseudonitzschia has increased since the 1950s, coincidentally with increasing Mississippi River nitrate flux and decreasing Si:N ratios (Parsons et al., 2002). Some Pseudonitzschia species are known to produce toxins, so that there may be a direct link between Si limitation and toxic diatom blooms (Dortch et al., 2001). Evidence from the northern Gulf of Mexico indicates that


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diatom sinking contributes significantly to the vertical carbon flux that leads to hypoxia (Turner et al., 1998; Turner, 2001). Most of the sinking diatoms are moderately to heavily silicified and their growth is

stimulated by high Si:N ratios (Dortch et al., 2001). Consequently, changes in the riverine nutrient ratios can influence the vertical flux of carbon and the severity of hypoxia.

10 8 6 4 2 0

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10 8 6 4 2 0

S cenario 4

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

Years Fig. 5. Simulated changes in the average oxygen concentration of the lower water column (10–20 m), at a station within the core of the Gulf of Mexico hypoxic zone (C6; Fig. 2). A solid line at 2 mg O2 l 1 denotes the operational limit of hypoxia. In the standard model simulation, forcing functions included the observed time-series of temperature, riverine freshwater discharge and nitrate concentrations over the 45-year period 1955–2000. Scenario 1 assumes a 30% decrease in the average Mississippi River discharge (a conservative lower estimate based on the Canadian model projections; Wolock and McCabe, 1999). Scenario 2 assumes a 20% increase in the average Mississippi River discharge (based on Miller and Russell, 1992). Scenario 3 assumes a 4 8C increase in the average surface and bottom temperatures of the northern Gulf of Mexico (IPCC, 2001). Scenario 4 assumes a 20% increase in the average Mississippi River discharge, and a 4 8C increase in the average surface and bottom temperatures of the northern Gulf of Mexico.


6. Outlook In a series of modelling studies, Justic´ et al. (1996, 1997, 2002) examined the potential impact of future climate change on productivity and hypoxia in the northern Gulf of Mexico (Fig. 5). Model scenarios were based on projected changes in the Mississippi River discharge, nitrate flux, and ambient water temperatures. The results were compared to the standard model simulation, in which the forcing functions included the observed time-series of temperature, riverine freshwater discharge and nitrate concentrations over the 45-y period 1955–2000. Simulation of the standard model identified the mid 1970s as a start of the recurring hypoxia in the lower water column, and predicted a total of 19 y with hypoxia between 1955 and 2000 (Fig. 5). These results are in good agreement with the reports of the first hypoxia events in the northern Gulf of Mexico (Rabalais and Turner, 2001), and are supported by the retrospective analyses of sedimentary records (Turner and Rabalais, 1994). For a scenario with 30% decrease in the average Mississippi River discharge (Scenario 1), the model predicted a total of 8 y with hypoxia, i.e. a 58% decrease in frequency relative to the standard model. For a 20% increase in the average Mississippi River discharge (Scenario 2), the model predicted a 37% increase in the number of years in which hypoxia occurred. For a scenario with 48C increase in the average temperatures of the northern Gulf of Mexico (Scenario 3), the model predicted a 32% increase in the frequency of hypoxia. Finally, for a scenario with 48C increase in the average temperatures of the northern Gulf of Mexico and a 20% increase in the average Mississippi River discharge (Scenario 4), the model predicted 31 y with hypoxia, i.e. a 63% increase in frequency relative to the standard model. Hence, depending on the assumptions about changes in freshwater inflow and temperature, major increases and decreases in the frequency of hypoxia are possible. The degree to which coastal eutrophication will be affected by future climate variability will vary from one system to another, depending on the characteristics of the physical environment and the current eutrophication status. The northern Gulf of Mexico and the Hudson River estuary are classic examples of riverdominated ecosystems that have become eutrophic in

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response to increased anthropogenic nutrient inputs. Yet, these two systems seem to show the opposite responses to climate-driven changes in freshwater inflow (Table 1). While increased freshwater inflow stimulates phytoplankton growth and increases eutrophication in the northern Gulf of Mexico (Justic´ et al., 1996; Rabalais et al., 1998), it decreases residence times in the Hudson so that phytoplankton growth is inhibited and the estuary becomes less eutrophic (Howarth et al., 2000). Hence, predicting future coupling between climate variability and coastal eutrophication remains a challenge. Whatever climate-eutrophication interactions may prevail on a global scale, there will be local estuaries and coastal ecosystems that will display the opposite effects.

Acknowledgements This research was funded in part by the NOAA Coastal Ocean Program grants for hypoxia studies (NGOMEX 2000). Three anonymous reviewers provided critical and insightful reviews of an earlier draft of this manuscript.

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