Global Ocean Ecosystem Dynamics (U.S. GLOBEC) is a component of the U.S. Global Change Research Program, with the goals of understanding and ultimately predicting how populations of marine animals (holozooplankton, fish and benthic invertebrates) respond to natural and anthropogenic changes in global climate. Although coastal regions dominated by large rivers are exceptionally important to global production of marine animals, no GLOBEC program to date has focused on these issues. As a first step towards establishment of such a program, a workshop supported by NOAA's Center for Sponsored Ocean Research (CSCOR) and U.S. GLOBEC was held at the Louisiana Universities Marine Consortium (LUMCON) in January 1999. The scientific goal of the workshop was to identify and explore the relationships between large rivers and marine populations and how these relationships might be affected by climate changes. Twenty-nine scientists attended (see Appendix 1).
Coastal regions dominated by large rivers are important to the biological production of the world's oceans for several reasons, including:
These systems are good indicators of global change because they quickly respond to variations in weather and climate:
In the U.S., the Mississippi River is the major large river, with discharge equal to approximately 2.2 times that of the Columbia River and 2.9 times that of the Yukon River. The Mississippi River supports much of the biological production in the northern Gulf of Mexico. There is a direct relationship between the flux of inorganic nitrogen into the northern Gulf of Mexico via the Mississippi River and the primary production observed within a 6900 km2 area around the delta (Lohrenz et al. 1997).
High rates of primary production stimulate and support high rates of zooplankton production. Concentrations of copepod nauplii are >1000 l-1 at some plume edge stations during the summer (Dagg and Whitledge 1991) and even during the less productive winter period are often >100 l-1 (Dagg et al. 1987). There is a characteristic group of copepod species in the vicinity of the Mississippi River plume which is numerically dominated by Temora turbinata, Eucalanus pileatus, Centropages furcatus, Paracalanus spp., and in the lowest salinity waters, Acartia tonsa. Paracalanus spp. is often highly selected as a food for larval and juvenile fish (Govoni et al. 1983; 1986). Egg production rates of these species are closely linked to their food regime (Dagg 1988), responding dramatically to increases in food concentration associated with river plumes. Population responses in these copepods are especially rapid in summer because of high temperatures (>30oC). In higher salinity waters directly beneath the river plumes, a more oceanic community dominates, including Eucalanus attenuatus, Calanus tenuicornis, Phaenna spinifera, and two Candacia species (Ortner et al. 1989; Dagg 1995).
It is reasonable to expect a relationship between fisheries production and riverine nutrient inputs into this system (Nixon 1988). This is especially true because other sources of nutrients are less significant than in temperate or higher latitude regions. Fish production is high in the vicinity of Mississippi River plumes and farther afield (e.g. Grimes and Finucane 1991; Ortner and Dagg 1995). Approximately 20% of the U.S. commercial fishery landings by dollar value are from the northern Gulf of Mexico and there are major recreational fisheries in this region. Approximately 90% of the commercial fisheries from the Gulf of Mexico comes from what has been referred to as the "fertile crescent," the area affected directly by the Mississippi River (Anon. 1995). The largest volume fishery in the region is gulf menhaden, Brevoortia patronus, a pelagic planktivore. Menhaden recruitment increased in the mid-1970s (Govoni 1997). Inshore fishery-independent surveys for the same time period support this observation (Chesney et al. in press). The inshore fishery-independent data for Louisiana and data from bycatch studies in coastal Louisiana also suggest an increased abundance of pelagics and a decrease in demersal nekton (Adkins 1992; Chesney et al. in press). In both fishery-independent surveys and bycatch studies, relative CPUE for menhaden increased significantly (Gunter 1936; Anonymous 1992; Chesney et al. in press). During the same period, there has been an increase in the CPUE of another small pelagic fish, the bay anchovy Anchoa mitchilli (Chesney et al. in press). In contrast, some demersal fish appear to have decreased during the same time period. A dramatic example is the Atlantic croaker, Micropogonius undulatus which had its CPUE decrease from 207.4:6.0 in a comparison of bycatch rate between the 1930s and 1990s (Gunter 1936; Anonymous 1992). Another dramatic example is star drum, Stellifer lanceolatus, 30.6:0.3. Taken together, these data suggest that an ecosystem shift towards a system with increased abundance of pelagics and a decline in the abundance of demersal fish may have occurred in the northern Gulf of Mexico. Such a shift has been observed in other systems where eutrophication has been combined with heavy exploitation rates of the fisheries but the mechanism for these shifts is not well understood (Caddy 1993).
Input of dissolved inorganic nitrogen from the Mississippi River to the Gulf of Mexico has increased dramatically during the past several decades. The large drainage basin of the Mississippi River encompasses an intensive agricultural region which has a moderate population density. During recent years, the net input of dissolved nitrogen to the land within the drainage basin of the Mississippi River has been 2220 kg N km-2y-1 (Howarth et al. 1996). This nitrogen comes from fertilizer application (1840 kg N km-2y-1), nitrogen fixation from crops (1060 kg N km-2y-1) and atmospheric deposition of anthropogenic nitrogen (620 kg N km-2y-1). The net deposition is less than the total input because the region exports 1300 kg N km-2y-1 as food and feed. Approximately 25% of the net anthropogenic input to the drainage basin, 565 kg N km-2y-1, is eventually delivered via the Mississippi River system to the coastal zone of the Gulf of Mexico (Howarth et al. 1996). This results in the delivery of 1.82 Tg N y-1 (130 Gmol y-1) to the northern Gulf of Mexico (Howarth et al. 1996). Riverine concentrations of nitrate at Southwest Pass are commonly >100 µM (Dagg and Whitledge 1991). This nitrogen could support 30 gC m-2 yr-1 of new production if it were uniformly distributed over the entire Louisiana-Texas shelf west of the Mississippi River (Dagg and Whitledge 1991). In reality, the region of direct stimulation is much smaller and the degree of stimulation within that region is much larger.
In addition to an overall stimulation of biological production at all trophic levels, the influx of large amounts of new nitrogen preferentially stimulates, for reasons that aren't completely understood, the "classical" food chain (N-P-Z) rather than the microbial web more typical of oligotrophic waters (Legendre and Rassoulzadegan 1995). Thus, not only is the overall production of the northern Gulf stimulated by the riverine nutrient inputs but the type of food web that is stimulated supports a more efficient transfer of fixed carbon from phytoplankton to fish.
Additional stimulation of productivity may be associated with small scale physical structures. There is evidence that riverine fronts and boundaries associated with the Mississippi River provide sites for enhanced feeding and growth of zooplankton and immature fish, and ultimately for enhanced fisheries recruitment (Grimes and Kingsford 1996; Grimes and Finucane 1991). Nevertheless, the evidence for these enhancements in larval fish is not apparent for all species (Govoni 1997) and an increase in predation induced mortality may offset gains in individual growth and feeding.
A fraction of the organic material from this highly productive water column sinks to the bottom and fuels the annual development of an extensive zone of bottom-water hypoxia (Rabalais et al. 1994). A shelf-wide survey of the distribution of oxygen from the Mississippi River to the Louisiana-Texas border has been made each summer since 1985. Hypoxia was observed in bottom water during each summer survey with the exception of 1988, a summer of exceptionally low freshwater input from the Mississippi River. The area of hypoxia has been as large as 18,000 km2 (Wiseman et al. 1997). Based on this record, hypoxia has existed in this region for more than a decade. These data also indicate significant variability in the extent and distribution of hypoxia. To date, this variability is not completely predictable because the complex of factors, including organic matter input, trophic process in the pelagic environment, the degree of stratification and input of mixing energy, and the strength and direction of prevailing winds which affect the distribution and transport of river water, are not completely understood. Statistical analysis shows a significant correlation between river flow and oxygen deficit in bottom water on the inner shelf, but with a two month lag (Justic et al. 1993). Prior to these systematic surveys, data are less extensive but direct measurements indicate that some hypoxia existed in bottom waters of Louisiana as early as 1973 (Renaud 1986).
Longer records have been obtained from bottom cores taken on the Louisiana continental shelf. Cores representing the past 100 years of sediment accumulation show increasing concentrations of organic matter over this time. Virtually all of this material is marine in origin (Eadie et al. 1994). Stable isotope signatures and accumulation patterns of organic carbon indicate water column productivity has increased significantly since the 1960s, a period that coincides with a doubling of the nitrate loading from the Mississippi River. Presence of glauconite, a mineral associated with hypoxic conditions, suggests hypoxia existed but was less common before the early 1940s (Nelsen et al. 1994). The foraminifera assemblage from these cores indicates there was a significant shift in species composition at about this same time, with more recent community structure closely resembling that currently found in hypoxic waters (Nelsen et al. 1994). Current knowledge of the specific consequences of bottom water hypoxia for the ecosystem and economics of the northern Gulf of Mexico are summarized in a separate report (Diaz and Solow 1999).
In addition to directly stimulating marine populations via nutrient inputs, large rivers establish physical structures that enhance biological interactions. For example, yellowfin tuna, a major commercial fishery in the Gulf, spawn near Mississippi River plumes (Lang et al. 1994) and recreational anglers harvest several tunas and tuna-like fishes in the Gulf. There is some evidence that tunas may select oceanic features within which to spawn and, by enhanced feeding in such regions, promote survival and growth of early life stages (Richards et al. 1989). Prominent oceanographic features of the Gulf of Mexico (i.e. the Loop Current and associated fronts, Mississippi River plumes and associated fronts, warm and cold core eddies) are habitats where physical and biological conditions potentially can influence growth and survival of tuna larvae. These features, and the magnitude of phytoplankton production in the Gulf of Mexico, are likely to be affected by climatic changes. In the permanently stratified open Gulf, the mixed layer deepens to the nutricline only with the intrusion of the cold air masses and their accompanying strong winds and evaporative cooling. The consequent nutrient additions to the euphotic zone have appreciable effects upon upper water column biology (Ortner et al. 1984). Offshore spawning by some Gulf fisheries species is associated with the passage of cold fronts. In non-ENSO years, such fronts pass over the Gulf about every 10-14 days in winter. In ENSO years, intensity and frequency increase.
Effects of large rivers are not confined to near-field environments but can be observed over large spatial scales. In coastal systems of the Gulf of Mexico, there is a general pattern of increasing oligotrophy with distance from the Mississippi River, ending with the coral reef systems of south Florida and the Yucatan. This pattern, however, is distinctly non-uniform both temporally and spatially because discharges interact with major oceanographic features. For example, a band of low salinity Mississippi River water, with elevated chlorophyll concentrations, was observed off Miami (Ortner et al. 1995) and off Cape Lookout, NC (Atkinson and Tester 1994) in September 1993. Transport time from the river mouth to NC was approximately one month. During this particular period, winds in the northern Gulf of Mexico had an abnormally strong westerly component. River water, typically transported along the shelf to the west of the Mississippi delta, was instead transported offshore and east, and became entrained in the Loop Current which then transported it along the west Florida shelf towards and through the Florida Straits. Production processes in the west Florida shelf region are stimulated by local upwelling but interactions with river water provide an additional stimulus on some occasions. In other years, when transport of Mississippi River water is more typically to the west, freshwater signals are commonly observed on the shelf as far away as the Texas-Mexico border. Interaction of river water with spin-off eddies from the Loop Current is common during these years, and rich riverine waters are entrained and transported far out into the Gulf. Clearly, the biological impacts of large rivers can occur over large spatial and temporal scales, and these impacts may be sensitive to climatic changes and changes in nutrient dynamics within the drainage basin.
Weather and climate forcings to this system are tractable for study and are linked to broader scale processes. For example, river plumes are positively buoyant and their transport and mixing processes are highly sensitive and immediately responsive to changes in local wind fields. Satellite imagery clearly shows that buoyant plumes respond, on the scale of hours, to changes in winds. Larger scale forcings, such as ENSO events, can have effects by modifying precipitation patterns over the drainage basin or by affecting the intensity and duration of winter storms. On longer time scales, it has been shown that anomalies in sea-surface temperature in the Gulf of Mexico are significantly correlated with the Pacific Decadal Oscillation (Enfield and Mestas-Nunez 1999).
In summary, new nutrients supplied to coastal waters by large rivers support a disproportionately large amount of zooplankton and fish production. The northern Gulf of Mexico is dominated by the large inputs of freshwater and nutrients from the Mississippi River. There are demonstrated relationships between these inputs and population responses in zooplankton and fish and there are suggestions of an ecosystem shift towards a system more dominated by pelagic fish species.
Mechanisms are being discussed for reducing nitrogen inputs from the drainage basin to the gulf. If reductions are accomplished, the ecosystem responses will undoubtedly include a reduction in productivity and a reduction in the extent and duration of bottom-water hypoxia. Quantitative understanding of these relationships is required. Other responses are unclear. This is an ideal place for examination of biological responses within a river dominated system to weather, climate, and other environmental changes.
A workshop was held in January 1999 to discuss relationships between the Mississippi River, the production of marine populations and ecosystem responses in the Gulf of Mexico, and to discuss how these relationships might be affected by changes in weather and climate.
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