The world's most productive fisheries are always associated with large inputs of nutrients from either upwelling or land runoff. Coastal regions dominated by large rivers are important to the biological production of the world's oceans because these rivers generally carry large amounts of "new" nitrogen. 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. River waters stimulate high rates of primary production on the shelf, which in turn stimulate and support high rates of zooplankton production and fisheries production. 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 highly selected as a food for larval and juvenile fish. Egg production rates of these species are closely linked to their food regime, 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 (> 30 oC). These copepods directly or indirectly support large populations of fish. 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. Fisheries data suggest that an ecosystem shift towards a system more dominated by pelagic fish species (the gulf menhaden, Brevoortia patronus, vs. the demersal Atlantic croaker, Migropogonius undulatus) may have occurred in the northern Gulf of Mexico, possibly associated with increased nutrient input to the region.
Input of dissolved inorganic nitrogen from the Mississippi River to the Gulf of Mexico has increased dramatically during the past several decades. 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. A fraction of the organic material from the highly productive water column sinks to the bottom and fuels the annual development of an extensive zone of bottom-water hypoxia.
Weather and climate forcings to this system are tractable for study and are linked to broader scale processes. River plumes are positively buoyant and their transport and mixing process are highly sensitive and immediately responsive to changes in local wind fields. Satellite imagery shows buoyant plumes respond, on the scale of hours, to changes in winds. Larger scale forcings such as ENSO events, modify precipitation patterns over the drainage basin and affect 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.
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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