US GLOBEC Newsletter 14: GOMEX Meeting Report

| GLOBEC - Gulf of Mexico Report of a U.S. GLOBEC Workshop 13 - 15 January, 1999 by Michael Dagg
Also in This Issue .... LTOP in the NEP Zooplankton in OR Coastal Zone El Niño Affects Marine Nekton GLOBEC Studies in the CGOA GoMex Workshop - Summary Cod & Climate Change Program Notes From the Field .... Sea Scallop Fishery Closures
Coastal regions dominated by large rivers are important to the biological production of the world's oceans for several reasons: Large rivers are distinctly different from smaller ones because their productive influences extend well out over the continental shelf; Many of the world's large rivers discharge high nutrient loads into otherwise oligotrophic regions; Boundaries / fronts associated with large river plumes are important micro- and meso-scale regions for biological production, animal aggregation, and fisheries recruitment; New nitrogen associated with riverine discharge generally supports the classic NPZ food web, resulting in high fisheries production. These systems are good indicators of global change because they quickly respond to variations in weather and climate: Climatic or anthropogenically caused changes of small magnitude throughout the drainage basin of a large river can result in a large response in the coastal shelf environment because of the magnifying or concentrating effects associated with drainage; characteristics of discharged river water result from an integration of processes that occur over broader time and space scales. As a result, riverine discharges of freshwater and nutrients are responsive to changes in rainfall in the drainage basins, and riverine discharge of nutrients is responsive to anthropogenic activities in the drainage basin. Buoyant plumes are quickly responsive to wind. The processing and fates of plume materials are dependent on plume location (over deep vs. shallow water), so a change in the wind regime can significantly alter the ultimate fate of discharged materials. In the U.S., the Mississippi River is the major large river, with discharge equal to approximately 2.2 x that of the Columbia River and 2.9 x 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 km area around the delta. High rates of primary production stimulate and support high rates of zooplankton 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. 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. Fish production is high in the northern Gulf of Mexico, a region that supports approx. 20% of the U.S. commercial fishery landings by dollar as well as major recreational fisheries. Approx. 90 % of the commercial fisheries from the Gulf of Mexico come from 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 partonus, 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. This may have implications for the commercially valuable shrimp fisheries in this region. A workshop supported by NOAA - COP and GLOBEC was held at the Louisiana Universities Marine Consortium (LUMCON) on January 13-15, 1999 to identify and explore the relationships between large rivers and marine populations and how these relationships might be affected by climate changes. 27 scientists from the U.S. and Canada attended. Topics discussed in the workshop include: Scales of Impacts (Temporal and Spatial) NUTRIENTS We know that the Mississippi River contributes more than 90% of the riverine fresh water input to the northern Gulf of Mexico, but assessing the importance of the associated nutrient input to the ecosystem, and specifically to fisheries recruitment, requires information on other nutrient inputs. A more refined N-budget is required. PHYSICAL AND BIOLOGICAL PROPERTIES We know that dissolved inorganic nitrogen in river plumes is taken up by phytoplankton within 100-200 km from point of discharge and within time scales of days to weeks but responses in copepod and fish populations occur over significantly broader temporal and spatial scales. It is important to more rigorously define the scales of both primary and secondary impacts. HYPOXIA We know that bottom-water hypoxia, ultimately fueled by riverine nutrients, develops on the shelf almost every summer and that there are temporal lags and spatial offsets between nutrient inputs and hypoxia that need to be better understood. Processes And Mechanisms DISTRIBUTION OF PLUMES AND ASSOCIATED MATERIALS Processing and fates of riverine materials are dependent in part on where the river plumes flow. Processes occurring in and beneath plumes over deep water are different from processes occurring in and beneath plumes over shallow coastal water. We need to know more about the physical and meteorological processes affecting the transport, retention and mixing of discharge plumes and their associated suspended sediments and nutrients. BIOLOGiCAL RESPONSES We know discharges of fresh water, dissolved nutrients and suspended sediments are highly variable but non-linearly related with precipitation in the drainage basin. Water column stratification is affected by patterns of fresh water discharge, and meteorological conditions significantly affect the transport of discharged water and associated materials. We need to know more about responses in animal populations to these highly variable forcings. BENTHIC - PELAGIC COUPLING / VERTICAL FLUX / HYPOXIA The shelf of the northern Gulf of Mexico is broad and shallow, implying important linkages between the pelagic and benthic environments. Increased nutrient inputs to the northern Gulf of Mexico have resulted in increased biological productivity (eutrophy), and some fraction of this water column production sinks to the bottom, supporting benthic production and creating an oxygen demand. We need to know more about coupling between the productivity and structure of the pelagic community, vertical flux, and hypoxia. We need to know more about the ratio of benthic to pelagic production on the shelf, and the volume and quality of fish habitat. SMALL SCALE FRONTAL PROCESSES We know that frontal boundaries are regions of enhanced biological processes. We need to know more about the significance of the high gradient environments between river plumes and receiving waters to the overall enhancement of biological production, especially fisheries recruitment, in the northern Gulf of Mexico. TRANSPORT AND FISHERIES RECRUITMENT Understanding recruitment processes requires knowledge of physical transport. In the northern Gulf of Mexico, transport refers to at least four distinguishable processes: Transport within the plume itself; Cross-shelf transport; Alongshore transport within the Louisiana Coastal Current; and Transport from the inner shelf into estuarine juvenile habitats. Depending upon the life history of the particular species of interest, one or more of these processes must be quantitatively understood to predict recruitment pathways. Links to Climate and Global Change CLIMATE SHIFTS We know that river plumes, coastal transport processes, and shelf-slope exchanges are all sensitive to local wind regimes that could change with climate shifts. There is evidence that precipitation within the drainage basin has increased over the past two decades and global models indicate this pattern will continue. There is also evidence that climate change can directly affect processes in the open gulf. We need to know how climate change will modulate the characteristics of river forcing and influence the physical - biological couplings of the shelf environment, and how large-scale climatic processes, through their effects on the timing and/or scale of the Mississippi River input, will affect the population dynamics of zooplankton and fish. ANTHROPOGENIC ADJUSTMENTS Changes to characteristics of the river discharge can be anthropogenically induced. Dissolved inorganic nitrogen has increased dramatically in recent decades because of the application of nitrogenous fertilizers within the drainage basin. Patterns of river discharge have been altered by the construction of a river levee system that effectively (with rare and highly visible exceptions) prevents river flooding. There is consideration being given to mechanisms for reducing nitrogen inputs as a means of reducing hypoxia in the northern Gulf of Mexico. We need to know more about the relationships between nutrient loading, food web structure and hypoxia in the northern Gulf of Mexico. Target Taxa Gulf menhaden, Brevoortia patronus, and Atlantic croaker, Micropogonius undulatus, can serve as target fish species to address these questions in the northern Gulf of Mexico. Both species of fish mature rapidly, reaching sexual maturity within two years, making them highly responsive to processes affecting physical - biological coupling. Recruitment of gulf menhaden, a planktivorous fish, has apparently increased significantly with increasing nutrient input to the northern Gulf. In contrast, populations of croaker, a demersal species, have declined dramatically over the same period. This suggests a shift in community structure has occurred, a shift involving a decline in demersal production and an increase in pelagic production. Commercially, the most important demersal species (three species of shrimp) do not appear to have been affected but data are incompletely analyzed at this time. Conclusions and Recommendations The workshop attendees were in agreement that GLOBEC and NOAA should be encouraged to develop a full science plan on the themes discussed in this workshop. A GLOBEC workshop report is being prepared by M. Dagg, P. Ortner, and J. Torres and is expected to be published within the next several months. Last updated: 14 October, 1999 Please address any comments, suggestions or questions to: GLOBEC Webmaster