Working group discussions were held under three headings:
- Large scale natural and anthropogenic linkages;
- Trophic interactions and community structure; and
- Frontal processes, plume dynamics and near-field responses
Separate reports were presented in a plenary session and later synthesized into the
summary presented here.
From a human perspective, the Mississippi River has both positive and negative aspects on animal populations in the northern Gulf of Mexico. On one hand, riverine inputs and associated riverine processes result in biological stimulation with associated enhancements to fisheries production. On the other hand, organic materials from this enhanced production fuel development of a large hypoxic zone which has deleterious effects on animal populations. Questions developed by workshop participants considered both perspectives and fell into three broad categories:
I - SCALES OF IMPACTS
(TEMPORAL & 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, requires information on other nutrient inputs. A more refined N-budget is required.
- What are the nutrient inputs to the shelf from the river, and from atmospheric,
lateral, and groundwater sources, and what are the important / dominant scales of these inputs ??
- What is the contribution of shelf-slope exchanges to the nutrient budgets of the
shelf and the open Gulf ??
- PHYSICAL & 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 impacts (distribution of near-field properties) and of secondary impacts (distribution of far-field properties).
- What are the temporal and spatial distributions of the Mississippi River plume and
its associated materials (nutrients and suspended sediments) ??
- What are the spatial and temporal distributions of biological communities stimulated by riverine nutrients ??
- What contribution does the riverine nutrient input make to support of the biological communities in the far-field (on the shelf to the E and W, in the open Gulf), and
what are the temporal and spatial patterns ??
- 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.
- What is the magnitude and what are the time and space scales of shelf hypoxia ??
II - 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.
- What are the physical and meteorological processes affecting the transport,
retention and mixing of discharge plumes and their associated suspended sediments and
- What are the important processes for transport of Mississippi River or shelf water
into the central Gulf ??
- BIOLOGICAL RESPONSES: We know river discharge is highly variable. Associated with this there is significant variability in nutrient and suspended sediment discharge, both highly but non-linearly correlated with precipitation in the drainage basin. Water column stratification is also affected by patterns of fresh water discharge. Meteorological conditions can significantly affect the transport of discharged water and associated materials. For these and other reasons, biological responses are likely to be highly variable.
- How does the short-term, seasonal, annual and decadal variability in discharge of
fresh water, suspended sediments and nutrients affect production of animal populations ??
- How do these inputs and patterns control the structure of the food web (diversity, number of trophic levels, species composition or functional groupings) and the spatial structure of biomass and production ??
- How will changes in the amounts and rates of nutrient delivery impact community structure and animal production, especially fisheries community structure and production ??
- BENTHIC-PELAGIC COUPLING, VERTICAL FLUX and 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. In other systems, nutrient increases beyond a certain level lead to radical shifts in ecosystem structure (dystrophy).
- What are the linkages between riverine inputs, physical processes on the shelf, the productivity and structure of the pelagic community, vertical flux, and hypoxia ??
- How does the development of hypoxic areas on the shelf affect overall system production, the ratio of benthic to pelagic production, and the volume and quality of available fish habitat ??
- Where along the continuum between eutrophy and dystrophy does the "fertile crescent" fall and what are the controlling processes ??
- SMALL-SCALE FRONTAL PROCESSES: We know that frontal boundaries are regions of enhanced biological processes.
- What significance are the high gradient environments between river plumes and
receiving waters to the overall enhancement of biological production, especially fisheries production, in the northern Gulf of Mexico ??
- TRANSPORT and FRONTAL PROCESSES: 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 including quasi-persistent features such as the mesoscale gyre that occurs northwest of the Southwest Pass; 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.
- How do transport and retention processes interact with growth and survival of the target zooplankton and fish species to affect recruitment ??
III - 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 which 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.
- How will climate change modulate the characteristics of river forcing and influence the biological-physical couplings of the shelf environment ??
- Do the large scale climatic processes, through their effects on the timing and / or scale of the Mississippi River input, affect food web structure / fishery yield ??
- 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.
- What effects will changing agricultural practices have on nutrient loading, on food web structure, and on hypoxia in the northern Gulf of Mexico ??
- How will the large-scale climatic processes identified as affecting the Mississippi River drainage basin and the northern Gulf of Mexico affect the production and species composition of animal populations ??
IV - 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 throughout its entire life, has apparently increased significantly with increasing nutrient input to the northern Gulf. In contrast, populations of croaker, a planktivore during larval stages but a demersal carnivore as an adult, have declined dramatically over the same time. This suggests a shift in community structure has occurred, a shift involving a decline in demersal production and an increase in pelagic production. Gulf menhaden live approximately three years, whereas Atlantic croaker live longer, five-to-six years.
Larvae and postlarvae of both fish feed on copepod nauplii in surface waters. Both are
dependent on the copepod community of mid-salinity waters associated with river plumes: Temora turbinata, Centropages furcatus, Eucalanus pileatus,
Paracalanus spp., and Acartia tonsa. These copepods will serve as target zooplankton species. Juveniles and adult menhaden are planktivores, juvenile croaker consume benthic invertebrates, and adults consume benthic invertebrates and small fish.
V - CONCLUSIONS and RECOMMENDATIONS
The workshop attendees were in agreement that U.S. GLOBEC and NOAA should be
encouraged to develop a full science or implementation plan on the themes discussed in this
Last updated: 4 March, 2000
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