The full suite of carbon exchanges among the 36 most important components of the Chesapeake Bay mesohaline ecosystem is estimated to examine the seasonal trends in energy flow and the trophic dynamics of the ecosystem. The networks provide information on the rates of energy transfer between the trophic components in a system wherein autochthonous production is dominated by phytoplankton production. A key seasonal feature of the system is that the summer grazing of primary producers by zooplankton is greatly reduced due to top—down control of zooplankton by ctenophores and sea nettles. Some of the ungrazed phytoplankton is left to fuel the activities of the pelagic microbial community, and the remainder falls to the bottom where it augments the deposit—feeding assemblage of polychaetes, amphipods, and blue crabs. There is a dominant seasonal cycle in the activities of all subcommunities, which is greatest in the summer and least in the cold season. However, the overall topology of the ecosystem does not appear to change substantially from season to season. Matrix operations can be employed to assess the various direct and indirect pathways by which each trophic group obtains energy. Often, indirect linkages reveal interesting differences. For example, although the bluefish and striped bass are both piscivorous predators, 63% of bluefish intake depends indirectly on benthic organisms, whereas striped bass depends mainly on planktonic organisms. Nearly all higher trophic species exhibit significant indirect dependencies upon the upper components of the microbial loop, especially during summer. The complicated trophic network can be mapped into an eight—level trophic chain in the sense of Lindeman. Such analysis reveals that detritivory is about 10 times greater than herbivorous grazing in the Chesapeake system and that 70% of detritus results from internal recycle. Annual efficiencies of trophic levels decrease as one ascends the chain. Major seasonal shifts in trophic efficiencies at higher levels appear to be modulated by how effectively microscopic zooplankton (mostly ciliates) are cropped by their predators. Average trophic efficiency is 9.6%. Despite the existence of eight trophic levels, the average level at which each species feeds always remains below 5. One "pest" species (the coelenterate Chrysaora quinquecirrha) feeds rather high on the trophic pyramid and may exert a heretofore unappreciated level of control on the planktonic food chain. The number of cycles present in the network is surprisingly few, despite the fact that a relatively large and seemingly constant amount (23.2%) of total system activity is devoted to recycling. This combination of factors possibly indicates a stressed ecosystem. A study of the rate—limiting links in the seasonal networks of recycling of material within the plankton reconfirms the shift of predator control from crustaceous zooplankton in springtime to the sea nettle (Chrysaora quinquecirrha) during summer months. The collection of cycles present in the system is disjoint; ...

As in other eutrophied estuaries and coastal embayments, persistent hypoxia now routinely develops during summer in the mesohaline portion of the Neuse River estuary (North Carolina, USA). In response to interannual differences in hydrography, summer 1997 exhibited much more intense and widespread hypoxia than summer 1998, permitting inferences about impacts of hypoxia on food web dynamics by comparing system changes across these two summers. The trophic structure of the Neuse estuary now resembles the generic pattern for a degraded temperate estuary with (1) intense planktonic algal blooms and similarly high production of free-living bacteria, (2) trivial levels of abundance of rooted aquatic plants and benthic macroalgae, (3) depleted apex predators, and (4) functional extinction of the historically dominant benthic grazer, eastern oysters. Detailed carbon-flow models, based on comprehensive field data, demonstrated large differences between the two summers in trophic transfers and system dynamics. Largely because of greater mortality of benthic invertebrates from more intense hypoxia, total biomass of heterotrophs declined over summer by 51% in 1997 as compared to only 17% in 1998. Because net primary production increased over summer and herbivory in this system is predominantly benthic, the fraction of primary production consumed by herbivores declined over summer by 35% in 1997 and 29% in 1998. Influx of juvenile fishes and their rapid growth in the estuarine nursery over summer led to increases in energy demand by demersal fishes of 380% and 507% in the successive summers. Thus, hypoxia-enhanced diversion of energy flows into microbial pathways away from consumers and mass mortality of benthic invertebrates from bottom hypoxia occurred at the season of greatest demand by predatory fishes and crabs using the estuary as nursery. Average residence time of carbon in the ecosystem declined by 51% in 1997 and 29% in 1998. Total system throughput declined over summer 1997 while increasing in 1998, indicating the reduced capacity of the system to transfer carbon to higher trophic levels in the more hypoxic summer. Late-summer trophic pathways were characterized by greater numbers of cycles, but flows became increasingly dominated by microbial loops rather than transfers to consumers. Ecosystem trophic efficiency was only ϳ4%, lower than other estuaries similarly analyzed. System properties indicative of resiliency of system function including development capacity, ascendancy, and flow diversity declined over summer 1997, while increasing or declining less in 1998. Thus, intensification of hypoxia caused dramatic reduction in the ecosystem's ability to transfer energy to higher trophic levels and rendered the ecosystem potentially less resilient to other stressors.

ABSTRACT. This paper presents results on the comparat~ve ecology of 4 tidal estuaries: the Ythan In Scotland, the Ems-Dollard in The Netherlands and the Swartkops and Kromme estuaries, both located in the Cape Province, South Africa. Flow models (in carbon) were constructed for each of these and analysed by means of network analysis. Specific attention was given to the trophic structure, and to the structure and magnitude of cycling in each estuary. We found that the trophic structure is similar for all 4 estuaries, but that differences exist in the rates of primary production and its utilization, standing stocks, in the systems' trophic efficiencies, and in the detrivory:herbivory ratios. Three aspects of cycling are considered: (1) the number of cycles and their distribution against cycle lengths; (2) the Finn cycling index (FCI) and (3) the whole system average path length (APL). Cycling analysis revealed that the FCI and the APL were highest in the most polluted estuary, the Swartkops, and lower in the other less stressed systems. The composite cycled flows revealed similar structures with virtually all components participating in the recycling of material in the estuaries. Global system properties such as total system throughput, development capacity, ascendancy, relative ascendancy and indices of flow diversity have also been calculated by means of network analysis. Results show that despite the high productivity, high FCI and longer path length of the Swartkops estuary, i t exhibits the lowest relative ascendancy, ~ndicating ~t to be more impacted than any of the other 3 estuaries.

Catch per unit effort was obtained for the nekton of the Swartkops estuary near Port Elizabeth by means of gill netting. Fifty gill net catches were made in which 1 269 fish were caught in areas representative of the mouth, middle and upper reaches of the estuary. Pomadasys commersonni, already shown to be the most abundant angling species in the estuary, dominated gill net catches both in terms of mass (29%) and numbers (17%). The family Mugilidae (five species) comprised 25% of the mass and 42% of the numbers caught. Environmental factors and availability of food that might determine distribution of fish in the estuary are discussed. The largest mean mass per individual was found in Chanos chanos (4 473 g), Elops machnata (1 656 g) and Va/amugil buchanan; (1 580 g). Lowest mean masses were recorded for Monodactylus falciformis (63 g), Rhabdosargus holubi (88 g) and Liza dumerili (94 g).

No abstract