The relative sensitivities and discrimination abilities of the Daphnia magna 48-h lethality assay of sediment pore water, the Photobacterium phosphoreum 15-min bioluminescence inhibition (Microtox@) assay of sediment pore water and the Chironomus tentans 10-d growth reduction assay of whole Detroit River sediments were investigated and predictive relationships developed. While all three assays demonstrated the toxicity of some sediments and all of them identified the most and least toxic sediments, the D. magna lethality assay was the least sensitive and discriminatory. The Microtoxa assay was the most sensitive. Based on lethality, the C. tentans assay was less sensitive than D. magna, but growth inhibition was sensitive and the most discriminatory of the three assays. Chironomids were not observed in the sediments that caused a 30% decrease in growth of C. tentans relative to that on control sediment in which chironomids were observed. This sediment toxicity is also approximately that which kills D. magna. Some locations were deemed very toxic by one or two assays but not toxic by the others. The results of all of the assays were correlated, but none of the assays accurately predicted the results of the other two. It was concluded that the D. magna lethality test could be used to predict which sediments were so toxic that benthic insects would not be expected to be present. A principal components analysis demonstrated that the MicrotoxO assay gave sufficiently different information that it could be included in a battery of sediment toxicity tests designed to accurately classify the toxicities of sediments.

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The Louisiana continental shelf in the northern Gulf of Mexico experiences bottom water hypoxia in the summer. In this study, we applied a biogeochemical model that simulates dissolved oxygen concentrations on the shelf in response to varying riverine nutrient and organic carbon loads, boundary fluxes, and sediment fluxes. Five-year model simulations demonstrated that midsummer hypoxic areas were most sensitive to riverine nutrient loads and sediment oxygen demand from settled organic carbon. Hypoxic area predictions were also sensitive to nutrient and organic carbon fluxes from lateral boundaries. The predicted hypoxic area decreased with decreases in nutrient loads, but the extent of change was influenced by the method used to estimate model boundary concentrations. We demonstrated that modeling efforts to predict changes in hypoxic area on the continental shelf in relationship to changes in nutrients should include representative boundary nutrient and organic carbon concentrations and functions for estimating sediment oxygen demand that are linked to settled organic carbon derived from water-column primary production. On the basis of our model analyses using the most representative boundary concentrations, nutrient loads would need to be reduced by 69% to achieve the Gulf of Mexico Nutrient Task Force Action Plan target hypoxic area of 5000 km(2).

Semicontinuous cultures of Lake Michigan algal assemblages were run at O.l5*day-' dilution rates under P limitation for 41 days. In control cultures, nutrients were supplied to the entire culture once per day. In patch cultures, 10% of the cultures received a high concentration IO-min pulse each day. Patch and control cultures received equal nutrient flux and achieved equal biomass levels; however, their species composition was different. Patch cultures were dominated by the filamentous blue-green alga Schizothrix calcicola and control cultures were dominated equally by S. calcicolu, Nitzschia aciculuris, and a group of phytoflagellates. These results, as well as shifts in subdominants, demonstrate the effect of small-scale, short term nutrient patches on species composition. Similar patch and control unialgal cultures revealed lower cell nutrient quotas for patchy cultures.Microvideographic observations of Daphnia magna suspended in three concentrations of Chlamydomonas sp. allowed quantification of rates at which algal cells were collected, swept close by, and released via bolus rejection or outwashing of unpacked cells. Outwashing release rates of presumably unharmed cells increased from 3 1 to 7 1% of the collection rate as food concentration increased. It appears that algal outwashing is a significant mechanism for eliminating excess food. An important implication is the potential algal nutritional consequence of this very close nondestructive encounter.