Summary 1. The native amphipod Diporeia spp. was once the dominant benthic organism in Lake Michigan and served as an important pathway of energy flow from lower to upper trophic levels. Lake‐wide surveys were conducted in 1994/1995, 2000 and 2005, and abundances of Diporeia and the invasive bivalves Dreissena polymorpha (zebra mussel) and Dreissena rostriformis bugensis (quagga mussel) were assessed. In addition, more frequent surveys were conducted in the southern region of the lake between 1980 and 2007 to augment trend interpretation. 2. Between 1994/1995 and 2005, lake‐wide density of Diporeia declined from 5365 to 329 m−2, and biomass (dry weight, DW) declined from 3.9 to 0.4 g DW m−2. The percentage of all sites with no Diporeia increased over time: 1.1% in 1994/1995, 21.7% in 2000 and 66.9% in 2005. On the other hand, total dreissenid density increased from 173 to 8816 m−2, and total biomass increased from 0.4 to 28.6 g DW m−2. Over this 10‐year time period, D. r. bugensis displaced D. polymorpha as the dominant dreissenid, comprising 97.7% of the total population in 2005. In 2007, Diporeia was rarely found at depths shallower than 90 m and continued to decline at greater depths, whereas densities of D. r. bugensis continued to increase at depths greater than 50 m. 3. The decline in Diporeia occurred progressively from shallow to deep regions, and was temporally coincident with the expansion of D. polymorpha in nearshore waters followed by the expansion of D. r. bugensis in offshore waters. In addition, Diporeia density was negatively related to dreissenid density within and across depth intervals; the latter result indicated that dreissenids in shallow waters remotely influenced Diporeia in deep waters. 4. With the loss of Diporeia and increase in D. r. bugensis, the benthic community has become a major energy sink rather that a pathway to upper trophic levels. With this replacement of dominant taxa, we estimate that the relative benthic energy pool increased from 17 to 109 kcal m−2 between 1994/1995 and 2005, and to 342 kcal m−2 by 2007. We project that previously observed impacts on fish populations will continue and become more pronounced as the D. r. bugensis population continues to expand in deeper waters.

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ABSTRACT. Increases in nutrient concentrations in the Mississippi River over the past 35 yr have led to speculation that primary production of organic carbon has been elevated as a result of increased nutrient fluxes that have occurred in the northern Gulf of Mexico coastal ecosystem. However, studies thus far have not provided direct demonstration of temporal relationships between measured primary production in continental shelf waters and nver-borne nutrient fluxes. This ~nvestlgation co~npared temporal variations in primary production with associated annual and interannual changes in riverborne nutrient inputs. Prlmary production In shelf waters near the river delta were found to b e signlflcantly correlated with nitrate (NO1-) + nitrite (NO? ) concentrations and fluxes over a 6 yr period from 1988 to 1994. Although light limitation was probably an important factor during winter months, a positive correlation was demonstrated between river inputs of NO,-+NO; and primary production for data collected from other times of the year. Peak nutrient inputs generally occurred in the spring. The magnitude of the riverborne NO? +NO, inputs averaged 106% of estimated nitrogen requirements for phytoplankton in the river-impacted region, considerably greater than in Amazon shelf waters, which have been less subject to anthropogenic nutrient increases. The possibility exists that further Increases In anthropogenic nutrients in the ~i s s i s s~p p iRiver could lead to h~g h e r and more widespread primary production, and this may intensify and extend the depletion of oxygen that has already been observed in the Louisiana shelf ecosystem. However, such a prediction is difficult because relationships between increasing nutnent inputs and pnmary production are unlikely to be linear, and a complete understanding of processes intermediate between primary production of organic matter and oxygen depletion in bottom waters on the Louisiana shelf is still lacking.

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