1985. Seasonal patterns in lipid content of Lake Michigan macroinvertebrates. Can. I. Fish. Aquat. Sci. 42: 1827-1832.Lipids in several taxaof macroinver%ebrates from Lake Michigan were determined seasonally to help define the role of these animals in transferring energy in the lake. Ranges of mean lipid content relative to ashfree dry weight (AFDW) throughout the year were as follows: amphipods (Pontopsreia hoyi), 21-54%; lumbriculid oligochaetes (Stylodrilus heringianus), 12-19%; tubificid oligochaetes, 9-22%; chironomid larvae, 9-39%; and Mysis relicta, 26-37%. A pattern of increasing lipid content during spring was observed for P. hoyi, chironomids, and M. relicta, but seasonal changes were not apparent for oligochaetes. Excluding M. relicta, P. hoyi accounted for about 65% of the biomass and 70% of the energy contained in southern Lake Michigan benthic macroinvertebrates and may assimilate up to 3Q% of the total bioavailable organic detritus settling into the offshore hypolimnion. Calculation of the predation loss rate (production rate minus nonpredatory death rate) of P. hoyi indicated that about 13 OQOcal.rn-' (2.1 g AFBW-m-') of this amphipod may be consumed annually by fish and other predators in southern Lake Michigan.On a d6termin6 sur une base saisonniere la teneur en lipides chez plusieurs taxons de macroinvertebr6s pr6sents dans le lac Michigan pour mieux d6finir le r61e jouk par ces animaux dans le processus de transfer% df6nergie dans le lac. ha teneur moyenne en lipides par rapport au poids sec sans cendre (PSSC) avari6 pour toute Ifann6e de 21 A 45 % chez Ifamphipode Pontoporeia hoyi, de 12 A 19 % chez %'oBigochete Stylodrilus heringianus (Bumbricuiid6s), de 9 ii 22 % chez les oligochGtes de la famille des tubificides, de 9 a 39 % chez les larves de chironomides et de 26 B 37 % chez Mysis relicta. On a observe en ce qui concerne P. hoyi., les chironomid6s et M. relicta une augmentation de la teneur en lipides au printemps, mais i B n'y a pas eu de changements saisonniers apparents chez ies oligochGtes. Mis part M. relicta, P. hoyi repr6sentait environ 65 % de la biomasse et 70 % de B'energie contenue chez les macroinvert6br6s benthiques presents dans B e secteur sud du lac Michigan et il peut assimiler jusqu'P 3Q % des d6chets organiques utilisabies qui sont prodbaits dans IfhypoIimnion au large. he caicul dba taux de pertes de P. hoyi par prbdation (taw de production moins taux de mortalit6 non attribuabie a la pr6dation) a montre que les poissons et les autres pr6dateurs pr6sents dans la partie sud du lac Michigan peuvent consommer cet amphipode au rythme d'environ 13 000 cal .m-2 (2,l g-m-' PSSC) par ann6e.

Microtechniques with disposable, calibrated, capillary pipets were developed to measure amounts and classes of lipids in individual Lake Michigan benthic invertebrates. After lipids from an animal were partitioned into 100 µl of extraction solvent and purified, measured portions were weighed with an electrobalance. Lipids in some samples were also characterized by thin layer chromatography with flame ionization detection (TLC‐FID). The extraction method was calibrated gravimetrically with cod liver oil and evaluated with a dry, ground fish sample. Lipid content was proportional (r = 0.99) to fish tissue dry weight over the range of 0.3–13 mg. TLC‐FID measurements correlated well (r = 0.98) with gravimetric analysis for portions of extracts of individual amphipods (Pontoporeia hoyi) and oligochaetes (Stylodrilus heringianus). TLC‐FID analysis indicated that <5% of the measured gravimetric weights was due to the inclusion of nonlipid material in the extracts.

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The relative importance of water and ingested food as possible avenues of contaminant accumulation was examined. Mysis relicta was exposed to two representative non-polar contaminants: benzo(a)pyrene (BaP) and 2,2',4,4',5,5'-hexachlorobiphenyl (HCB). The accumulation was examined kinetically in the presence and absence of two food types, Daphnia pulex (water flea) and Tabellaria flocculosa (a diatom). The availability of either food enhanced HCB accumulation and reduced BaP accumulation. Feeding did not affect HCB elimination but enhanced BaP elimination. The BaP was eliminated primarily as metabolites. Essentially all of the elimination occurred via the fecal route for both compounds. The fraction of the accumulation via ingestion of contaminated food was greater when mysids fed on Daphnia than Tabellaria and was greater for HCB than BaP.