Phosphorus ization, and Biogeochemica n in the Great Lakes'p2 Claire L. Schel~ke,'*~ Eugene F. Stoermer,' Gary L. Fahnenstielr3ts and Mark Haibach4 Great Lakes Research Division, %he University sf Michigan, Ann Arbo~; MI 48189, USA Schelske, C. h., E. F. Stoermer, G. L. Faknenstiel, and M. Haibach, 1986. Phosphorus enrichment, silica utilization, and bicsgeochemical silica depletion in the Great Lakes. Can. ). Fish. Aquat. Sci. 43: 407-41 5. Our hypothesis that silica (Si) depletion in Lake Michigan and the severe Si depletion that characterizes the lower Great Lakes were induced by increased phosphorus (P) inputs was supported by bioassay experiments showing increased Si uptake by diatoms with relatively small P enrichments. We propose that severe Si depletion (Si concentrations being reduced to 10.39 mg SiBr-ha' prior to thermal stratification) results when P levels are increased to the extent that increased diatom production reduces Si concentrations to limiting levels during the thermally mixed period. Large P enrichments such as those that characterized the eastern and central basis of bake Erie and bake Ontario in the early 1970s are necessary to produce severe Si depletion. It is clear that severe Si depletion in the lower lakes was produced by P enrichment because inflowing waters from Lake Huron have smaller P concentrations and larger Si concentrations than the outflowing waters of either bake Erie or Lake Ontario. Severe Si depletion probably began in the 1940s or 1950s as the result of increased P loads from expanded sewering of an increasing urban population and the introduction of phosphate detergents. The model proposed for biogeochemical Si depletion is consistent with previous findings of high rates of internal recycling because, under steady-state conditions for Si inputs, any increase in diatom production will produce an increase in permanent sedimentation of biogenic Si provided some fraction of the increased biogenic Si production is not recycled or unless there is a compensating increase in dissolution of diatoms. Notre hypothese voulant qkee I'appauvrissement en silice (Sij du lac Michigan et celui tres marque des Grands lacs d'aval aient ete provoqu6s par une augmentation des appsrts de phosphore (P) a kt$ confirrnke par des essais biologiques msntrant qu'un enrichissement relativement faible en phosphore entrainerait une augmentation de I'assimilation de la Si par les diatomees. Nous formulcsns I'hypothese qu'il y a appauvrissernent tres important en Si (teneur ere Si 1 8,39 mg Si02-L-' avant la stratification thermique) quand les teneurs en P augmentent suffisamment pour que la production accrue de diatomees reduise la teneur en Si aux niveaux limitatifs pendant la pkriode de melange thermique. D'importants enrichissements en P, de I'ordre de ceux qui ont caracterise les bassins est et central des lacs Erie et Ontario au d$but des annkes 70, sont nkcessaires pour provoquer un epuisement pouss6 du Si. Il apparait clairement que l'epuisement pousse du Si des lacs d'aval a et6 cause par l'enrichissement en P car les eaux provenant du lac Huron prksentent des teneurs en P moins 6lev6es et des teneurs en Si plus klevhes que celles s'6eoulawt des lacs Erie et Ontario. Cet epuisement du Si a probablement debut6 au cours des ann6es 40 et 58 suite a l'augmentation des rejets de phosphore causee par la hausse du volume des eaux usees qui a accompagne la croissance des centres urbains, et suite A I'arrivee sur ie march6 des dktersifs au phosphate. Le modele propos4 pour I'epuisemewt bioghochimique du Si est coherent avec la dkcouverte awterieure de taux eleves de reeyclage interne car, en conditions d'equilibre pour les apports de Si, toute augmentation de %a production de diatomkes se traduit par une hausse de la sedimentation prmanente du Si biogene, 3 condition qu'une certaine partie de la production accrue de ce Si ne soit pas recyclee ou 2 rnoins qu'il y ait compensation par une plus grande dissolution des diatornees. Received November ir 9, % 984 Accepted May 7 7, 1985 ('$8069) 'This paper was presented at the P-Plankton Dynamics Sym- posium at the 27th Conference ow Great Lakes Research, htema- tional Association for Great Lakes Research, Brock University, St. Cathasrimes, Bnt., May 1984. 'Contribution No. 416 of the Great W e s Research Division, University of Michigan, and Contribution No. 46 at the Great Lakes Environmental Research Laboratory, NBAA, Ann Arbor. "ABso SchmAB of Natural Resources, University of Michigan. "Also Department of Atmospheric and Oceanic Science, University of Michigan. 'Also Great Lakes Environmental Research Laboratory, NOAA, Awn Arbor. t is well known that nutrient enrichment, particularly phssphsms (P) enrichment, affects silica (Si) uptake by diatoms. Seasonal Si uptake by diatoms is well known from the classical studies of the English Lake District (see Reynolds 1984). It also has been suggested by Kilham ( 197 1) that Si demand could be u s 4 as an index of environmental enrichment. Schelske (197%) props& that areal Si and nitrate uptake could be used to assess trophic state in the upper Great Lakes, and Bobson et al. ( 1974) compiled data based on sea- sonal changes in Si and nitrate concentrations in the Great Lakes that were consistent with this concept. Previous studies Can. J. Fish. Aqua. Sci., V01. 43, 1986