Nutrient limitation of benthic algae in Lake Michigan: The role of silica

In the Laurentian Great Lakes, phytoplanktongrowth and biomass are secondarily limited by sil-ica (Si), as a result of phosphorus (P) enrichment.Even modest levels of Penrichment can induce sec-ondary Silimitation, which, in turn, promotes a shiftfrom the native diatom phytoplankton ﬂora to chlo-rophyte and cyanobacteria species. However, verylittle is known about thenutritional statusof benthicpopulations and their response to nutrient enrich-ment. Two experiments were performed in the lit-toral zone of Lake Michigan where nutrients weredelivered to in situ benthic algal (episammic andepilithic) assemblages using nutrient-diffusing sub-strata. In order to test the hypothesis that benthicalgae in Lake Michigan are Si limited, a 2 3 fac-torial experiment was used to deliver all combin-ations of Si, N, and P to resident assemblagesgrowing on artiﬁcial substrata composed of natural(Si rich) versus calcium carbonate (Si poor) sand. Asecond experiment utilized a serial enrichment toevaluate the role of Si in mediating changes in taxo-nomic composition. These ﬁndings indicate thatbenthic algae in Lake Michigan exhibit signs of sec-ondary Si limitation, and that their response to en-richment is similar to the phytoplankton. Moreover,natural sand substrata may provide a source of Si toresident benthic algae.Key index words: benthic algae; Great Lakes; silicaLimited information exists on the nutritional ecol-ogy of benthic algae relative to that known about phy-toplankton (Stevenson et al. 1996, Wetzel 2001). Whileempirical relationships between water-column nutri-ents and periphyton biomass have been established(Cattaneo 1987, Dodds et al. 2002), there is little con-sensus on the role nutrients play relative to other fac-tors that may regulate benthic algae in aquatic systems(Biggs 1995, Lowe 1996). Nonetheless, a variety ofmethods have been developed to evaluate the growthresponse of benthic algae to nutrient enrichment,while maintaining a high degree of environmentalrealism (Pringle 1987). These methods include dosingof artiﬁcial enclosures or streams (Bothwell and Jasper1983), nutrient-releasing artiﬁcial substrata (Fairchildand Lowe 1984, Pringle and Bowers 1984), lake en-closures (McCormick and O’Dell 1996), and whole-lake manipulations (Schindler 1975, Bergmann andWelch 1990).In the Great Lakes, phytoplankton growth and bi-omass is secondarily limited by silica (Si) as a result ofphosphorus (P) enrichment that has been acceleratedby human activities (Schelske et al. 1986). In turn, theoccurrence of Si limitation promoted a shift from thenative diatom phytoplankton ﬂora to chlorophyte andcyanobacteria species (Schelske and Stoermer 1971,1972). A variety of complementary studies have beencarried out over the past 20 years that support theSi-limitation hypothesis, and the existence of thisphenomenon is now recognized as a key biogeochem-ical feature of the Great Lakes ecosystem (Schelskeet al. 2006). However, few studies have evaluated thenutritional status of the benthic algal counterpart inLake Michigan, despite the fact that the assemblage isdominated by diatoms (Kingston et al. 1983). Carrickand Lowe (1988) conducted an in situ bioassay indicat-ing that benthic algae in Lake Michigan became moreSi limited following enrichment with both nitrogen (N)and P, resulting in a shift from diatom dominance tothat of chlorophyteandcyanobacterial species (Carricket al. 1988). Carrick and Lowe (1988) did not, howev-er, evaluate all possible combinations of Si, N, andP, but rather used a more simple serial enrichmentdesign.Thus, the hypothesis being tested here is that en-richment with key plant nutrients (all combinations ofSi,N, and P)promotessecondary nutrient limitationofbenthic algae in Lake Michigan. Three specific ques-tions were addressed through this research. First, isbenthic algal growth limited by Si, N, and P (singly orin combination)? Second, is the enrichment responseof benthic algae inﬂuenced by substratum type (natu-ral silicate vs. calcium carbonate)? Third, is the re-sponseofbenthicalgaetoenrichmentcomparablewith