Effects of nutrient limitation on the release and use of dissolved organic carbon from benthic algae in Lake Michigan

Compared to phytoplankton, relatively little information exists about the importance of benthic algae as a source of dissolved organic C (DOC) in lakes. We enriched outdoor tanks with N and P in a full factorial design (unenriched, +N, +P, N+P) to evaluate effects of nutrients on the release, composition, and decomposition of DOC from Cladophora glomerata (L.) Kutz. in Lake Michigan. After 10 d of enrichment, biomass-specific rates of gross primary productivity (GPP) were significantly greater in the N+P treatment than in the +N or +P treatments. The fraction of fixed C released as DOC was similar to 37% of GPP in the N+P treatment, which was similar to material collected from the lake (initial samples). Biomass-specific rates of DOC release did not vary significantly with treatment, whereas GPP declined in the nutrient-limited treatments (+N, +P, and unenriched). Consequently, DOC release represented a higher percentage of GPP (>100%) under severe nutrient limitation. Specific ultraviolet (UV) absorbance, an indicator of aromatic compounds, indicated that DOC from material grown in the N+P treatment (DOCN+P) was more labile than DOC from material grown in the + N treatment (DOC+N). Both DOCN+P and DOC+N supported an exponential increase in bacterial density during a 19-d incubation, but DOCN+P was used more rapidly and supported higher bacterial density than DOC+N. Cladophora has become a major contributor to primary production in the shallow nearshore areas of the lower Laurentian Great Lakes, and our data suggest that >= 1/3 of this productivity is available for the benthic microbial loop as DOC. The amount of DOC released remains relatively constant as nutrient limitation reduces productivity, but the nutrient status of the alga affects the quality of DOC as a substrate for heterotrophic bacteria.