The ecological significance of ground water-lake interactions was determined by comparing epibenthic algal biomass, species composition, diversity, ground water flux, pore water, and overlying lake water chemistry among five equal depth sites in Sparkling Lake, Wisconsin.

Epibenthic algal biomass and ground water seepage flux were positively correlated (r=0.466; p<0.001) indicating that the flow of water across the sediment-water interface enhanced epibenthic algal growth. Epibenthic algal biomass was significantly greater at sites characterized by high ground water discharge (chlorophyll-A range 4.0 to 65.0 milligrams per square meter; ground water flux range 5.0 to 74.0 milliliters per square meter per minute) than at sites characterized by low ground water discharge (chl-A range 2.0 to 21.0 mg per square meter; ground water flux range 1.5 to 3.5 ml per square meter per minute) or low ground water recharge (range -0.5 to -1.5 ml per square meter per minute). Pore water nutrients differed among sites. Pore water PO4 was significantly greater at high ground water discharge sites (range 29.2 to 110.7 micrograms phosphorus per liter) than at other sites (range <10.0 to 27.7 micro g P per liter), respectively. Pore water NH3-N, was significantly lower at high ground water discharge sites (range <10.0 to 566.0 micro g N per liter) than at low ground water discharge and recharge sites (range 61.4 to 1464.9 micro g N per liter).

Species diversity and richness of epibenthic algal communities in seepage lakes are also influenced by ground water-lake interactions. The species composition among sites were distinct. Regions of high ground water discharge were dominated by several species of diatoms indicative of high phosphorus concentrations, whereas communities at low ground water discharge and recharge sites consisted mainly of diatoms and cyanobacteria indicative of high nitrogen. Species diversity (H') and richness in regions of high ground water discharge were significantly lower than in regions of low ground water discharge and recharge.

These results suggest that the spatial heterogeneity and chemical characteristics of ground water flow may significantly influence epibenthic algal biomass. Furthermore, observed differences in species composition among sites also suggests important repercussions on epibenthic algal diversity within lakes. Results support the "paradox of enrichment,² which predicts lower diversity as a consequence of increased limiting nutrients. Moreover, whereas increased ground water flux lowers diversity at a single site, the distinctiveness of the community contributes to the overall lake species diversity.