The effects of grazing by the snail Lymnaea and enrichment with N and P were examined in the epilithic algal community of an arctic lake. Increased nutrients raised chlorophyll CL levels and primary production and favored the growth of filamentous green algae and coccoid blue-greens.Intensive grazing reduced algal concentration and activity. Grazing increased the relative abundance of small green and blue-green coccoid cells at the expense of larger (>20 pm) diatoms.Photosynthetic activity of snail feces was 77-79% that of ungrazed algae per unit mass. Split chamber experiments and measurements of dissolved phosphorus indicated that nutrients released in grazing may be tightly recycled within snail guts or on the rock surface, rather than transferred into the water. Radioisotope tracing was used to develop a simple energy budget for Lymnaea. Despite viable gut passage of some of the algae. the snail was an efficient grazer, assimilating 68% and indbrp&ating 25% of ingested c&b&.

The effects on phytoplankton and limnetics of two different types of suspended sediments and their interactions with P loading were tested in a small North Carolina Piedmont lake. Limnocorrals were used in a complete, triplicated six-treatment, blocked design. Treatments were loaded with P, kaolinitic clay (K), K+P, montmorillonitic clay (M), and M+P. A4 caused more turbidity and stayed in suspension longer than K. Consequently, the light-dependent parameters, net community productivity (NCP), chlorophyll concentration, and algal density were lowest in the M and highest in the P treatment. Combined P and clay loading promoted clearing for both sediments and mitigated their effects on algal densities and NCP. Flagellated algae and nonfilamentous cyanophytes dominated the control community. The P treatment had blooms ofAnabaena. Without fertilization, both clays resulted in sparse, flagellate-dominated communities. The M+P community, like that of the P treatment, was dominated by Anabaena, but total algal densities were suppressed. In contrast, the K-t-P community lacked Anabaena and was similar to the control in algal quantity and composition.

Independent and interactive effects of phosphorus and clay loading on pelagic community organization and productivity were tested in a small Piedmont lake in North Carolina. Twelve limnocorrals (2-m diam, 3 m deep) were used for field manipulations.Treatments (in triplicate) were unaltered controls, P loading of 3.3 mg m-2 d-l, kaolinite clay loading of 100 g m-2 d-l, and combined clay and P loading. Fertilization with P significantly increased rates of turbidity reduction in comparison to controls (Secchi depth increasing at 7.2 vs. 5.8 cm d-l) and also significantly lowered sustained turbidity in treatments under clay loading. Clay loading reduced net community productivity (NCP), Chl a concentrations, and algal ccl1 numbers. Suspended clay also caused a shift in algal community composition;Trachelomonas superba and other flagellates replaced the otherwise dominant blue-green Spirulina major. Fertilization with P increased NCP and algal densities and favored development of the N,-fixing blue-green algae Anabaena spiroides and Anabaena circinalis. Combined P and clay loading produced intermediate values of turbidity, NCP, and Chl a. Simultaneous clay loading eliminated the influence of P fertilization on algal community structure, yielding an assemblage dominated by flagellates. Clay turbidity also caused a shallowing in the daytime distribution of zooplankton.

Effects of the grazing activity of the gastropod Lymnaea elodes (arctica?) on animal community structure in the rocky littoral of an arctic lake were examined. In situ caging experiments on natural substrates with various snail densities resulted in shifts in faunal biomass abundance and distribution. Chironomids living in fixed tubes (Paratanytarsus) were greatly reduced at high snail loads. Free—living scrapers (Corynoneura, Crictopus cf. sylvestris, Zalutschia trigonocies, and others) fared only slightly better, while predators (Arctopelopia and Ablabesmyia) were relatively less sensitive to changes in snail density. Trichoptera larvae, Acari, and copepods were unaffected by the snails. Ostracoda and Cladocera biomass was significantly reduced by Lymnaea. All of the less sensitive groups were relatively more mobile. Reduced algal biomass and chlorophyll concentration were correlated with increased snail densities. Lymnaea production was limited by conspecific density and food.