The Nitella which grows in Mountain Lake, Giles County, Virginia, was investigated and found to be Nitella megacarpa (T.F.A.) em. R.D.W. after N. furcata subsp. megacarpa and not N. flexilis (L.) Ag. as previously reported. Internodal cell length was found to increase with depth, pigment content changed with depth, biomass distribution had decreased since an earlier study, primary productivity was low and was associated with the depth of maximum biomass and pigment content, and the furcations of the branchlets increased in shallower water. / M.S.

The first comprehensive year-round investigation of Mountain Lake, Virginia since 1970-71 was begun in March 1985 and continued through November 1987. Monthly studies suggested that this unique natural lake is undergoing change. Whereas inorganic nitrogen, orthophosphate, Secchi disc transparency, and primary productivity continue to suggest an oligotrophic condition, important limnological changes are indicated by the greater seasonal hypolimnetic oxygen depletion and extractable chlorophyll a which at times approach mesotrophic levels. In addition, the decrease in alkalinity, the increase in sulfate levels, and the increase in pH fluctuations compared to the past, suggest that Mountain Lake is susceptible to damage or change by acid rain and perhaps other perturbations that effect pH alkalinity, or hardness. Changes were also noted in phytoplankton composition. One hundred sixty-three taxa were identified which included 56.4% Chlorophyta, 17.8% Chrysophyta, 13.5% Cyanophyta, 8% Pyrrophyta, 3.7% Cryptophyta, and 0.6% Euglenophyta. Sixty percent of these taxa were new reports for Mountain Lake. Dominant pyrrophytes included Gymnodinium sp. and Peridinium wisconsinense. Dominant chlorophytes included Scenedesmus bijuga, Quadrigula chodatii, and Oocystis pusilla. Important cryptophytes were Cryptomonas erosa var. reflexa, C. pusilla, and C. pyrenoidifera. The most abundant cyanophyte was Aphanocapsa elachista var. conferta. The only diatom (chrysophyte) that ever dominated was Cyclotella meneghiniana. Studies of ammonia uptake by the phytoplankton, using the ammonia analog [¹⁴C]methylamine (MeA), revealed that during thermal stratification, phytoplankton succession was related to rapid ammonia uptake. Each successive phytoplankton community had significantly higher Vmax values for ammonia (methylamine) until midsummer when ammonia-nitrogen levels were at a seasonal low. At that point, the Vmax values dramatically declined and the community became dominated by blue-green algae. When allochthonous inputs of inorganic nitrogen renewed the levels of ammonia-nitrogen, Vmax values increased and the community changed, replacing the blue-green algae. Communities dominated by green algae (Chlorophyta) had higher MeA (and thus ammonia) uptake rates than those dominated by cryptophytes or blue-green algae (Cyanophyta). The effect of various treatments and inhibitors on MeA uptake by the phytoplankton support the conclusion that ammonia uptake is a chemical, light-independent, energy-dependent process. / Ph. D.

The present paper summarizes observations made on Mountain Lake, Virginia during 1961, 1962, and 1963. It includes data on the annual cycle of physical and chemical events occurring in the lake, and will serve as a point of departure for future, more specialized studies there. Observations made on the profundal bottom fauna of the lake during the same period are also reported. / Master of Science

Seasonal effects on protozoan colonization of polyurethane foam (PF) units were investigated in.an oligotrophic lake, Mountain Lake, Virginia. PF units were placed into the lake's pelagic water to simulate barren habitat islands suitable for colonization by protozoa. The results were interpreted with respect to the MacArthur-Wilson equilibrium theory of island biogeography. Results showed seasonal changes can cause a substantial amount of instability in colonization curves. Within a season, changes in the lake's planktonic species pool were more prominent during the decay of the thermocline than during Fall or Spring overturn and Summer stratification. For different seasons, the relationship between changes in the lake's planktonic species pool and species accrual on PF units was positive, for total species and for ciliate species. For different seasons, the relationship between changes in colonization rate and species accrual was consistent with MacArthur-Wilson theory. In contrast, for different seasons, the relationship between changes in equilibrium number of species and species accrual was inconsistent with MacArthur-Wilson theory, and this places in question the interseasonal predictive value of colonization curves. For some situations where the MacArthur-Wilson equation for insular colonization did not describe adequately the colonization a modified equation was developed. The modified equation was used to account roughly for species which may rapidly colonize and persist in PF unit communities and described adequately more total species colonization curves than the unmodified equation. Moreover, the modified equation was a useful tool for the interpretation of PF unit colonization by protozoa. / Master of Science