Suzanne N. Levine

Retention of spike additions of soluble phosphorus in a northern eutrophic stream

We evaluated P transport through a 3rd-order eutrophic stream in 2 seasons, December 1994 and September 1997, by adding spikes of dissolved P (orthophosphate) and dye tracer (Rhodamine WT) to a 3-km reach. The December experiment occurred under ice cover when biological growth was minimal; during the September experiment plant growth was abundant. Both the dye and P plumes were dispersed in space and time as a result of mixing and transient storage; only P was retained in the stream reach. Initial reversible short-term P retention in the stream reach was demonstrated in both seasons. In December, 36% (318 g) of added P was retained for about 2.5 h, then released over the next 12 h. In September, 39% (116 g) of added P was retained over 12 h, with negligible subsequent release. All the P added in December was exported from the reach within 24 h; 38% of added P (4 mg P/m2) was still retained in the stream reach at the end of the 48-h September experiment. Phosphorus retention was apparently influenced by flow, temperature, concentration gradient, and biological activity. Short-term retention probably resulted from sorption by inorganic sediments and organic biofilms; long-term retention may have been a result of biological uptake. Instream P retention processes may not be capable of reducing P transport significantly during high flows and cold temperatures, but may temporally attenuate inputs delivered during small stormflows in the growing season.

Phosphorus, Nitrogen, and Silica as Controls on Phytoplankton Biomass and Species Composition in Lake Champlain (USA-Canada)

Abstract The long-standing assumption that the phytoplankton in Lake Champlain are phosphorus limited was tested through measurement of physiological indicators of phosphorus status (alkaline phosphatase activity and orthophosphate turnover time) and enrichment experiments conducted four times during the growth season. Phosphorus addition to experimental carboys incubated 4–5 days in situ substantially increased phytoplankton biomass relative to controls in June, but had only a mild impact in July and September, and no effect in May. Nitrogen addition augmented biomass in one of four experiments (in June), while silica had no impact at any time. In summer and fall, addition of N and P in combination always yielded more phytoplankton biomass than singular P addition. In spring, even combined addition of N, P, and Si failed to stimulate phytoplankton growth. The phytoplankton groups responding to fertilization were largely the same as those that flourished in controls (diatoms and green algae), suggesting that enclosure was a more powerful determinant of species composition than nutrient inputs. Orthophosphate turnover times and levels of alkaline phosphatase activity in the lake indicated spatial and temporal variability in phytoplantkon P status, with P sufficiency as common as P deficiency. We conclude that multiple interacting factors influence the abundance and species composition of phytoplankton in Lake Champlain. In spring, phytoplankton growth is not limited by N, Si, or P, but by some factor yet to be determined (perhaps light or temperature). In summer, P is the principal limiting nutrient, but N exerts an influence that deserves further investigation.

The Impact of Zooplankton Grazing on Phytoplankton Species Composition and Biomass in Lake Champlain (USA-Canada)

Abstract Rates of grazing on phytoplankton by macrozooplankton (cladocerans and copepods > 220 μm in length) and microzooplankton (animals