Nutrient‐Phytoplankton Relationships in the Holland Marsh, Ontario

During a limnological investigation of the Holland River and Cook Bay of Lake Simcoe, seasonal changes in phytoplankton biomass and species composition in the lower Holland River (Holland Marsh) were evaluated with reference to data collected on nutrient concentrations and other environmental factors. Over the whole year, Chlorophyceae comprised about ½ of the average standing crop of phytoplankton, Bacillariophyceae, about ⅓, and Cyanophyceae, < °. In spring and fall, diatoms (mainly Stephanodiscus hantzschii) were dominant and constituted at times >90% of the total phytoplankton biomass which was as high as 3.2 x 10 7 μm 3 /ml in May of 1972. The high standing crops in summer (1.0—3.0 ? 10 7 μm 3 /ml) were dominated by chlorococcalean forms (especially Scenedesmus quadricauda) with smaller volumes of diatoms (mainly Melosira granulata and M. ambigua) and blue—green algae (Oscillatoria articulata and O. tenuis) present. Despite summer inorganic—N concentrations of <10 mg N/1, phytoplankton species succession continued with several taxa developing maximum abundance during the period of low nitrogen concentrations. Although the winter phytoplankton showed more diversity, with diatoms, chrysomonads, dinoflagellates, cryptomonads, and euglenoids all prominent in the plankton, the biomass was considerably lower at 0.16—2.9 X 10 6 μm 3 /ml. Maximum chlorophyll a concentrations in the Holland Marsh (as high as 358 mg/l) compare with data in the literature from highly eutrophic waters. Phaeopigment concentrations ranged as high as 81 mg/l and comprised up to 58%, but averaged °15% of total uncorrected chlorophyll a during the ice—free summer period. Over the year, chlorophyll a content of phytoplankton ranged from 0.14% to 1.65% of fresh weight. Despite almost identical species composition of the phytoplankton (°90% S. hantzschii by volume) in May and November, the chlorophyll a content as a percentage of fresh weight was almost 3 x greater in November than in May. According to the influence of total daily incident radiation and inorganic N concentrations as controlling factors of chlorophyll a content of phytoplankton, three conditions were recognized in the Holland Marsh: (1) chlorophyll a contents were intermediate in spring under highest daily incident radiation (favoring low chlorophyll contents) and ample supplies of inorganic—N (favoring high chlorophyll contents); (2) chlorophyll a contents were lowest in summer when inorganic N was in scarce supply and daily incident radiation was high; and (3) contents were highest in fall and winter when supplies of inorganic—N were ample and daily incident radiation was lowest. Inorganic—N most likely limited summer phytoplankton growth. The spring phytoplankton maximum (S. hantzschii) was likely limited by SiO 2 in 1971 and by P in 1972. The extent of depletion of soluble reactive P by diatoms in the autumn was controlled by silica concentration. Some evidence is presented which suggests that the initial effects of improved nutrient retention within intensively cultivated and urban areas on the Holland River watershed would be realized as a shift in the limiting nutrient for the spring phytoplankton bloom in the Holland Marsh and southern Cook Bay, from SiO 2 to P.