Year: 1987
Abstract:
The primary purposes of the Baptiste Lake Study, carried out between March, 1976 and March, 1979 were to determine the trophic status of the lake, and to develop and employ methods for assessing the impact of existing or future shoreline and watershed development on that status. In order to determine the trophic status of the lake an intensive limnological study was undertaken; the latter included weekly observations of phytoplankton carbon fixation, phytoplankton standing crop, taxonomic changes and changes in the physico-chemical environment. In order to assess existing cultural impacts to the lake, the sedimentary record was analyzed to determine any historical changes in the fertility of the system. In order to predict the effect of future development, a detailed nutrient budget was constructed for the lake and a comparative nutrient loading analysis undertaken.
Baptiste Lake drains a large watershed and, as such, has a large external loading of nutrients. Prior to European settlement this loading, primarily from a boreal mixed-wood forest, was large enough to ensure that the lake was in a eutrophic condition. This interpretation has been confirmed by the sediment core analysis. The clearing and conversion of forested land to agricultural use has had a measurable impact on stream nutrient loadings, the net result of which has been to elevate the trophic status of the lake to a more eutrophic condition. By comparison, the estimated nutrient loadings from cottages were relatively small, although the question of nutrient bio-availability was not assessed in this study.
The average total phosphorus (TP) export coefficient for forested lands was 0.14 kg ha-1 yr-1 , and that from agricultural land was 0.27 kg ha-1 yr-1 • These data are similar to the mean coefficients presented by the US EPA (1974) and Uttormark (1974). The average total nitrogen (TN) export coefficient for
forested land was 2.50 kg ha-1 yr-1 and for agricultural land 2.13 kg ha-1 yr-1 ; these differences between land use types were not statistically significant. These coefficients are lower than those measured in many other locations, and the TN pools differ significantly in composition. The TN loads were primarily composed of organic nitrogen (approximately 75% for agricultural streams and 90% for forested streams). The TN:TP ratios in both stream basin types were high and suggest phosphorus deficiency, however, inorganic N:P ratios were below 7.2 and suggest nitrogen deficiency. This conundrum will remain until some understanding of the bio-availability of the organic nitrogen component is attained.
The atmospheric input of nutrients was measured in devices similar to those described by Likens et al. (1979) for the Hubbard Brook Study. The annual input of TP was estimated at 30.4 mg -2 -1 m yr , which is lower than that occurring in many other regions. Most of the deposition occured during spring, sunmer, and autumn, with little input over winter. Groundwater flows into the lake were estimated by a mathematical model developed by Crowe and Schwartz (1981). They estimated that approximately 13% of the annual hydraulic loadings could be derived from groundwater. However, because of the
diversity of hydrostratigraphic. units surrounding the lake basin, each with different hydrochemical characteristics, we did not attempt to estimate nutrient loading from this source. Direct evidence for groundwater seepage into the lake was produced as a result of discrete chemical testing in the north basin. Baptiste Lake has one seasonally stratified basin, and one temporarily stratified basin. The lake is partially meromictic as a result of incomplete spring mixing, and displays many of the characteristics of the classical eutrophic system. Hypolimnetic oxygen depletion occurred in the deep, southern basin and this resulted in a large accumulation of nutrients in the hypolimnion, presumably from sediment release. TP concentrations exceeded 500 mg
m-3 near the bottom in late summer and this hypolimnetic pool probably enriched surface waters as the thermocline was depressed.
Heavy blue-green algal blooms occurred during late summer, with peak surface chlorophyll ! concentrations observed in the high hundreds of mg m- a, The shallow, northern basin stratified for approximately three months each summer; this was also accompanied by oxygen depletion and nutrient accumulation. Isothermal conditions developed by late August in this basin so that the hypolimnetic nutrient supply enhanced the summer blue-green bloom, as opposed to enhancing the autumn pulse of phytoplankton as occurred in the southern basin.
The seasonal pattern of phytoplankton succession is similar to that described for eutrophic systems by Reynolds (1982). The open water season lasts approximately six months, extending from late April to early November. The spring diatom assemblage is dominated by Stephanodiscus astraea, with the pyrrhophyte Peridinium !Q.. ·making brief but important contributions to the biomass during May. An unidentified unicellular blue-green, now believed to be broken colonies of Microcystis aeruginosa, was also significant. An early sunmer minimum during June was characterized by a mixed conmunity which included ~- astraea, several cyanophytes, and the cryptophyte Cryptomonas ovata. The cyanophytes dominated from early July to mid-September, however, there was a shift from domination by Anabaena circinalis and Microcystis aeruginosa to Gomphosphaeria naegelianum and Aphanizomenon flos-aguae in August. The large pyrrhophyte Ceratium hirundinella was also present during July and August and was extremely important in terms of total cell volumes.
During the month of October, the phytoplankton was again dominated by the diatoms Melosira granulata and Stephanodiscus astraea, but also included the cryptophyte Cryptomonas ovata. The annual carbon fixation rate was determined by the radiocarbon method and estimated at approximately 302.5 g c m- 2 yr-1 • Most photosynthetic activity was concentrated in.the open water season, although some under-ice activity was recorded in late March as the photoperiod increased Extreme spatial heterogeneity, or patchiness, was observed in surface chlorophyll ~distributions in Baptiste Lake. The degree
of patchiness increased as the concentration increased, and these data pointed out the importance of statistical sampling design in the accurate trophic assessment of productive lakes.
