Plankton Dynamics in Mesotrophic Highland Reservoirs of the Colorado River System, Texas
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Various ecological and limnological factors affect plankton dynamics and nutrient processing in clear, hard water subtropical reservoirs where temperature and light usually are conducive to year-round phytoplankton growth. Selected reservoirs in the Highland Lakes system that are currently under a point-source discharge ban, yet are slated for future wastewater discharges, were studied by collecting natural plankton assemblages from the surface-mixed layer at various uplake to near-dam sites in the larger reservoir, Lake Lyndon B. Johnson (LBJ), and from the near-dam sites in the two smaller reservoirs, Inks Lake and Lake Marble Falls. My questions were: how do the nutrients, nitrogen (N) and phosphorus (P), affect phytoplankton growth; and to what extent, seasonally, does zooplankton grazing impact phytoplankton, since phytoplankton mortality is a major loss factor of productivity and biomass from the photic zone?
Nutrient bioassays were prepared by initially pre-sieving the water to remove adult zooplankton (>150 micron) and with the following treatments: control (no nutrients added); nitrogen (N); phosphorus (P); and N+P. The monthly bioassays were incubated 5-7 d in a growth chamber set at reservoir temperature and seasonal photoperiods. Phytoplankton growth was measured using daily in vivo fluorescence, and by initial and final extracted chlorophyll (Chl a). Phytoplankton growth rates differed statistically among treatments and generally were negligible in the controls and increased four-fold in the N+P treatments. In the single nutrient treatments (N or P), significant interactions occurred with N-limitation during the warmer, stratified months, and P-limitation during the cooler months when the vertical water column was fully mixed.
Phytoplankton mortality due to zooplankton grazing was assessed seasonally with natural zooplankton assemblages that were collected by vertical net tows at the near-dam sites of each reservoir. Once in the lab, a density gradient was prepared by adding progressively higher numbers of zooplankton (grazers) to each treatment (0-3X) and incubated from 24-48 h with the ambient phytoplankton assemblages collected from the near-surface mixed layer at various reservoir sites. The 0X treatment had no grazers added to the water and served as a control to assess the phytoplankton growth rate in the absence of grazers. The 1X treatment had about 7-10 grazers, the 2x had about 20 grazers, and the 3X had about 30 grazers added to the ambient phytoplankton assemblages. The grazing rate was determined by the change in phytoplankton growth between 0 and 3X treatments, with phytoplankton growth rate plotted as a linear function of zooplankton density. N and P were added to all treatments at approximately half the concentration of the [NP] in the nutrient bioassays to allow for phytoplankton growth and to dampen the effects of nutrient regeneration by zooplankton.
Phytoplankton mortality due to zooplankton grazing was significant by the reservoir copepod-dominated assemblages. In post-analysis, the phytoplankton growth rates in the controls were compared to the overall grazing rates to determine seasonal patterns of growth versus grazing, by season and site. In spring, phytoplankton growth exceeded zooplankton grazing from all sites except one site, and at all but two sites in the late summer. In the fall, winter, and early summer, zooplankton grazing rates were equal to or exceeded growth rates at most sites.
Currently, the Highland Lakes’ phytoplankton communities are constrained by both ambient nutrient limitation and zooplankton grazing. This study serves as an important assessment of primary producers and secondary consumers in subtropical reservoirs, prior to potential system eutrophication. The ecological and limnological processes studied here also may be useful as indicators to assess ecosystem health, climate, and anthropogenic changes within the context of aquatic food web ecology, limnology, and biogeochemical cycling.