Simulated terrestrial runoff triggered a phytoplankton succession and changed seston stoichiometry in coastal lagoon mesocosms
Climate change scenarios predict intensified terrestrial storm runoff, providing coastal ecosystems with large nutrient pulses and increased turbidity, with unknown consequences for the phytoplankton community. We conducted a 12-day mesocosm experiment in the Mediterranean Thau Lagoon (France), adding soil (simulated runoff) and fish (different food webs) in a 2 × 2 full factorial design and monitored phytoplankton composition, shade adaptation and stoichiometry. Diatoms (Chaetoceros) increased four-fold immediately after soil addition, prymnesiophytes and dinoflagellates peaked after six- and 12 days, respectively. Soil induced no phytoplankton shade adaptation. Fish reduced the positive soil effect on dinoflagellates (Scripsiella, Glenodinium), and diatom abundance in general. Phytoplankton community composition drove seston stoichiometry. In conclusion, pulsed terrestrial runoff can cause rapid, low quality (high carbon: nutrient) diatom blooms. However, bloom duration may be short and reduced in magnitude by fish. Thus, climate change may shift shallow coastal ecosystems towards famine or feast dynamics.
Application of a numerical model to predict impacts of climate change on water temperatures in two deep, oligotrophic lakes in New Zealand
We applied a numerical hydrodynamic model (DYRESM) to two large, deep New Zealand lakes that are characterised by deep thermoclines and high wind forcing, to assess their sensitivity to changes in climate. Modifications to standard model parameters were necessary for the successful application of DYRESM. Predictions from downscaled global circulation models suggest an increase in mean air temperature, rainfall, and wind speeds. Modelling the hydrodynamics of the lakes suggests that increasing air temperatures would offset the cooling influences of increased rainfall and river flows, resulting in warmer overall lake temperatures, and an earlier, longer, and shallower thermal stratification. These physical changes could affect phytoplankton production as their light limitation would decrease in duration and intensity. However, deeper mixing caused by increases in wind speed would negate this reduction of thermocline depth. While warmer air temperatures appear to be the dominant driver of changes in thermal structure, changes in other meteorological factors, especially wind speed, are important in predicting future hydrodynamics. Compared to large, deep lakes in the Northern Hemisphere, the predicted warming rates in Lakes Wanaka and Wakatipu are slower, due partly to a lower predicted rate of atmospheric warming and the absence of winter ice cover in these lakes.
Investigation of nutrient limitation status and nutrient pathways in Lake Hayes, Otago, New Zealand: a case study for integrated lake assessment
We present a case study illustrating an integrated approach to lake water quality assessment, for Lake Hayes. Otago, New Zealand. This approach improved understanding of the biological importance of several macro- an micronutrients and their major pathways (including groundwater). and, coupled with trophic state monitoring, could be the basis for more effective management of lakes exhibiting algal blooms. A nutrient enrichment bioassay experiment found that additions of zinc and nitrogen stimulated productivity, indicating, that phytoplankton growth in the lake may have been limited by these nutrients. Nutrient data confirmed the potential for nitrogen limitation at times. The main source of nutrients to the lake was the surface inflow, but almost 30% of the nitrate input entered the lake via a groundwater-fed spring. highlighting the importance of nutrients in groundwater as a potential driver of algal proliferation. A recent shift in phytoplankton community structure from dominance of cyanobacteria and desmids to dominance of mobile, mixotrophic dinoflagellates indicates that the dominant phytoplankter may no longer be restricted to using, only epilimnetic and inorganic forms of nutrients. This integrated assessment of Lake Hayes also suggested strong internal phosphorus cycling. which Could explain why phytoplankton blooms Continue despite improved catchment nutrient management.