Abstract
Increasing levels of environmental stress due to global warming and eutrophication, and concerns about an unparalleled global diversity loss, have triggered new interest in the question whether the stability of ecosystem properties depends on population dynamics of dominant species or on compensatory growth of rare species. Recent meta-analyses suggest that compensatory dynamics are rare in natural systems. Experimental results, however, indicate that the interdependence of stressor regime, species traits, and species richness determines which mechanisms stabilise communities. Stability will depend on population dynamics of dominant species, if they remain the best performers regardless of disturbance. If dominant species become rare or lost, compensatory growth of rare species will insure natural communities against complete failure. Salinity is an important stressor governing growth and distribution of phytoplankton in brackish ecosystems, and its impact on coastal aquatic ecosystems is likely to change due to global warming. We performed two short-term experiments to investigate the effects of salinity stress on community structure and biomass production of natural phytoplankton communities collected in tidally influenced and polymictic Lake Waihola (New Zealand). The lake was brackish when the inoculum for the first experiment was collected. The inoculum for the second experiment originated from a fresh water situation. In both experiments, the phytoplankton assemblage was exposed to a salinity gradient ranging from 0 to 5. To assess the importance of dominance and compensatory growth, we determined biomass production, species richness, diversity, evenness and dominance indices, and species specific growth rates.
Biomass production in our experiments was determined by dominant species.
Anabaena flos-aquae dominated in the first experiment, and
Asterionella formosa in the second experiment. Despite the importance of these species, we found significant growth responses of rare and abundant species. Even if these species showed high growth rates, biomass production was carried by the dominant species as long as the salinity level allowed them to grow. When the salinity level was detrimental to the growth of the dominant species, reduced dominance and increased diversity indices emphasised the importance of compensatory growth of rare species. The salinity stress applied in our experiments was strong enough to change the hierarchy of successful functional traits, which affected community structure and biomass production of the plankton communities. If the predicted sea water rise, increasing frequency of storm tides, rising water temperatures, and altered precipitation and run-off cause the salinity of coastal aquatic ecosystems to change, major changes in community composition, diversity and dominance structure of planktonic primary producers might be expected.