Stability of marine phytoplankton communities facing stress related to global change: Interactive effects of heat waves and turbidity

According to climate models, coastal ecosystems will face an increased frequency of heat waves and increased turbidity due to terrestrial sediment run-off induced by increasing precipitation. Several studies have examined the effects of heat waves and turbidity separately, whereas this study analysed the individual effects of both stressors as well as their interaction, because stressors affect communities differently when acting in combination. Using a factorial experimental design, we simulated heat waves (22 °C and 26 °C compared to an 18 °C control) and turbidity (sediment addition). The response of the phytoplankton community was analysed for the aggregate parameters biovolume and diversity index (H′), as well as for community composition. Heat waves had a significant negative effect on biovolume, whereas turbidity tended to affect biovolume positively. Repeated measures ANOVA revealed significant interactions of heat waves and turbidity for H′ and community composition. Strong heat waves (26 °C) alleviated the otherwise positive effect of turbidity on H′, i.e. highest diversity remained in the turbid control. Diatoms gained dominance in the control and the 22 °C heat wave treatment with Cylindrotheca closterium being the successful competitor. At 26 °C this species was lost and small flagellates dominated the experimental communities. Future increases in heat wave intensity and frequency may thus induce major changes in phytoplankton community structure whereas algae might profit from increased turbidity as an additional source of nutrients

Dominance and compensatory growth in phytoplankton communities under salinity stress

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

The influence of mixing frequency and depth on phytoplankton mobility

The influence of mixing frequency and depth on phytoplankton functional group composition (mobile versus immobile species) was studied by enclosure experiments in a shallow, stratified lake. Mixing events were artificially induced at intervals from 2–12 d. The mixing depth was increased from the natural level (4 m) to 6 and 9 m. The mobile phytoplankton in the experiments consisted of cyanobacteria and flagellates. Among the latter, large and rapid swimming species were represented by dinoflagellates. An increase of the relative abundance of gas vacuolated cyanobacteria occurred with increasing frequency of mixing. Additionally Reynolds' hypothesis predicting the occurrence of certain mobile phytoplankton genera in response to the mixing regime could be confirmed for the condition when mixing depth exceeds the euphoric depth

The influence of fluctuating light on diversity and species number of nutrient-limited phytoplankton

The influence of fluctuating light on diversity and species number of a natural phytoplankton assemblage competing for nutrients was investigated for 48 days under semicontinuous culture conditions. Light conditions were either changed periodically from high (65 μmol photons·m−2·s−1) to low intensity (15 μmol photons·m−2·s−1) at intervals of 1, 3, 6, and 12 days or fixed at constant light conditions of intermediate intensity (40 μmol photons·m−2·s−1). Fluctuating light at intervals of 1–12 days significantly affected phytoplankton diversity. The development of phytoplankton communities differed in treatments with different light regimes. In treatments with long light intervals, species abundance oscillated with the light phases. Differences in the temporal development of phytoplankton communities resulted in hump‐shaped relations between the interval length of the light phases and both species number and diversity index and can be explained by the intermediate disturbance hypothesis. Fluctuating light tends to sustain phytoplankton diversity under nutrient limitation if the light regime changes in the order of several days. This indicates that temporal changes in weather regime are important in preventing competitive exclusion of phytoplankton species in nature

Diversity in planktonic communities: an experimental test of the intertmediate disturbance hypothesis

According to Connell�s intermediate disturbance hypothesis (IDH), diversity within a community is maximal at intermediate frequencies and intensities of disturbances. In order to test the IDH, disturbances of different frequencies and intensities were imposed on natural plankton communities in controlled field experiments. These disturbances consisted of an artificial deepening of the mixed layer, leading to the dilution of epilimnetic populations and to a higher level of nutrients. Intervals between disturbances ranged from 2 to 12 d. Different intensities of disturbance were caused by differences in the experimental mixing depth (150 and 225% of the original epilimnion depth). Investigation focused on the effect that disturbances had on the diversity of natural phytoplankton communities. Additionally, we were interested in determining the effect of grazing by zooplankton. The results of the field experiments show for the first time the applicability of the IDH to phytoplankton within complete planktonic communities. Diversity showed a clear maximum at the intermediate disturbance interval of 6 d. Similarly, species number peaked at intermediate interval length (6-10 d)

Phytoplankton diversity of shallow tidal lakes: influence of periodic salinity changes on diversity and species number of a natural assemblage

The influence of periodic salinity changes was investigated for 42 days under semicontinuous culture conditions with phosphorus limitation using phytoplankton assemblages from Lake Waihola, a tidally influenced shallow lake. To simulate tidal effects on the phytoplankton community, salinity in the cultures was increased in pulses at different intervals (3.5, 7, and 14 days), and these cultures were compared with those that experienced constant freshwater conditions. Salinity pulses significantly affected competition and succession with a major loss in diversity during the first days of the experiment due to the initial pulse that caused a transition from freshwater to brackish conditions in the cultures. After this initial phase, diversity index (H') and species number (Scorr) decreased less rapidly. The loss in H' and Scorr over time was highest under constant freshwater conditions and lowest in the treatment with an interval of 3.5 days between salinity pulses. At the end of the experiment, the combination of initial loss in H' and Scorr and the time course of H' and Scorr resulted in a U‐shaped relation between the interval length of salinity pulses and both H' and Scorrtemp1.txttemp1.txt. Our results indicate that salinity pulses at intervals of a few days tend to promote phytoplankton diversity. If saline intrusions in coastal freshwater systems occur only at spring tides, this will lead to decreases in diversity and species richness

Energy-Dependent Bacterivory in Ochromonas minima - A Strategy Promoting the Use of Substitutable Ressources and Survival at Insufficient Light Supply

Phagotrophy and competitive ability of the mixotrophic Ochromonas minima were investigated in a three-factorial experiment where light intensity (low: 1.0 μmol m−2 s−1 and high: 60 μmol m−2 s−1 PPFD), nutrient concentration (ambient: 7.0 μmol N l−1, 0.11 μmol P l−1 and enriched: 88 μmol N l−1, 6.3 μmol P l−1) and DOC supply (without and with enrichment, 250 μmol C l−1) were manipulated. Ochromonas minima and bacterial abundance were monitored for 12 days. We found significant and interacting effects of light and nutrients on Ochromonas minima growth rate and abundance. At high light intensity, nutrient enrichment resulted in increased growth rates and population sizes. In contrast, reduced growth rates and population sizes were observed for nutrient enrichment when light intensity was low. Although, Ochromonas minima was able to ingest bacteria under both high and low light conditions, it grew only when light intensity was high. At high light intensity, Ochromonas minima grew exponentially under nutrient conditions that would have been limiting for photoautotrophic microalgae. In non-enriched low light treatments, Ochromonas minima populations survived, probably by using background DOC as an energy source, indicating that this ability can be of relevance for natural systems even when DOC concentrations are relatively low. When competing with photoautotrophic microalgae, the ability to grow under severe nutrient limitation and to survive under light limitation should be advantageous for Ochromonas minima