Grazing Induced Shifts in Phytoplankton Cell Size Explain the Community Response to Nutrient Supply

Phytoplankton cell size is important for a multitude of functional traits such as growth rates, storage capabilities, and resistance to grazing. Because these response traits are correlated, selective effects on mean community cell size of one environmental factor should impact the ability of phytoplankton to cope with other factors. Here, we experimentally apply expectations on the functional importance of phytoplankton cell size to the community level. We used a natural marine plankton community, and first altered the community’s cell size structure by exposing it to six different grazer densities. The size-shifted communities were then treated with a saturated nutrient pulse to test how the changes in community size structure influenced the mean community growth rate in the short-term (day 1–3) and nutrient storage capacity in the postbloom phase. Copepod grazing reduced the medium-sized phytoplankton and increased the share of the smallest (<10 µm3) and the largest (>100,000 µm3). Communities composed of on average small cells grew faster in response to the nutrient pulse, and thus confirmed the previously suggested growth advantage of small cells for the community level. In contrast, larger phytoplankton showed better storage capabilities, reflected in a slower post-bloom decline of communities that were on average composed of larger cells. Our findings underline that the easily measurable mean cell size of a taxonomically complex phytoplankton community can be used as an indicator trait to predict phytoplankton responses to sequential environmental changes.

Functional diversity alters the effects of a pulse perturbation on the dynamics of tritrophic food webs

Biodiversity decline causes a loss of functional diversity, which threatens ecosystems through a dangerous feedback loop: this loss may hamper ecosystems' ability to buffer environmental changes, leading to further biodiversity losses. In this context, the increasing frequency of climate and human-induced excessive loading of nutrients causes major problems in aquatic systems. Previous studies investigating how functional diversity influences the response of food webs to disturbances have mainly considered systems with at most two functionally diverse trophic levels. Here, we investigate the effects of a nutrient pulse on the resistance, resilience and elasticity of a tritrophic---and thus more realistic---plankton food web model depending on its functional diversity. We compare a non-adaptive food chain with no diversity to a highly diverse food web with three adaptive trophic levels. The species fitness differences are balanced through trade-offs between defense/growth rate for prey and selectivity/half-saturation constant for predators. We showed that the resistance, resilience and elasticity of tritrophic food webs decreased with larger perturbation sizes and depended on the state of the system when the perturbation occured. Importantly, we found that a more diverse food web was generally more resistant, resilient, and elastic. Particularly, functional diversity dampened the probability of a regime shift towards a non-desirable alternative state. In addition, despite the complex influence of the shape and type of the dynamical attractors, the basal-intermediate interaction determined the robustness against a nutrient pulse. This relationship was strongly influenced by the diversity present and the third trophic level. Overall, using a food web model of realistic complexity, this study confirms the destructive potential of the positive feedback loop between biodiversity loss and robustness, by uncovering mechanisms leading to a decrease in resistance, resilience and elasticity as functional diversity declines.

Nitrogen and phosphorus translocation of forest floor mosses as affected by a pulse of these nutrients

Aims Mosses are dominant in many ecosystems where nutrients from deposition are one of the main nutrient sources. However, it is difficult to evaluate mosses’ role in nutrient cycling without knowledge of how mosses use deposited nutrient inputs. To fill this gap, the present study aims to investigate: (i) how nitrogen (N) and phosphorus (P) concentrations of new-grown segments change along a gradient of N or P amount in a pulse treatment? (ii) how do a pulse of major nutrient (N or P) affect N or P translocation rate along a moss shoot? and (iii) to what extent do N or P translocation rates link to nutrient status of the new-grown segments of mosses? Methods We measured N and P concentrations of segments with different ages in two dominant forest floor mosses, Actinothuidium hookeri and Hylocomium splendens, on 8 days and 1 year after N and P pulse treatment with an in situ experiment in a subalpine fir forest in eastern Tibetan Plateau. Important Findings Both mosses were efficient in taking up nutrients from a pulse of either N or P. Nitrogen and P concentrations of new-grown segments were affected by nutrient pulse treatments. These N and P concentration changes were attributed to the initial N and P concentration of the young segments harvested 8 days after nutrient pulse treatments, suggesting that the captured nutrients were reallocated to the new-grown segments via translocation, which was largely controlled by a source–sink relationship. While no significant relationship was found between N translocation rate and N:P ratio of the new-grown segments, P translocation rate explained 21%–23% of the variance of N:P ratio of the new-grown segments, implying importance of P transport in supporting the new-grown sections. These results suggest that nutrient (N, P) translocation is a key process for mosses to utilize intermittent nutrient supply, and thus make mosses an important nutrient pool of the ecosystem.

Effect of a rainfall pulse on phytoplankton bloom succession in a hyper-eutrophic subtropical lagoon

In the present study, we sought to understand the succession of phytoplankton species, after a natural nutrient pulse, in a subtropical lagoon located in southern Taiwan. The lagoon was surrounded by aquaculture ponds. The present study was performed during the wet summer season, before and after an episode of heavy precipitation. Before rainfall commenced, both the phosphate concentration and the level of phytoplankton were relatively low. After heavy precipitation, physical and chemical measurements indicated that significant amounts of dissolved inorganic nutrients had drained into the lagoon. A phytoplankton bloom occurred; organism levels reached 77.6×105 cells L–1. The dominant organism was Chaetoceros curvisetus (99.3%). After the bloom ceased, the levels of inorganic nutrients, especially silicate, fell. Phytoplankton became of low abundance once more. At the end of our study period, the ecosystem was dominated once more by diatoms (75.8%); this may have been caused by a low-level nutrient pulse following rainfall that occurred one day before final sampling. Overall, our results suggest that the bloom succession of phytoplankton species was principally dependent on nutrient dynamics in the lagoon, which was associated with nutrients discharged from drainage after heavy rainfall.