Quantifying microplastic residence times in lakes using mesocosm experiments and transport modelling.

The microplastic residence time in lakes is a key factor controlling its uptake by lake organisms. In this work we have, for the first time, conducted a series of microplastic addition experiments in a 12 × 3 m lake mesocosm and traced its transport through the lake water column. This was combined with a 1D physically based random walk model of microplastic transport. Four experiments were conducted using three microplastic size ranges (1-5, 28-48, and 53-63 µm) over one year during thermal stratification and lake turnover. The results showed that the residence time in the water column largely depended on particle size and lake hydrodynamics, although the smallest particles were poorly represented by the model. Residence times in the mesocosm ranged between ∼1 day for the largest particles to 24 days for the small particles during summer. The modeled residence time were similar to the measured values of the 28-48 µm and 53-63 µm particles, but for the smallest particles residence times were calculated to be >200 d. The discrepancy is likely due to aggregation between the small microplastic particles and natural lake particles, which increases the microplastic settling velocity. Aggregation is favored for the small particles due their large surface area to volume ratio. In contrast, density instabilities in the water column during autumn likely led to turbulent convective mixing and rapid microplastic transport within the water column. This work shows that microplastic transport within lakes is complex and not fully understood, especially for the smallest sizes, and involves interactions between physical, physicochemical and biological processes.

Nitrogen enrichment leads to changing fatty acid composition of phytoplankton and negatively affects zooplankton in a natural lake community.

Secondary production in freshwater zooplankton is frequently limited by the food quality of phytoplankton. One important parameter of phytoplankton food quality are essential polyunsaturated fatty acids (PUFAs). Since the fatty acid composition of phytoplankton is variable and depends on the algae's nutrient supply status, inorganic nutrient supply may affect the algal PUFA composition. Therefore, an indirect transfer of the effects of nutrient availability on zooplankton by changes in algal PUFA composition is conceivable. While the phosphorus (P) supply in lakes is largely decreasing, nitrogen (N) inputs continue to increase. This paper presents data from a mesocosm field experiment in which we exposed phytoplankton communities to increasing N enrichment. As a consequence, the PUFA composition of the phytoplankton community changed. With increasing nitrogen fertilisation, we observed lower quantities of essential PUFAs, together with a decrease in the abundances of the dominant herbivorous zooplankton Daphnia sp. Their biomass was significantly correlated with phytoplankton PUFA content (C18:3 ω3, C20:5 ω3, C18:2 ω6). Our data therefore indicate that changes in nitrogen supply, together with the resultant changes in phytoplankton food quality, can negatively affect the secondary production of herbivorous zooplankton by reducing the availability of essential polyunsaturated fatty acids.

Water temperature and mixing depth affect timing and magnitude of events during spring succession of the plankton.

In many lakes, the most conspicuous seasonal events are the phytoplankton spring bloom and the subsequent clear-water phase, a period of low-phytoplankton biomass that is frequently caused by mesozooplankton (Daphnia) grazing. In Central European lakes, the timing of the clear-water phase is linked to large-scale climatic forcing, with warmer winters being followed by an earlier onset of the clear-water phase. Mild winters may favour an early build-up of Daphnia populations, both directly through increased surface temperatures and indirectly by reducing light limitation and enhancing algal production, all being a consequence of earlier thermal stratification. We conducted a field experiment to disentangle the separate impacts of stratification depth (affecting light supply) and temperature on the magnitude and timing of successional events in the plankton. We followed the dynamics of the phytoplankton spring bloom, the clear-water phase and the spring peak in Daphnia abundance in response to our experimental manipulations. Deeper mixing delayed the timing of all spring seasonal events and reduced the magnitudes of the phytoplankton bloom and the subsequent Daphnia peak. Colder temperatures retarded the timing of the clear-water phase and the subsequent Daphnia peak, whereas the timing of the phytoplankton peak was unrelated to temperature. Most effects of mixing depth (light) and temperature manipulations were independent, effects of mixing depth being more prevalent than effects of temperature. Because mixing depth governs both the light climate and the temperature regime in the mixed surface layer, we propose that climate-driven changes in the timing and depth of water column stratification may have far-reaching consequences for plankton dynamics and should receive increased attention.