Department of Physics, Chemistry and Biology (IFM)

It is well documented that shallow, eutrophic lakes often occur in two alternative stable states, where the clear water state is characterized by high transparency and abundant submerged vegetation while the turbid state is characterized by low transparency and high phytoplankton densities (Scheffer, et al., 1993).  In a situation of increasing nutrient loading, submerged vegetation can stabilize the clear-water state up to relatively high nutrient loadings, but once the system has switched to a turbid state, it takes a strong nutrient reduction to enable a return to the clear water state. Of these two states, both is natural but the clear-water state benefits waterfowl and is also attractive for human recreation.  A clear-water state also indicates a functional ecosystem where higher nutrient loads are not a big problem (Scheffer, 2004). Ecosystem responses, which alternative stable states are, is a nonlinear response to environmental change. This means that a lake is stable till a certain limit there the shift suddenly appears and there it is hard to get the ecosystem back to the desirable condition (in this case the clear water state). It is a sensitive, reversible system and the shift can have large consequences for eutrophic lakes that provides good habitats for birds (Hansson, et al., 2010).

Turbidity strongly affect transparency and light attenuation (light attenuation), which negatively affects the amount of light that reaches through the pelagic zone and reach the submerged vegetation. A low transparency and a high light attenuation affects submerged vegetation negatively (Faafeng & Mjelde, 1998). Turbidity shift naturally during the season and bring a natural “clear-water phase” in spring, which is frequently observed in meso- and eutrophic lakes (Lampert, et al., 1986). This temporary clear water state brings that secchi transparency increases and phytoplankton biomass decreases. This is caused by grazing of zooplankton (Lampert, et al., 1986) and enables a possibility for submerged vegetation to re-establish.

Phytoplankton biomass contributes to increase turbidity and is negatively affected by zooplankton biomass by grazing and filtering (Figure 1). Studies shows that zooplankton abundance and species correlates negatively with turbidity, especially for Daphnia spp (Levine, et al., 2005; Dejen, et al., 2004). This is a problem because Daphnia spp is particularly important for water clarity beacause they ingest up to 100 % of their body weight per day (Jeppesen, et al., 1994). Zooplankton is negatively affected by the number of small fish, which causes zooplankton levels to fluctuate during the spring and summer (Lathrop, et al., 1999). A lot of zooplanktivorus fish in a lake can also, except decrease zooplankton biomass, affect the amount of resuspended material by bio-turbulence (Figure 1) (Romo, et al., 2004 ). 

The positive effects of vegetation on the water clarity is a result of several different mechanisms: resuspension of bottom material is reduced by vegetation and the sedimentation rate increases due to lower water movement. Submerged vegetation provides a refuge against planktivorous fish for phytoplankton-grazing zooplankton and the same time as vegetation suppresses phytoplankton growth due to a reduction of nutrient availability (Figure 1). Water depth has a negative influence on submerged vegetation.

   All this leads to the aim of this thesis, which was to examine how interactions between phytoplankton, zooplankton and resuspended sediment influence light transmission during a critical phase for establishment of submerged vegetation.

Questions to be answered: i) Do zooplankton grazing affect the amount of phytoplankton biomass? ii) Do phytoplankton density and biomass contribute to total amount of resuspended sediment? iii) What do resuspended sediment consist of and how do that change during the season? iv) Is zooplankton biomass and density affected by resuspended sediment? v) Which variable contributes the most to inhibit light transmission?

 This was done by sampling and study the development in L. Tåkern during spring and summer of 2016, and by doing a laboratory experiment to examine how zooplankton is affected by sediment particles. This was done in L. Tåkern because it is known to have shifted between clear water and turbid states during the latest decades (Hargeby, et al., 2007).