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Discussion

The variables that contributed most to inhibit light transmission in L. Tåkern was phytoplankton biomass and the amount of detritus material. Phytoplankton biomass and density also contributed to total amount of resuspended sediment in the water volume. The content of resuspended sediment changed during the season, but consisted mostly of inorganic solids and detritus material. However, even if the biomass of phytoplankton was lower in the later part of the season, it still contributed to inhibit light transmission in the lake. The amount of inhibition of light transmission was at times that high that it should have affected the submerged vegetation

Zooplankton grazing showed to affect the amount of phytoplankton biomass, and created a “clear-water window” in May 2016. It is possible that this contributed to the establishment of submerged vegetation. Zooplankton biomass was in general not affected by resuspended sediment, but correlations at point 2 showed that larger Daphnia species were affected by high amounts of resuspended sediments. 

A lot more correlations was found at point 2. This is due to a higher impact from wind and waves that stirs up sediment and material in a higher extent then at point 1 (Lathrop, et al., 1999). Point 1 is more protected due to reed stands and shallow waters. Other factors that could affect are different amount of submerged vegetation that could give protection for zooplankton and therefore give a higher grazing pressure at phytoplankton biomass. If more sites were looked at during this experiment, even more differences and similarities would occur. Different sites in a lake brings naturally different conditions.

Differences and similarities for point 1 and 2.

Turbidity was always affected by total amount of resuspended sediment, phytoplankton density and velocity. Resuspended sediment was always be affected by organic content, phytoplankton density and total phosphorous. Resuspended sediment affected conductivity and transparency negatively. Phytoplankton in turn always increased turbidity, transparency, organic content and resuspended sediment. Chlorophyll levels increased organic content and had a negative correlation with detritus material. Transparency, light attenuation and calculated light attenuation was always affected by resuspended sediment, organic content, phytoplankton density and velocity. A lower transparency affected phytoplankton diversity and conductivity negatively.

Correlations found only in point 1:

Correlations between turbidity and inorganic material-, with a lower depth, wind probably has a stronger influence at stirred up sediment. Transparency, light attenuation and calculated light attenuation was correlated with temperature. Due to a lower depth, temperature should have a stronger influence at growth rate. Zooplankton density affected chlorophyll a negative. B. longirostris also increased with a higher water temperature here, so they might have been given a higher grazing pressure.

Correlations found only in point 2 (Light attenuation was also measured at point 2):

Turbidity was correlated with chlorophyll a, organic content, phytoplankton diversity, total phosphorous, transparency and conductivity. Resuspended sediment was correlated with turbidity, chlorophyll a, calculated light attenuation, inorganic content and phytoplankton diversity. Phytoplankton density was correlated with chlorophyll a. Chlorophyll a was correlated with turbidity, conductivity, transparency, total phosphorous, resuspended sediment, phytoplankton density and diversity. Transparency and calculated light attenuation was correlated chlorophyll a, total phosphorous, and wavelength, turbidity, absorbance and grazing pressure.

When analysing concentration of different seston fractions and how they contributed to the total amount of resuspended sediment, phytoplankton biomass had a bigger impact at the total amount in the beginning of the season at both sampling points. Which is interesting because literature shows that phytoplankton biomass increases during the season (Growth of Cyanobacteria for example) (O’Neila, et al., 2012). This is a result of the re-establishment of submerged vegetation in L. Tåkern during 2016. When looking at the concentrations, it is also clear that the amount of inorganic solids increases with higher wind velocity. Other trends were that amount of detritus material increased during the season. Results showed that even if phytoplankton biomass contributed little to the total amount of resuspended sediment, it did contribute in large extend to inhibit light transmission.

None of the experiments support for the idea that grazing Cladocerans were severely affected by suspended solids.

Overall it does not seem like Daphnia sp survival is negatively affected by temporary high amount of sediment, up to FNU level up till 150 and above. It seems like instead of filtering chlorophyll, they might be grazing on particles in the water volume.

Daphnia experiment conducted in June 2016 showed that number of surviving Daphnia’s increased significantly with amount of added sediment, turbidity and chlorophyll a levels. Chlorophyll a levels increased significantly with added sediment and turbidity levels. This experiment was conducted in darkness/twilight and half of the samples was shaken in lines and half in circles. Amount of organic/Inorganic material was not measured.

Daphnia experiment conducted in September 2016 showed that number of surviving Daphnia’s increased significantly with added sediment and inorganic content. There was no clear correlation between surviving Daphnia’s and chlorophyll a levels, but chlorophyll a increased with added sediment and inorganic content. This experiment was conducted in day light and shaken in circles.  


Responsible for this page: Agneta Johansson
Last updated: 05/28/17