A global increase in atmospheric CO2 and temperature is assumed to affect the marine ecosystems in numerous ways, e.g. by altering ocean circulation patterns and changing nutrient regimes. The changes are expected to impact heavily on both phytoplankton communities as well as the rest of the marine food-web. Based on previous experimental studies that have investigated the impacts of varied algae food-quality on zooplankton, this quantitative study hypothesizes that (i) the tested microzooplankton species Brachionus plicatilis (rotifer) and Euplotes sp. (ciliate) will show high population growth rates (g) when fed with Nannochloropsis sp. grown under nutrient replete conditions, (ii) that the species will show a population growth rate close to zero when fed with algae grown on phosphorous-deficient media and (iii) that microzooplankton will be negatively affected by the algae grown in nitrogen-deficient media. The study thus aims to investigate how changes in the balance of energy and several chemical elements in ecological interactions, ecological stoichiometry, affect the growth rates of algal grazers. The results show that food-independent factors had a large impact on growth rates and resulted in unexpected, deviating trends. However, as the growth rates for B. plicatilis fed with phosphorous-deficient algae were lower than those of B. plicatilis fed with nitrogen-deficient algae, there is some support for the / <p>The paper was written within the research-area of marine biology.</p>

The linear relationship between a conservative element and salinity during mixing of water masses is widely used to study biogeochemistry in estuaries and the oceans. Even though nutrient ratios are widely used to determine the limiting nutrient in aquatic environments, the rules of nutrient ratios change through the mixing of freshwater and seawater are still unstudied. This study provides general rules for nutrient ratios change via mixing. A simple mixing model is developed with the aims to illustrate that nutrient ratio is a nonlinear function of salinity, thus, shift in limiting nutrient over the salinity gradient can be simply a result of river water and seawater mixing, albeit complicated by biological consumption or remineralization. This model explains a natural phenomenon that rivers contain relatively high dissolved inorganic nitrogen (DIN) to soluble reactive phosphorus (SRP) ratios start to decrease the ratios as salinity increases when seawater contains higher SRP:DIN ratios. Although additional sources of P have been implicated as the cause for such change, this change can be a result of riverine water and seawater mixing. Four mixing rules are presented here to explain the factors governing the change in nutrient ratios vs. salinity; thus, answering why in some cases variations in nutrient loading and in other cases mixing triggers changes to seasonal limitation status in some estuaries. Shift in nutrient ratios can be explained by the change in nutrient inventories via mixing. After the P-limited riverine water shifts in N limitation by mixing with N-limited seawater, new production of the estuary in general becomes limited by the amount of N inputs from the riverine water and the seawater. The result may help to explain a current consensus that N and not P riverine loadings lead to eutrophication in estuaries which are influenced by P-limited riverine waters. Further, new production which is generated by N-limited riverine input and N-limited seawater input mainly depends on the amount of N inputs from the riverine water and the seawater.

Gaining knowledge about what factors determine benthic algal biomass and productivity is vital for understanding food webs in aquatic systems, especially in woodland streams with naturally low rates of primary productivity. The aim of this study was to investigate what factors determine algal growth in Swedish headwater streams. Nutrients, in terms of nitrogen (N) and phosphorus (P), and light are factors known to affect algal growth. By using nutrient diffusing substrate (NDS) and different gradients of light, it was possible to test the importance of these factors. To determine the effect of the experiment, the study was carried out in a forested reference stream, which is largely shaded with extreme low nutrient levels, and in a stream running through a clear-cutting plantation with high nutrient levels and incident light. In the forested reference stream it became clear that algal growth increased by experimentally adding N (P&lt;0.005), although light did not affect the productivity. In the stream running through the clear-cut, algal productivity increased significantly with higher levels of light (P&lt;0.005), regardless of nutrient addition. The results from this study suggest that light only becomes the depending factor when sufficient amounts of nutrients are available. However, it is still unclear at what nutrient levels this shift occur, and therefore future research is recommended.

In the work described in this thesis I studied how increases in nitrogen (N) inputs may affect plant community structure in boreal forest understorey vegetation. These phenomena were investigated in N fertilization experiments and along a national N deposition gradient. After five years of N additions, large changes in understorey vegetation composition were observed in the fertilization study. In plots that received 50 kg N ha'1 year"1 (N2), the abundance of the dominant species, Vaccinium myrtillus, decreased on average by 32 %. No decrease was observed in control plots during the same period. In contrast, the grass Deschampsia flexuosa responded positively to increased N input, being on average more than five times as abundant in the N2 treatments as in controls. Also an increase was seen in the incidence of disease caused by the parasitic fungus Valdensia heterodoxa on leaves of V. myrtillus following N additions. The parasite was on average nearly twice as abundant in N2 plots than in control plots. This could be explained by increased N concentrations in host plant tissue. Disease incidence also increased following experimental additions of glutamine to leaf surfaces of V. myrtillus, suggesting a causal connection between plant N concentration and performance of the fungus. The parasite also played a key role in the observed changes in understorey species composition. D. flexuosa was more abundant in patches in which V. myrtillus was severely affected by V heterodoxa. This suggests that V heterodoxa mediates the increased abundance of D. flexuosa following increased N additions. The fungus mediates changes in the composition of understorey vegetation mainly by increasing light availability via premature leaf loss of V. myrtillus. The incidence of disease due to the parasite was on average higher in large than in smaller N-treated plots, indicating that the response to N fertilization is spatially scale dependent. This shows that using small plot sizes in experiments that simulate changed environmental conditions may be problematic, as important interactions may be underestimated. Comparison of the occurrence of understorey species between regions with different rates of N deposition revealed that the occurrence of the two dwarf shrubs V. myrtillus and V. vitis- idaea was lower in regions with high N deposition compared to regions with low deposition. The opposite pattern was found for V heterodoxa. This is consistent with expectations from N fertilization experiments. For D. flexuosa no differences in occurrence were found between the different regions investigated. The effects on vegetation and mycorrhizal fungi observed following N additions were also found to be long lasting. Nine years after termination of the fertilization, no signs of recovery were detected, and nearly 50 years after termination characteristic signs of N fertilization were found among bryophytes and mycorrhizal fungi. This suggests that the time needed for re-establishment of the original biota following N-induced changes may be substantial. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 2002, härtill 5 uppsatser</p> / digitalisering@umu