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<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<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.

Algal biofuels present a promising future alternative to petroleum based fuels. Water nutrient recycling is a key step to increase the sustainability of algae biofuel production facilities. This thesis discusses the process of nutrient and water recycling in high rate algae raceway ponds (HRAP) fed primary treated municipal wastewater. Research was conducted primarily at the San Luis Obispo Water Resource Reclamation Facility (SLOWRRF). Nine 30 m2, 0.3 m deep HRAP’s were operated continuously from June 1, 2013 to April 17, 2014. The ponds were arranged in three sets of triplicate ponds, with two pond sets run on 3-day hydraulic residence time (HRT), and the third on a 2-day HRT. The biomass productivity of the 2-day HRT and 3-day HRT were compared. The two sets of 3-day HRT ponds were run in series to determine the effect on productivity associated with recycling growth media without supplemental nutrient addition. The first pond in series was referred to as round 1 and the second as round 2. Due to solids accumulation in the 2-day HRT ponds in summer proper biomass productivity values could not be determined. 4-inch standpipes were determined to cause the solids accumulation when large flocs were present in ponds. As a possible solution to the solids accumulation issue, a ramped standpipe was designed and installed in one pond per triplicate set. In winter the 2-day HRT pond was roughly 37% more productive than the 3-day HRT. In summer the round 1 (3-day HRT) ponds were roughly 33% more productive than the round 2 (3-day HRT) ponds. In winter the round 1 (3-day HRT) ponds were roughly 19% more productive than the round 2 (3-day HRT) ponds. The type of standpipe (ramped or 4-inch) did not cause a significant amount of solids accumulation in either of the 3-day HRT ponds. The type of standpipe did make a difference in the 2-day HRT ponds. On average the 4-inch standpipe pond had 35% higher TSS than the ramped standpipe ponds. In addition to these field experiments, laboratory aerobic degradation experiments were conducted to determine the nutrient release of previously digested sludge in aerobic ponds. Pretreatment of algae sludge did not have a significant effect on nutrient release of previously anaerobically digested algae sludge in aerobic conditions. The maximum soluble nitrogen generated in the aeration reactors was between 56% for the treated sludge, and 66% for the untreated sludge.

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