Clarifying the relationship between salmon farm nutrient loads and changes in macroalgal community structure/ distribution

This project was developed in response to questions raised by the salmon industry about the potential for adverse impacts on macroalgae as a result of the additional nutrient inputs from farming operations in marine systems. Two student projects were identified with the intention of further exploring the relationship between salmon farm nutrient loads and changes in macroalgal community structure and distribution. The research took two different but complementary directions, which comprised the body of research for two PhD students. The first project was primarily concerned with how additional nutrient inputs added directly to a subtidal reef system might affect macroalgal ecology and performance. The second project was a desk based modelling study focused on determining whether growing macroalgae in conjunction with salmon farming is a viable mechanism for reducing nutrient loads within a stylized waterbody, what species might respond most effectively to salmon farm derived nutrients, whether there could be additional benefits associated with such macroalgal culture (such as a secondary commercial crop or further environmental benefits associated with the growth of threatened/ ecologically valued species) and where the optimum location would be for growing such species. The outcomes of this research are described in more detail in the following report, both theses and any papers published or in press at the time this report was submitted have been attached as appendices. The key findings for each component are summarized below.

PhD 1 - Determining the effects of nutrient enrichment on rocky reefs communities (observational, experimental and predictive capabilities).

Nutrient sources were added to three reef systems to assess the effects of increased nutrient availability on macroalgal community composition. There was no effect on the abundance of canopy forming algae. The effect on the abundance of opportunistic species (e.g. filamentous algae) was not clear. Although opportunistic algae proliferated at one site where nutrients were elevated, this was not the case at the other reef locations. There was little evidence of change in macroalgal abundance, and reef function appeared fundamentally unchanged. Physiological differences were observed in several canopy-forming species that appeared to be in response to nutrient addition, suggesting that this may be a more sensitive early indicator of change than full evaluation of community structure. The physiological response data appears to suggest that the reef system in the upper part of the Channel may be more impacted by nutrients than the other reef systems in the study. Overall, the three reefs assessed in this study responded differently to equivalent nutrient additions (impacts), suggesting that broad-scale susceptibility/ resilience to nutrient inputs is contingent upon the prevailing environmental conditions and that future assessments may need to consider this.

PhD 2 – Farming macroalgae to mitigate coastal nutrification from finfish aquaculture: a modelling study

Modelling was used to assess the feasibility of growing algae in and around salmon farms in the D’Entrecasteaux Channel/Huon Estuary for integrated multi-trophic aquaculture (IMTA). The results indicate significant potential for algal growth in conjunction with salmon farming for the purposes of IMTA. However, the study also found that the location of algae culture within the system would differ if IMTA is for profit or for nutrient mitigation purposes.