To quantify how fish farming modifies short-term benthic carbon cycling in fjord environments and the role of hydrodynamics in modifying effects on the benthos, we assessed benthic ecosystem structure and respiration and used isotope labeled algae as a tracer to quantify C flow over 48 h through macrofauna and bacteria in sediments collected from beneath fish farm sites in (1) high water-flow areas, (2) low water-flow areas, and (3) two appropriate control sites situated downstream from the farms. Bacterial biomass was significantly greater in sediments collected from the fish farm sites relative to the controls. This was also true for sediment oxygen consumption (SOC) rates averaged over each 48 h investigation, which were significantly correlated with total benthic (macrofauna and bacteria) biomass. Short-term C-uptake rates by macrofauna were elevated in both fish farm treatments compared with bacterial C uptake and were significantly higher in sediments from the low flow fish farm site relative to both controls. While SOC rates were significantly higher in experiments using sediments from the low flow fish farm site; faunal abundance, biomass uptake, C uptake, bacterial biomass, and metabolism in sediments from both fish farm treatments were not significantly different from one another. Fish farming can dramatically alter benthic ecosystem functioning, and significant effects can occur around fish farms irrespective of the water-flow regime the farms are moored in.

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Sugar kelp Saccharina latissima is one of the dominating kelp species on sheltered sublittoral sites along the Norwegian coastline, providing important habitats and ecosystem services. In finfish aquaculture in Norway, infections caused by salmon lice Lepeophtheirus salmonis are one of the biggest challenges the industry is currently facing, and presently the most-used therapeutant is hydrogen peroxide (H 2 O 2). Commonly, this treatment involves bathing the fish in a solution of approximately 1700 mg H 2 O 2 l −1 seawater before releasing the solution into the surrounding waters. The present study was conducted to increase the knowledge on how these H 2 O 2 emissions from farm operations could impact juvenile S. latissima. This was achieved by determining the effects of a 1 h exposure to realistic H 2 O 2 levels on S. latissima mortality and photosynthesis. Effects on photosynthesis were determined by incubating plants at 3 time intervals postexposure. Toxicity potentials including lethal concentration for 50% of the population (LC 50) and effective concentration (EC 50) for photosynthetic capacity (P MAX) and efficiency (α) were determined based on these data. Juvenile S. latissima was highly sensitive, having an LC 50 of 80.7 mg H 2 O 2 l −1 , which is less than 5% of the dose commonly used at farms and emitted to the environment. A concentration of 85 mg l −1 caused an immediate 90% reduction in both P MAX and α. The EC 50 was found to be 27.8 and 35.4 mg l −1 for P MAX and α, respectively. This may indicate that natural S. latissima populations in the vicinity of fish farms can be negatively affected by H 2 O 2 bath treatments.

Large-scale finfish farms are increasingly located in dispersive hard-bottom environments where Laminaria hyperborea forests dominate; however, the interactions between farm effluents and kelp forests are poorly understood. Effects of 2 levels of salmonid fish-farming effluents (high and low) on L. hyperborea epiphytic communities were studied by sampling canopy plants from 12 sites in 2 high-energy dispersive environments. Specifically, we assessed if farm effluents stimulated fast-growing epiphytic algae and faunal species on L. hyperborea stipes—as this can impact the kelp forest community composition—and/or an increased lamina epiphytic growth, which could negatively impact the kelp itself. We found that bryozoan biomass on the stipes was significantly higher at high-effluent farm sites compared to low-effluent farm and reference sites, resulting in a significantly different epiphytic community. Macroalgal biomass also increased with increasing effluent levels, including opportunistic Ectocarpus spp., resulting in a less heterogeneous macroalgae community at high-effluent farm sites. This habitat heterogeneity was further reduced by the high bryozoan biomass at the high-effluent sites. Such changes in the epiphyte community could have implications for the faunal community that relies on the epiphytes for food and refuge. On the kelp lamina, no clear response to farm effluents was found.