Highly probable - Kelp beds are within 40 m of light up to lower story younger individuals the salmon farm, and therefore close enough thus promoting growth of younger kelp in the to receive some nutrients. Fish biomass is lower story. Uncertainty is high, as population sufficient to produce sufficient nutrients bi- decline due to epiphytic algae has not been yearly in the summer when background levels actually studied. are low. The intensity and geographic extent is medium because it is likely that only the 5.2.0 Blade erosion increases due to epiphytic algae section of the kelp bed nearest the farm increases. would receive nutrients. Increases in epiphytic Highly probable - This has been docu- algae occurs when background nutrient levels mented in previous research investigations. increase by 80% for extended periods of time Researchers have only found an increase in during periods of high light, and this is 4.5 µM epiphyte abundance and host erosion due to at this site (summertime). For one quarter of an increase in dissolved nutrients (not total the production cycle of this value is exceeded lamina coverage). Intensity and geographic when fish biomass peaks (approximately 4 extent are low, because the entire lamina is months). Uncertainty is low, because this has not covered with epiphytic algae and the entire already been shown to occur on seaweeds in bed is not affected. The cycle of high nutri- proximity to the salmon farms. ent pulses from fish farms repeats bi-yearly. Uncertainity is low as this has been docu- 5.1.1 Epiphytic algae promote light limitation. mented in previous investigations. Highly probable - Once laminae are covered with organisms, photosynthesis is impaired. 5.2.1 Sori production changes due to blade erosion Intensity and geographic extent are low, Highly probable - At the time of the nutrient because the entire lamina is not covered with pulse from the salmon farm, L. saccharina is epiphytic algae and the entire bed is not affect- producing sori. Intensity of impact is medium. ed. Researchers have only found an increase Researchers have only found an increase in in epiphyte abundance and a decrease in pho- epiphyte abundance and host erosion due to tosynthesis in the host with nutrient pulses. In an increase in dissolved nutrients (not total addition, the high nutrient pulse occurs only lamina coverage). If the epiphytic algae grow four months every second year. Uncertainty over sori tissue or areas where sori would have is low, because scientific evidence supports a been produced, and this tissue is consumed, decrease in photosynthesis with shading. then, sori production would be restricted. Impact is not high, as the kelp can live three 5.1.2 Lamina production changes due to light limita- to four years and the pulse occurs twice in four tion caused by epiphytic algae. years. Uncertainity is high, because no one Medium probability - Late spring and early has correlated epiphytic algae with host sori summer is the rapid growth period for L. sac- production. charina. When the nutrient pulses are highest at summer end, it is, therefore, likely laminae 5.2.3 Kelp productivity changes as a result of have already grown to their maximal extent. changes in sori production. Therefore, a reduction in photosysthesis would Highly probable - Intensity of impact is low not greatly affect kelp production. Intensity and because most likely only the section of the geographic extent are low, because the entire kelp bed nearest the salmon farm would be lamina is not covered with epiphytic algae and impacted by dissolved nutrients. The nutrients the entire kelp bed is not affected. The cycle of would be taken up by the kelp and epiphytes high nutrient pulses from fish farms repeats bi- at the front of the bed leaving few nutrients yearly. Uncertainity is medium, as researchers for the kelp in the reminder of the bed. As have examined both nutrient pulses, epiphytic well, the high nutrient load during adequate algae and seaweed growth, but not at the time lighting occurs only four months every second of maximal fish farming dissolved nutrient year, whereas kelp live three to four years. releases. Gametophytes have a wide dispersal range (possibly greater than 200 m), so if the affected 5.1.3 Kelp productivity changes as a result of kelp did not produce sori, it is likely that new changes in lamina production from epiphytic recruits would be obtained from the unaffected algae. kelp outside of the range of the fish farm. This Low probability - Intensity and geographic would be the case in a healthy population. In extent are low, because the entire lamina a population affected by sea urchin limited sori is not covered with epiphytic algae and the production could set back attempts to main- entire bed is not affected. Researchers have tain or restore kelp populations. Uncertainty is only found an increase in epiphyte abundance high, as the effect of epiphytes and dissolved and a decrease in photosynthesis in the host nutrients on kelp population decline due to with nutrient pulses. In addition, the high limited sori production has not been examined nutrient pulse occurs only four months every in the field. second year. Furthermore, a decrease in lamina biomass in the upper story would open GESAMP Reports and Studies No 76 ASSESSMENT AND COMMUNICATION OF ENVIRONMENTAL RISKS IN COASTAL AQUACULTURE 151 5.3.0 Dissolved nutrients affect C and N storage because of the adequate available back- Medium probability - Under normal circum- ground nutrients from spring upwelling condi- stances in the region, L. saccharina grows tions, and optimal temperature and lighting during the high light period until nitrogen conditions. Due to the optimal environmental stores are depleted. At the same time, carbon conditions, in this area, severity of this effect is stored so that dissolved nitrate can be stored on kelp would be marginal. Uncertainty is low during the dark period that occurs the follow- because the knowledge level concerning kelp ing winter and early spring. Biyearly on the and background nutrients is high. salmon farm, the kelp receives farm derived nutrients. Then it can grow without the need 5.4.1 Increase in lamina growth causes light limita- to consume its nitrogen stores. It continues to tion. grow until the end of its growth period when High probability - Previous research investi- sori are produced. Some evidence exists that gations have clearly shown that increases in indicates that carbon is not stored when kelp lamina production can cause self shading of is fertilized in this way. Intensity is expected to upper story individuals and shading of lower be medium because the ammonium from the and younger story individuals. Intensity is fish farm would be taken up by the kelp at the expected to be low, because the size of the front of the bed to leave few nutrients for the kelp in the bed in this area is already con- rest of the bed. This pattern will occur as long sidered to be the largest in Norway due to as the fish farm is in the area. Uncertainty is adequate nutrients, lighting and optimal tem- high, as little field work as been done in this perature. Uncertainty is low due to the level of area. understanding and available information. 5.3.1 Lamina production changes due to changes in 5.4.2 Lamina production changes due to light limita- C and N storage. tion. Low probability - Carbon that is stored in Highly probable - It is well known that Laminaria in the summer is used for nutri- light limitation negatively affects kelp growth ent uptake in the spring and for growth under unless carbon stores are adequate. Intensity low light conditions. If carbon is not stored, is expected to be low, because the size of the the production of the new laminae may be kelp in the bed is already considered to be the decreased. Intensity is expected to be medium largest in Norway due to adequate nutrients, because the ammonium from the fish farm lighting and optimal temperature. Uncertainty would be taken up by the kelp at the front of is low due to the high level of understanding in the bed to leave few nutrients for the rest of this area. the bed. Therefore, most of the bed would not be affected by the nutrients. This pattern 5.4.3 Kelp productivity changes as a result of will occur bi-yearly as long as the fish farm changes in lamina production. is in the area. Uncertainty is high, because Low probability - In the area of interest, kelp researchers have not investigated the effect are already the largest in Norway. Lamina pro- of low C stores on regeneration in perennial duction is expected to be optimal in spite of kelp. fish farming nutrients because of the adequate available background nutrients from spring 5.3.2 Kelp productivity changes as a result of upwelling conditions, and optimal tempera- changes in lamina production. ture and lighting conditions. Hence, dissolved Low probability - Little information exists nutrients from the fish farm would not greatly relating to the fitness of Laminaria without suit- affect the kelp. Uncertainty is low because the able carbon stores at the time of nitrate avail- knowledge level concerning kelp and back- ability due to upwelling. Intensity is expected ground nutrients is high. to be medium because the ammonium from the fish farm would be taken up by the kelp at 5.5.0 Sori production changes as a result of changes the front of the bed leaving few nutrients for in lamina production the kelp in the rest of the bed. In addition, the Highly probable - Sori are produced on effect is expected only once every two years, lamina, so it is logical to assume that sori but it will endure as long as the fish farm is production will change if lamina production present.