Freshwater aquaculture, focused on Atlantic salmon (Salmo salar L.), has become a common practice in Northern Europe since the late 1960s. From a water quality perspective, nutrient addition from waste (faecal matter and uneaten feed) is a major concern, but the impact of this pollution on aquatic ecosystems, especially fresh waters has only rarely been quantified (Gou and Li, 2003; Lin and Yi, 2003).

ENSIS has conducted two projects which have used palaeolimnological techniques, principally diatom analysis, to assess the ecological response of freshwater lakes to fish farming activities. These projects have been undertaken in Scotland. One of these studies focused on a large (>1km2), deep (18 m) loch in north-west Scotland, referred to as Loch A to preserve anonymity (Sayer et al., 2012). A fish farm was established on Loch A in 1985 and continues to operate to the present day. The study sought to determine the nutrient status of Loch A before fish farm expansion so that reference conditions could be defined according to EU Water Framework Directive criteria (European Union, 2000), and to ascertain whether expansion posed a threat to water quality.

A Glew core (32 cm in length), was collected from the deepest area of the loch, some distance (~300 m) away from the fish farm in order to represent whole-loch as opposed to local conditions. The core was dated by spheroidal carbonaceous particle (SCP) analysis and 36 samples were analysed for diatoms spanning the entire length of the core. The SCP-derived chronology revealed a reasonably consistent and low sediment accumulation rate over the last 150 years of 1.16–1.67 mm yr-1. Major changes in the diatom stratigraphy were observed above 5 cm sediment depth corresponding to a date of 1984±3 yr (below). The assemblages shifted from dominance by planktonic species typical of nutrient-poor environments, i.e. Cyclotella comensis agg. and Cyclotella kuetzingiana var. planetophora, and a number of non-planktonic species notably Stauroforma exiguiformis, to dominance by Asterionella formosa and Discostella pseudostelligera, taxa associated with mesotrophic conditions. A diatom–total phosphorus (TP) transfer function was used to evaluate the eutrophication history of the loch. This showed relatively stable TP (~6 µg L-1) before 1985, with an increase to 9 µg L-1 in 1995 and a further increase to 12 µg L-1 at the surface (below). The latter value compared well with the measured annual mean TP for the loch in 2002 which was 12 µg L-1.

Fish Farm diatoms
 

The palaeoecological data provide evidence that Loch A has changed ecologically in response to recent enrichment suggesting that it is no longer in its reference state. The major shifts occurred after 1985 and were, therefore, coincident with the arrival of the fish farm suggesting that this has been the major cause of degradation. Oligotrophic lochs such as Loch A appear to be highly sensitive to eutrophication, most likely because of their extremely nutrient-poor, natural condition. Indeed, similar diatom shifts have been observed in other formerly oligotrophic Scottish lochs influenced by nutrient enrichment (Bennion et al., 2004). On the basis of our findings further expansion of fish farm operations at Loch A was not recommended.

References

Bennion H., Fluin J. & Simpson G.L. (2004) Assessing eutrophication and reference conditions for Scottish freshwater lochs using subfossil diatoms. Journal of Applied Ecology, 41, 124-138.

European Union, (2000) Establishing a framework for community action in the field of water policy. Directive of the European Parliament and of the Council 2000/60/EC. PE-CONS 3639/1/00 REV 1, Luxembourg.

Gou, l. & Li, Z. (2003) Effects of nitrogen and phosphorus from fish cage-culture on the communities of a shallow lake in middle Yangtze River basin of China. Aquaculture, 226, 201-212.

Lin, C. K. & Yi, Y. (2003) Minimizing environmental impacts of freshwater aquaculture and reuse of pond effluents and mud. Aquaculture, 226, 57-68.

Sayer, C.D., Bennion, H., Davidson, T.A., Burgess, A., Clarke, G., Hoare, D., Frings, P & Hatton-Ellis, T. (2012) The application of palaeolimnology to evidence-based lake management and conservation: examples from UK lakes. Aquatic Conservation: Marine and Freshwater Ecosystems, 22, 165-180.