Open access publications in the SLU publication database

The report deals with nutrient load, especially phosphorous, from fish farms in dams. The aim is to give suggestions on methods and guidelines on how to determine
the reference conditions, classification of impact, and control programs. At present, guidelines are missing for how to evaluate ”ecological potential” and how
to design control programs. This report is a first step towards such guidelines.

In a case study, different models for simulating nutrient load and effects in dams are compared. The phosphorous load from the watershed of Mjölkvattnet were simulated using a source apportion model and three different calibrations
using a mass balance model of Vollenweider type. The highest tolerable nutrient load, were calculated using the four different model-setups.

Depending on choice of model and calibration, the highest acceptable fish production in the dam would range between 184 and 432 tons per year. The differences in model results can be explained by that the source apportion model is calibrated against local conditions, while the Vollenweider model is based on measurements in lakes that describes a condition for a typical lake with a set of characteristics,
rather than the lake or dam at study.

The source apportion model has the advantage that it can be used to evaluate the effect of point sources, for example fish farms, on the nutrient budget at different locations in a larger watershed. The source apportion model can also be used in lakes and dams that has not attained equilibrium after a change in nutrient loadings.

In the report the following is suggested about methods and definitions:
* Maximum ”ecological potential” for a dam is defined as the condition of a pristine lake of the same type but with the new water level fluctuation regime.

* The reference condition in a dam is defined as the condition before the establishment of the fish farm, given that the dam is free from other sources of nutrient
loading.

* Preferably, measurements made before the establishment of a fish farm should be used when deciding on a reference value for total phosphorous (Tot-Pref) in a dam. As a secondary solution the model used in the evaluation of the environmental criteria, partially based on water absorbance, can be used. As an alternative the reference value can be estimated from a source apportion model.

* The cut-off level for ”good ecological potential” in dams is defined as two times Tot-Pref of the dam. In the case of water with phosphorus levels of just a few μg l-1 it is suggested to use a cut-off level of 5 μg P l-1.

* Before and after the establishment of a fish farm, a sampling program is implemented that contains both frequent sampling and a yearly more comprehensive program containing extra parameters. The samples should be taken locally at the fish farm, in the whole dam, and at the outlet of the dam.