Förväntade effekter på bentisk primärproduktion och fiskbiomassa av en tröskeldamm i reglingsmagasinet Grundsjön

Scheduled water level changes in lakes regulated for hydropower production often exposes large parts of the lake bottom to air. Lake sediments are then left to dry out, having negative consequences for lake food webs. In Grundsjön (Härjdalen, Sweden), Fortum are now considering building a threshold dam to prevent an area in the lake from drying up, with the aim of reducing the environmental impact of hydropower on the lake ecosystem. A more stable environment, without drying, enables sediment to accumulate over time and bottom-dwelling (benthic) algae, insect larvae and crustaceans to establish. Increased production of benthic organisms should have positive effects on the food supply for fish. In this report, we give a brief summary of the state of knowledge regarding fish and primary production in clear water lakes. Based on existing knowledge, we also estimate how fish and specifically benthic primary production can be affected by the construction of two different threshold dams in Grundsjön. Our estimates suggest there could be ~20 times as much benthic primary production in one dam and about 5 times as much in the other dam, compared to the current situation (no threshold dams). We expect the same increase in fish biomass since our calculations show a proportional increase in fish biomass following an increase in benthic primary production