Jorma Keskitalo

Recovery of the fish community and changes in the lower trophic levels in a eutrophic lake after a winter kill of fish

A severe oxygen deficit induced a fish kill in the eutrophicated two-basin Lake Äimäjärvi in southern Finland during winter 2002–2003, resulting in cascading effects on the lower trophic levels of the lake. Pikeperch disappeared from the lake and bleak and white bream decreased to low numbers. The recovery of the populations of other species started immediately when strong year-classes of roach and perch appeared in summer 2003 and onwards. A sharp increase in the growth of perch and roach was recorded, and perch became the dominant fish species during 2004–2006. Consequent responses after the fish kill included increased Secchi depth, expansion of submerged macrophytes, decreased nutrient concentrations and reduction of Cyanophyta from the more eutrophic northern basin of the lake, and a temporary increase in the size of Daphnia in the early summer 2003. However, the ecosystem of the lake returned gradually to the earlier structure and level of eutrophication. Bluegreen algae have returned since 2005, the water has become more turbid, macrophytes declined in 2007 and the fish community was again dominated by small cyprinid fishes in 2008.

Effects of Biomanipulation on Fish and Plankton Communities in Ten Eutrophic Lakes of Southern Finland – Hydrobiologia (2006) 553: 67–88

The effects of biomanipulation were studied in ten Finnish lakes to determine responses in fish and plankton communities and water quality after mass removal of cyprinids. From 1997 to 2001, the fish communities shifted from the dominance of large cyprinids to an explosion of small cyprinids and a higher proportion of piscivores in effectively biomanipulated lakes (>200 kg ha 3 yr). The biomass of cyanobacteria decreased, and the duration of the blooms shortened and shifted towards the autumn. Decreased concentrations and slower cycling of nutrients and increased grazing by cladocerans probably affected the declined biomass of cyanobacteria. Less intensive sediment disturbance and increased phosphorus-retention in fast growing fish biomass may have turned the role of the fish assemblage from ‘nutrient recycler’ to ‘nutrient storage’. Increased potential grazing pressure, higher proportion of edible algae, and lower chlorophyll a:total phosphorus ratio indicated strengthened herbivore control. A high mass removal catch in relation to trophic state, low background turbidity, and bearable external loading favoured the successful biomanipulation, whereas intensive cyprinid reproduction, high nutrient loading and non-algal turbidity hindered the recovery. Three important issues should be noticed before biomanipulation in Finland: (1) careful selection of target lake, (2) well-planned, effective and long-lasting biomanipulation and (3) sustainable management of piscivores.

Effects of mass removal of fish on the nutrient status of a eutrophic lake in southern Finland

The nutrient status in a lake i s dependent not only on the externalloading, but also on the ecosystem stmcture (CARPENTER et al. 1992, SCHEFFER 1998, JEPPESEN et al. 1998, MEIJER et al. 1999). At a constant level of nutrient 1oad, function of the different trophic levels of phytoplankton, macrophytes, zooplankton and fish can directly or indirectly change the amount, ba1ance and circulation of nutrients (ANDERSSON et al. 1988, SCHEFFER 1998). Biomanipulation by removing cyprinids can change the nutrient status o f a lake in several ways. The total amount o f nutrients in the water pool decreases as the nutrients bound in the fish biomass are removed and as the fish-induced nutrient excretion and the release from the sediment declines (T Á TRAI & [STVÁNOVITS 1986, BRABRAND et al. 1990, HORPPILA & KAIRESALO 1990). !f the cyprinid migration i s lessened between habitats due to reduced food competition and increased predation pressure, the circulation o f nutrients decreases (BRABRAND & FAAFENG 1993, HORPPILA et al. 1998). Ifthe average individual growth rate increases due to younger age structure and released resources, fish bind nutrients into the fast-growing biomass (CARPENTER et al. 1992, KRAFT 1992). The cyprinid reduction indirectly changes the nutrient status by increasing the proportion of large cladocerans, which retain more phosphorus (P) than nitrogen (N) compared to small cladocerans (CARPENTER et al. 1992). Higher grazing rate and reduced sediment disturbation may increase the Secchi depth, enab1ing the invasion o f vegetation, which especially reduces N-concentration (SCHEFFER 1998). With decreasing phytoplankton biomass, sedimentation declines, which reduces P-flux from the bottom sediment (SCHEFFER 1998). Lake Ãimãjãrvi was biomanipulated during 1997-2001, and the measures and the biological responses are presented in more detail in RAsK et al (2003). The decrease o f phytoplankton biomass, the increase of crustacean zoop1ankton, and the shift o f blue-green algal blooms toward autumn were the most obvious responses. In the present study, we calculated the amounts of nutrients (P and N) removed from the lake through the mass removal fish catches, related this nutrient removal to the nutrient balance of the 1ake, and examined the responses in water quality. We hypothesised that biomanipulation would decrease nutrient concentrations through the fish removal and increase Secchi depth due to increased grazing by crustacean zooplankton. The changed nutrient cycling or increased grazing should reduce chlorophyll a : total P ratio (MAZUMDER 1994, MEIJER et al. 1999, SARVALA et al. 2000). Eventually, the nutrient retention should increase (J EPPESEN et al. 1998).