Interactions between Phytoplankton, Zooplankton, and Fish in the Nutrient Rich Shallow Lake Hjarbæk Fjord, Denmark

Abstract

Abstract In the shallow, nutrient‐rich dammed‐up lake Hjarbæk Fjord the balance between phytoplankton, zooplankton, and fish was completely disturbed during the summer period. Extensive growth of small blue‐green algae ( Oscillatoria cfr. trichoides, Pseudanabaena galeata ) and chlorococcal green algae ( Scenedesmus spp., Monoraphidium contortum, Actinastrum hantzschii ) induced a pH rise to 10.5 in July. Fish were driven into the rivers, and many that did not escape died. After the fish had disappeared daphniae multiplied explosively and grazed away the major part of the phytoplankton. Ammonia concentrations rose and oxygen concentrations dropped to noxious levels for highly sensitive fish in August–September. Larger phytoplankton species ( Aphanizomenon flos‐aquae, Pediastrum boryanum, Pandorina morum , a. o.) were not grazed by the daphniae and gave rise to a maximum of copepods in September. Fish returned to the lake in October, when pH, ammonia, and oxygen levels had normalized.