Lake Constance has undergone significant changes over the past century, which have profoundly affected its ecosystem, including the water flea (Daphnia) community. These tiny crustaceans are important aquatic grazers controlling algal growth and a crucial food source for many fish species in the lake, including the iconic Lake Constance whitefish (Coregonus sp.). Understanding how Daphnia communities change and evolve helps us to predict their future development and the resulting effects on the lake ecosystem.

In the middle of the last century, during the eutrophication phase (i.e., increased nutrient levels) of the lake, the native Daphnia species, D. longispina, was largely replaced by the invasive D. galeata. However, after efforts to restore the lake’s nutrient balance (re-oligotrophication), D. longispina became the dominant species once again. More recently, the lake has faced new challenges, including the invasion of quagga mussels, which compete directly with Daphnia for food (phytoplankton), and the spread of the three-spined stickleback into the pelagic zone, which preys on larger Daphnia species. Additionally, rising temperatures due to climate change are favoring species with other temperature preferences and shift abundance peaks, timing of reproduction etc. These factors could have contributed to the invasion and dominance of another species, Daphnia cucullata, in recent years. Because of their smaller size and possibly because of stronger defenses (pointy helmets and long spina), D. cucullata is harder for whitefish larvae to prey on, which may have played a role in the decline of whitefish populations. Finally, the three Daphnia species present in the lake are known to hybridize, which could influence adaptive processes such as defense mechanisms against fish predation, thereby exerting a further influence on the rest of the food web.