Biological effects: How is climate change affecting aquatic life?

The changing climate also affects life in and around water. This is referred to as the biological effects on ecological water quality. This page delves into the effects of climate change on common flora and fauna in urban surface water: blue-green algae, aquatic plants, fish, exotic species, and mosquitoes.

More blue-green algae and duckweed

Climate change may result in more nutrients ending up in the water, thus raising its phosphorus and nitrogen concentrations. This can lead to eutrophication. Stagnant water adds to this effect. Eutrophication fosters the growth of blue-green algae, a cyanobacterium. This bacterium is part of the phytoplankton family, as are algae and weeds. Blue-green algae have a negative impact on the use of surface water. In many cases, blue-green algae tend to grow easier than other phytoplankton species. One of the reasons is that they can form thick floating layers on the water surface. Such layers shade out the phytoplankton species underneath, thus hampering their growth. Furthermore, climate change can cause an extension of the growing season, thus facilitating the growth of phytoplankton in spring and autumn. Milder winters also affect water plants in urban water: plants that hibernate in or on the water benefit most from mild winters. For example, duckweed is increasingly prevalent in minor bodies of urban water, as the plant no longer dies off in winter.

More rampant water plants

The growing nitrogen and phosphorus concentrations in water resulting from climate change are affecting the diversity of aquatic plants. If aquatic plants just manage to beat algae in the nutrient competition, a dominant aquatic vegetation may form. Often, the water system has already grown more turbid, thus limiting light penetration. Rampant species such as waterweeds and hornwort have little or no need to root in the soil and require little light to grow. Thus, they will gradually out-compete benthic and slow growing species. This effect is enhanced as such rampant species no longer die off in winter.

Fish mortality and changes in fish species

Climate change will lead to increased fish mortality in summer, whereas winter mortality will decrease. In summer, fish mortality will increase when high temperatures and stagnant water cause anoxia. Anoxia can also occur in winter, when bodies of water are covered with ice and snow for a prolonged period of time. Particularly in shallow water, this may lead to fish mortality. However, as prolonged icy periods tend to occur less and less frequently, winter fish mortality will decrease.

Climate change will not only impact fish mortality but also fish species' composition. Cold fish species will disappear. Benthic fish such as carp and bream, which live at or near the bottom of water, will become more prevalent. In addition, higher temperatures will result in longer spawning seasons. Thus, surface water will be inhabited by small fish for a prolonged period of time. As these small fish eat zooplankton, less zooplankton will be left to eat the phytoplankton. Thus, the water will contain more phytoplankton.

More exotic species and mosquitoes

Climate change is not the main cause of the increased prevalence of exotic flora and fauna in urban waters. However, it may affect species populations. For example, in milder winters, exotic plant and wildlife species will survive in urban surface water and start to dominate other species. Such exotic species include the American crayfish, water lettuce, floating pennywort, and common water hyacinth. Currently, such species often die in winter, but in milder winters, the chances are that they will survive even better here.

Mosquito nuisance can occur at locations where drained rainwater remains stagnant for too long. This mainly occurs when small, above-ground drainage systems become clogged, e.g., with large quantities of sediment running off during torrential rain.