Eutrophication and climate change

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Although the integration is highly complex, consideration is required of the degree to which existing eutrophication problems are likely to be influenced by climate change-induced variation of marine and coastal ecological, physical and chemical systems, and, in particular, the degree to which climate change will exacerbate future eutrophication problems and possibly increase the proliferation of harmful algal bloom. In marine and coastal waters, eutrophication occurs as a result of the additions of chemical nutrients, principally nitrogen and phosphates, which can promote excessive amounts of algal growth and may result in undesirable ecological balance and degraded water quality.

Warm early summers may favour spring spawning zooplanktivourous fish at the expense of autumn spawning species in oligotrophic lakes. This may lead to altered nutrient status and nutrient and phosphate rations. Changes in climate patterns and related runoff regimes can significantly influence nutrient losses from catchments. Future levels will be sensitive to: changes in the timing of seasonal and annual events (spring runoff, autumn low flow, ice and snow cover), and the frequency and severity of extreme events (floods, droughts, erosion).

Impacts of increased water temperatures may increase productivity, leading to higher phytoplankton biomass and the occurrence of phytoplankton blooms. Warm early summers may favour spring-spawning zooplanktivorous fish at the expense of autumn spawning species in oligotrophic lakes. This may lead to altered nutrient status and nutrient and phosphate ratios

Warm and dry years are associated with reduced influxes of nutrients (particularly nitrate) in runoff, but may also lower minimum water levels, increasing re-suspension (particularly phosphate) from bottom sediments in shallow lakes. Changing nutrient and phosphate ratios which may lead to phytoplankton dominance shifts, with lower relative nutrient availability.

Climate scenarios point to moderate increases in mean annual river flow to the Black and Baltic by the late 21st century. Reduced influence of snow melt will increase the synchronisation between precipitation and stream discharge, reducing and extending summertime base flow but increasing wintertime runoff.

Coastal areas may become more vulnerable to rainstorms and flood events, with severe effects on agricultural activities in former floodplain areas, as well as nutrient losses. Responses of nutrient losses to climate change will be faster and more direct in small agricultural catchments; internal material cycles will lead to more complicated dynamics in large catchments.

There are concerns as to whether land use changes will result in large eutrophication effects, such as the growth of bio-fuels and bio-energy crops. To what degree will hotter, dryer summers in regions that are already eutrophic, like the Baltic or Black Seas, result in changes to coastal water quality? What are the likely impacts of increased stratification on inshore waters? What options are available to mitigate increased eutrophication?

Some of these questions could be answered by examining historical time-series data. Others by using coupled climate-catchment-ecosystem models, or perhaps even using experimental mesocosms. It is essential that combinations of methods be used. It can also be difficult to separate the symptoms of eutrophication from climate change impacts. For example; while nutrient enrichment is a first-order cause of eutrophication; weather, wind, storminess, local physiographic conditions and cloud variation also determine the extent to which nutrient enrichment is likely to become problematic.