Eutrophication

Case studies in UK

Case Studies Elsewhere

The marine environment has a capacity to absorb and recycle a considerable amount of natural waste products (those produced by marine animals themselves, for example) via well-developed detritivore communities. The simplest form of marine pollution arises from local or more widespread overloading of this capacity, through disposal into coastal waters or at sea of excessive amounts of inorganic nutrients (from sewage or agriculture) or organic material (from sewage or industry). The addition of excess inorganic fertilisers to cropland where the nutrients cannot be held within the soil results in seepage of nutrients into the groundwater as well as into surface run-off water. Either through ground seepage or drainage into the river systems, these excess nutrients are ultimately delivered to the coastal waters. In addition, nutrient rich sewage water is run off into the river and coastal waters, sewage sludge is dumped in coastal waters (although this will cease in the near future) and large volumes of untreated or only partially treated sewage waste enters coastal waters from the numerous coastal towns and villages deemed too small to warrant the construction of a complete treatment plant. Industrial organic waste may also enter coastal waters, either through the sewage system or from coastal water dumping of solids.

Excessive nutrient inputs may alter the balance of food chains, eliminate sensitive species, and change the composition of benthic communities, in severe cases completely altering their structure. The increase in the levels of macronutrients (particularly nitrogen and phosphorus) in European coastal waters results in the excessive growth of ephemeral species of macroalgae (commonly referred to as green tides where the effects are visible on the shore). The increased nutrient levels can also result in increased turbidity of the coastal water due to more prolific growth of phytoplankton. Both these possibilities could result in damage to kelp biotopes.

The competitive advantage given to mussels in the vicinity of sewage outfalls (filtering the plume of particulate matter) has significant ecological connotations, especially for marine benthic algae (Fletcher, 1996). For example, reports suggest that mussels are unsuitable substrata for many algae; they compete for available substratum space, have been reported to cover the surfaces of algae, and act as very efficient filters of potentially settling spores. Not surprisingly, therefore, several authors have linked increased mussel populations with declines in algal settlement and establishment, leading to the impoverishment and elimination of the macroalgal biotopes. The effects of eutrophication have been best studied in the Baltic, where there are no kelps, but their ecological role is played by Fucus vesiculosus, which grows subtidally there (see below).

Case studies UK

None known

Case studies elsewhere

Baltic Sea

During the last few decades, eutrophication of the Baltic Sea has increased dramatically. Estimates of nutrient input and measured nutrient concentrations in the open water in the 1990s are many times those in the 1950s - before large-scale nutrient input started. The increased nutrient availability in the photic zone of the open Baltic has led to increased pelagic primary production and consequent sedimentation of organic matter. Studies in locally eutrophicated coastal areas from all over the Baltic Sea region have shown the same general pattern of changes in the macroalgal vegetation:

  • decreased occurrence of perennial red and brown algae, especially of Fucus vesiculosus - which may affect the entire ecosystem as many epiphytic and free-living invertebrates are dependent for at least part of their lives on these plants for substratum, food and shelter
  • increased occurrence of fast-growing filamentous algae
  • the formation of loose-lying algal mats, the decomposition of the lower layers of which result in anoxia in the covered substratum

The lower limit of occurrence of F. vesiculosus and the depth at which the maximum biomass of the species occurred became significantly more shallow between the 1940s and 1984, but there was little additional change by 1996 (Ericksson et al., 1998). The most probable factor influencing the depth changes in the population distribution was decreased light penetration in the water column, caused by a general increase in phytoplankton production.

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