In the Water Law Act of 18 July 2001 (Polish Journal of Laws Dz.U.2001.115.1229), eutrophication (hypertrophication) is defined as waterenrichment with nutrients, particularly with nitrogen or phosphoruscompounds, which causes accelerated growth of algae and higher plants,compromises the balance of the aquatic environment and deteriorates thewater quality.
Inaddressing the problems of eutrophication, its causes must be clearlyseparated from its consequences. The causes of eutrophication areexcessive nutrient levels in water reservoirs and water courses. Thecauses of excessive influx of biogenic compounds can be either naturalor anthropogenic (excessive anthropogenic fertilisation is also referredto as hypertrophication). Agriculture is the main anthropogenic causeof eutrophication. Natural fertilisers, if applied in excess, carry thesame loads of biogenic compounds as artificial fertilisers. Nitrogen andphosphorus ? the key chemical elements in the process of eutrophication? are washed from fields to surface water (surface runoff andprecipitation), and the nutrient overload interferes with the naturaldistribution of macroelements.
OptimumN:P proportions for proper growth of cyanobacteria, alga(phytoplankton) and water plants is 16:1. If this balance is lost,phytoplankton will bloom or die. If the N:P weight ratio is more than 7,it indicates phosphorus deficiency, if the N:P weight ratio is lessthan 7, nitrogen deficiency is likely. It is the deficiency ofphosphorus which most commonly inhibits algal blooms. There are severalreasons: phosphorus is a biogenic compound which is naturally present atthe lowest concentrations, its percentage share in the produced organicmass is the lowest, and is the only nutrient which is not gaseous.Because of the quantitative C:N:P proportions in phytoplankton cells,the primary productivity of an ecosystem depends on the presence of eventhe smallest quantities of phosphorus. If 1 kg of phosphorus isintroduced to a water reservoir, as much as 1-2 mg of fresh algae masscan grow under favourable conditions; if 1 kg of nitrogen is introduced,the biomass will increase by around 16 kg, which, and when it decays,it will elevate the overall chemical oxygen demand (COD) burden(commonly used to indirectly measure the levels of organic compounds inwater) by over 20 kg O2.
Undernatural conditions, the growth of phytoplankton biomass in the BalticSea follows a specific pattern, conditional on the fluctuations innutrient levels.Intensive, but short-term diatom spring blooms aretypical. The diatom bloom reduces the concentrations of nutrients, whichincrease again in the autumn during water exchange and nutrient runoff.After the diatom spring bloom, other algae start and continue to bloomfrom mid-summer until autumn. In winter, in conditions of limited accessto light and low temperatures, phytoplankton vegetation is inhibited.The ecosystem regenerates itself, nutrients levels are restored(decaying microalgae from the autumn bloom; supply of nutrients releasedfrom the sea bottom by microbiological decay of dead diatom from thespring bloom). Within an annual cycle, natural water enrichment of theBaltic ecosystem comes in two cycles (natural eutrophication).
Waterbloom is when water changes its colour. According to various sources,water bloom becomes visible at cyanobacteria and algae levels of1.000-30.000/mL. During algal bloom, there can be as many as 10 millionunicellular organisms in 1 ml of water. ?20-50 mg of chlorophyll per 1 m3of water is the threshold value of water bloom. Another water bloomcriterion is the total mass of alga and cyanobacteria: 3 - 10 g/m3 in eutrophicated water, and 100-500 g/m3 in water which is critically contaminated with biogenic compounds.
The bloomingprocess has 5 distinct stages:
Algaeblooms caused by natural eutrophication are time- and space-limited andcarry beneficial ecological consequences; anthropogenic eutrophicationinvolves dynamic over-fertilisation of inland waters and sea water(which cannot be mitigated by the natural rebalancing mechanisms) andthe increase of phytoplankton and zooplankton biomass(hypertrophication).
Algalbloom is the direct consequence of eutrophication. Interference withthe natural ecosystem, reduced oxygen levels (oxygen is consumed whenorganic matter sinks to the bottom and decays, which is particularlyevident in oxygen-poor deep water, leading to large hypoxic areas ofoxygen deficits, or oxygen deserts, which cover up to 100,000 km2of the Baltic Sea deep water), loss of fauna living in deep water,reduced light penetration in deep water which limits the growth ofmacroalgae and high plants (including focus species/Fucus vesiculosus and Fucus serratus, and seagrass/Zostera marina)are the indirect consequences. Eutrophication in the Bay of Puck hasbeen evident since 1970s; eel, whitefish, pike, and garfish populationshave been declining.
Primary production of the Baltic Sea ecosystem has increased by around 30-70% in the 21stcentury, zooplankton growth increased by around 25%, zoobenthosproduction doubled, and organic carbon sedimentation increased by70-190%. What is also important, biomass increase caused byeutrophication is accompanied by general loss of biodiversity.
Asmentioned before, nutrient runoff from agricultural production is oneof the major anthropogenic factors that contribute to eutrophication(hypertrophication). In fact, as much as 50-80% of nitrogen influx comesfrom agricultural areas (land cultivation, fertilisation, slurrystorage, industrial livestock production). Although municipal waste isthe main source of phosphorus influx, agriculture is the main donor ofphosphorus in the Scandinavia, where biological waste water purificationis commonly applied. Large nutrient influx per unit of area (kg N orP/ha/year) is typical for densely populated regions with a large shareof agricultural land. Within the last 40 years, nitrogen and phosphorusinflux from households and industry has been considerably reduced overthe past 40 years; nutrient influx from agricultural sources has beenmaintained at a constant level.
Seaeutrophication is the final stage of the process of increased nutrientrunoff from a number of inland ecosystems.Surface water areparticularly exposed since the consequences of eutrophication can beproportionally more severe.
Landecosystem eutrophication is another environmental phenomenon whichdeserves more attention. Land ecosystem eutrophication can affectforests and grasslands where excess phosphorus stimulates the growth ofplants which prefer phosphorus-rich environments and compete withspecies of low tolerance to high phosphorus levels. Loss of plantspecies is accompanied by the loss of animals which used to feed onthese plants.This has a negative impact on the food chain in general,which further reduces the biocenotic diversity of eutrophicatedecosystems.