1. Definition of Eutrophication Developed by Richard Sandford with contributions from Martin Bloxham and Paul Worsfold, Eutrophication in the Sea of Azov. Source: SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAG

2. 1.1 What is eutrophication? Eutrophication as a phenomenon is described in different ways by limnologists and marine scientists. Most limnologists consider eutrophication as an increase in the rate of supply of organic matter to an ecosystem. For marine scientists, eutrophication (GESAMP, 1990) is “used simply to mean ‘enhanced nourishment’ and refers to the stimulation of aquatic plant growth by mineral nutrients, particularly the combined forms of phosphorus or nitrogen”. 1.2 Why is there a difference in approach? Though widely associated with pollution, eutrophication is also a natural process in lakes as they gradually accumulate carbon, nitrogen and phosphorus. Lake ecosystems can recycle some of the nitrogen and phosphorus compounds deposited in the lake bed and this promotes more production of organic material. The organic material builds up on the lakebed and contributes to the lake’s own demise. Bogs are shallow lakes that have filled with detritus (often peat) in this manner. The process may take hundreds or thousands of years and lead to new land formation when the bogs themselves are colonised by trees. Eutrophication of a lake may therefore be considered as an increase in organic matter in the system. This may be a result of direct river or sewage discharges but is most commonly associated with the increase in plant production caused by an increased supply of nitrogen and phosphorus compounds, essential nutrients for plant growth. The problem is that human intervention is accelerating this natural process, leading ultimately to the accelerated decline of lakes. For more information on the classification of lakes, please click here. here In the marine environment, eutrophication has only been recognised in the past three decades. It was first identified in coastal lagoons of the US eastern seaboard through the presence of unusually intense blooms of phytoplankton. Since eutrophication has been documented in coastal seas throughout the world – this is not to say that all coastal seas are eutrophied! In all cases, eutrophication is seen as a non-natural phenomenon associated with an increase of essential nutrients, nitrogen and phosphorus compounds, utilised for phytoplankton growth. Reference: GESAMP (1990) (IMO/FAO/UNESCO-IOC/WMO/WHO/IAEA/UN/UNEP Joint Group of Experts on the Scientific Aspects of Marine Pollution) The State of the Marine Environment. Rep. Stud. GESAMP No. 39, 111pp, London. Definition of Eutrophication

3. 1.2.1 Oligotrophic Lakes Oligotrophic lakes are generally deep with steep sides and relatively small drainage areas, although some oligotrophic lakes are no more than shallow granite pans. Nutrient levels are low. Water is clear blue with high light penetration due to the low number of planktonic or attached algae that can be supported by the low nutrient levels. Water transparency (measured by Secchi disk) is typically over 8 m. Hypolimnetic and benthic 0 2 levels do not vary much from saturation throughout the year (10 + 10 %). Biomass at all trophic levels is low. Question: What is Secchi disk? Definition of Eutrophication

4. 1.2.2 Eutrophic Lakes Eutrophic lakes are often shallow, usually less than 10 m deep with gradually sloping edges and a large drainage. Nutrient levels are high. Primary productivity is also high with an abundance of planktonic or attached algae. Surface blooms of blue-green algae are common. Large variation in 0 2 levels. Depression in the hypolimnion (0-100 %) and supersaturation in the epilimnion (100-200%). Low water clarity with light penetration often not reaching the thermocline or lake bed. Water transparency (measured by Secchi disk) is typically less than 2 m and in extremely eutrophic conditions is only a few centimetres. Sediments of eutrophic lakes become enriched with nutrients as organic matter from the photic zone accumulates. Eutrophic lakes have a biomass at all levels of the food chain, including fish. Summer and winter fish kills are typical in eutrophic waters. Definition of Eutrophication

5. 1.2.3 Mesotrophic Lakes Mesotrophic lakes are intermediate between oligotrophic and eutrophic lakes. They can be defined as having a Secchi disk depth of 2 - 8 m. Definition of Eutrophication

6. 1.2.4 Dystrophic Lakes Dystrophic lakes contain humic acids leached from decaying aquatic vegetation in the watershed. Humic acid stains the waters a characteristic yellow- brown colour. They are usually shallow and unproductive although some are highly productive and contain high levels of floating blue-green algae. Small dystrophic lakes are common in mountain regions where leachate from pine forests and bogs provide a continuous supply of humic acids. Definition of Eutrophication

7. Question: What is Secchi disk?