13.1 Introduction

13.1 Introduction

Eutrophication increases the amount of new particulate organic matter in water and the subsequent sedimentation (see also Chapter 12). In estuaries, coastal areas, shallow open sea areas and many lakes a large portion of the autochthonous and allochthonous particulate organic matter reaches the surface of the bottom sediment. Organic matter and nutrients and their processes in sediments therefore play an important role in the nutrient dynamics of these shallow systems. Increased concentrations of labile sediment organic matter leading to higher sediment O₂ consumption and the eventual depletion of dissolved O₂ in bottom water are a common consequence of eutrophication. The exhaustion of O₂ weakens the ability of sediments to retain nutrients. Thus, increase in the release of nutrients from the bottom sediments back to water column is a response to eutrophication.

The bottom area of marine and lake waters can be categorized as erosion-, transport- and sedimentation bottoms. At the erosion bottoms the erosion conditions prevail, whereas at the transport bottoms the settled fine particulate matter can stay for long periods, even for decades, before it is transported to the sedimentation bottom. The fine inorganic and organic particulate matter is eventually buried at the sedimentation bottoms, i.e. areas, where nearbottom water currents can not re-suspend the fine-grained particles back to water. The whole bottom area of the water system does not efficiently release nutrients from sediments to water even under anoxic conditions. The major part of the release usually occurs at the sedimentation areas, where the concentrations of organic matter and nutrients are high in sediments. At the sedimentation areas the renewal of near-bottom water is slow and sediment O₂ consumption is high and, thus, they are sensitive to O₂ depletion and the subsequent high release rate of nutrients.

The sensitivity of water masses to an excess nutrient load and subsequent eutrophication is largely determined by the geomorphological and hydrographic properties of an area. The longer the residence times of the water and the stronger the stratification due to changes in water density, the greater is the sensitivity. In lake systems water density is largely governed by temperature, but in a brackish water body, for example in the Baltic Sea, the main factors controlling density are salinity and temperature. These result in strong stratifications, namely halocline and thermocline, in the water column. As a consequence, the separate surface and deeper layers may prevent the mixing of O₂-rich surface water with the bottom water for years or parts of a year. Due to its unique stratification properties, the Baltic Sea is sensitive to hypoxia i.e. low oxygen concentrations and the high release rates of phosphorus (P) from the sediments [100]. Changes in hydrographic features affecting stratification in the water body have an effect on biogeochemical processes in the sediments, which may, in turn, affect the pool of P in the water.

13.1 Introduction