The photosynthetic process in aquatic ecosystems is responsible for fixing approximately 40% of our planet's yearly amount of carbon available for the production of new living matter, with about 48.5 Pg C yr-1 fixed in the aquatic ecosystems [1-3]. Carbon fixation in the aquatic environment, mediated by the utilization of solar radiation, takes place in both the water column and the benthos. While water-column autotrophic organisms (mainly phytoplankton) are responsible for most of the share in carbon fixation, benthic organisms (i.e., macrophytes and microalgal communities) are involved in about 10% of the total production [4,5]. Although this latter amount is globally less than that due to phytoplankton, marine macrophytes also provide food (directly or through detritus) to a wide variety of invertebrates and fish in the coastal ecosystems [6]. Benthic microalgal communities, on both hard and soft substrata, also serve a crucial ecological function in shallow freshwater and marine habitats. They constitute the local basis of the food webs in shallow areas, which are recognized as having high secondary production (e.g., of fish and their prey). In these areas, the microphytobenthic (MPB) communities may account for 50% or more of the total primary production, equalling or exceeding the productivity of the water column [7].

Even though solar radiation is attenuated in the water column (see Chapter 3) it penetrates to a depth that will vary, among other things, according to the location (e.g., oceanic vs. coastal), latitude and concentration of particulate and dissolved matter. The euphotic zone in the water column (i.e., 1 % of surface PAR, 400-700 nm) can vary from few centimetres in estuarine waters or lakes with a heavy load of DOM [8,9] to more than 100 m in the open ocean [10]. Hence ultraviolet radiation (UVR, 280-400 nm) can penetrate accordingly to comparable depths [11] (see also Chapter 3). In coastal waters, biologically effective ultraviolet B radiation (UV-B, 280-315 nm) reaches only to 1 m depth, as in the Baltic Sea [12], whereas in the Mediterranean it can penetrate as deep as 20 m [13]. This variability is also observed in other environments. For example, in a study carried out in freshwater Japanese ponds and lakes, Hodoki and Watanabe [14] determined that the 1 % of surface UV-B varied from 0.3 to 2 m, depending mostly on the concentration of chlorophyll-a (chl-a) and particulate organic carbon present in the water body. The photic zone in the benthic environment extends to ca. 3 mm into the sediment. Fiber optic microsensor measurements have shown that UVR can penetrate down to at least 1.25 mm of this zone, and through scattering it can even exceed the incoming UVR by up to 50% [15,16]. In addition, as the water column in estuaries and embayments is often shallow, and regularly absent in intertidal areas, UVR can reach high levels at the sediment surface. Thus, and in view of this background, UVR should be considered a very important environmental factor that can affect different metabolic and physiological processes in autotrophic organisms living in the water column and in the benthos.

In this chapter we will discuss the role of UVR in affecting the photosynthetic process in phytoplankton, MPB, and macroalgae. This is especially important as the effects of UVR on the photosynthesis of these organisms may have a considerable impact on higher trophic levels of the aquatic ecosystem (Chapter 12), as well as in climate change (Chapter 17) and biogeochemical cycles (Chapter 5).

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