Effects of natural light on nitrogen dynamics in diverse aquatic environments

Solar radiation is a major force affecting the ecosystem dynamics of aquatic and terrestrial ecosystems. As the source of energy for photosynthesis, it allows plants to assimilate nutrients into energy-rich organic material at the base of food webs and provides energy for biogeochemical processes. The importance of the quality and quantity of light has been studied extensively in terms of its relationship to photosynthetic processes (e.g. KlRK 1994). The effect of light on nitrogen cycling in aquatic ecosystems has been less studied and has not been defined clearly, except for several studies of nutrient uptake as related to primary production processes (e.g. COCHLAN et al. 1991 and references cited therein). Light provides energy for photosynthesis and increases inorganic nutrienr uptake, but excess light can also inhibit phytoplankton growth and nutrient uptake. These effects depend nor only on the intensity of the light but also on its spectral characteristics and the physiology of component organisms. The effects of light on heterotrophic nutrient regeneration processes are less apparent than are those for phytoplankton uptake processes because heterotrophic organisms do not usually depend on light for energy. For this reason, metabolic studies of heterotrophic aquatic organisms are usually conducted under dark conditions. However, light could affect biological nutrient regeneration mechanisms and rates in a variety of indirect ways. To develop an understanding of rhese effects, we must define rhe mechanisms of food web interactions and photochemical stimuli that may affect them. Additionally, ultraviolet light may affect nitrogen regeneration by photochemically converting dissolved organic nitrogen (or complexed inorganic forms) to ammonium or nitrate in surface waters (BusHAw er al. 1996). Research on microbial ecology over the past few decades has expanded our concepts of energy and nutrient transformations in aquatic ecosystems (POMEROY 1974, AzAM er al. 1983, SHERR et al. 1988). The current paradigm is that a large portion of the energy and nutrients fixed by primary producers in aquatic ecosystems passes through small organisms that constitute the "Microbial Food Web (MFW)". Details of this changing paradigm are still being defined for aquatic ecosystems. For example, small phytoplankton and bacteria are grazed by protists or microzooplankton, which are eaten by crustacean zooplankton or other metazoan invertebrates before the energy becomes available for higher trophic levels such as fish. With more trophic levels involved in energy and nutrient transformations, the relative quantity of fixed organic matter that is metabolically released as inorganic nutrients and carbon dioxide is increased ( e.g. SuzuKI et al. 1996). I f materials pass through the MFW before being incorporated into larger units of biomass, such as zooplankton, fixed nutrients will be recycled to a greater extent than if zooplankton or macrobenthos consume the phytoplankton directly. The community composition and trophic dynamics of autotrophic and heterotrophic organisms (e.g. HAGA et al. 1995, MILLER et al. 1995) and their responses to available light (LIPSCHULTZ et al. 1985) must therefore be studied to understand the effects oflight on nutrient transformations and cycling rates in aquatic ecosystems.