Solar radiation as an ecosystem modulator

Robert G. Wetzel

Table of contents

Abstract 5

1.1 Introduction 5 1.2 Size matters – radiation attenuation in relation to loadings of organic matter 6 1.3 Precipitation matters – importance of frequency and intensity of influents 7 1.4 Direct effects of UVR 9 1.5 Allochthonous vs. autochthonous organic matter – key UV-VIS mediated processes regulate heterotrophic utilization 9 1.5.1 Alterations of enzymatic accessibility by the macromolecules 10 1.5.2 Photolysis of humic macromolecules 10 1.5.3 Photolysis of dissolved organic nitrogen and phosphorus compounds 11 1.5.4 Complete photolysis of humic substances to CO and CO2 11 1.5.5 Less direct but important biogeochemical interactions of UVR 12 1.6 Recalcitrant organic matter, metabolic stability, and photolysis 13 References 15

Abstract

Solar radiation is the fundamental ecosystem modulator. Nearly all generation of organic matter is photosynthetic and as such the distribution of light in aquatic ecosystems is critical to regulation of major energetic inputs. However, simultaneously specific components of solar radiation, in particular the UV, function as both an accelerator of microbial degradation by enhancing bioavailability of complex organic substrates to microbes and by complete photolysis and oxidation of components of organic macromolecules to CO2 and other inorganic forms of nutrients. Alterations in UV intensities impinging upon and within inland aquatic and coastal marine ecosystems by natural or anthropogenic causes will modify the rates of metabolism and biogeochemical processes associated with these macromolecules. This cascade of effects can greatly modify the functioning of natural ecosystems.

1.1 Introduction

In the subsequent chapters of this volume, detailed evaluations provide a summary of contemporary understanding of the properties of ultraviolet radiation (UVR) in aquatic ecosystems and its effects on aquatic organisms. Here I attempt to provide an overview of the coupling of these properties to emphasize how individual effects of UVR are integrated and, at the ecosystem level, provide a master level of regulation of ecosystem biogeochemical cycling, energy fluxes, productivity, and system evolution.

In regard to these detailed treatments of specific components of solar radiation and their effects, it is useful to emphasize several related universal characteristics of aquatic ecosystems. Namely, ecosystems are biological systems, ecosystems are biogeochemical systems, and the cycling of materials and energy in ecosystems is regulated by a highly variable set of intercoupled physical, chemical, and biological parameters. It is extraordinarily important to evaluate the influences and changes of UVR in the ecological contexts of a highly dynamic, changing environment – dynamic spatial and especially temporal scales. The question then is whether UV effects within the ecosystems are so variable that analyses are chaotic or whether certain stoichiometric analyses allow quantitative predictions of generic system responses to changes in UVR.

The approach taken is to first analyze our present understanding of how UVR influences ecosystem processes and how these processes are intercoupled with other related influences of those processes, such as climatic or atmospheric processes related to UVR. Finally, can one reasonably predict how ecosystems of different characteristics will respond to changes in atmospheric or aquatic conditions that alter UVR.

1.2 Size matters – radiation attenuation in relation to loadings of organic matter

Nearly all UV-C (< 280 nm) is absorbed by the stratospheric gases and by the water of aquatic ecosystems. Although relatively little UV-B (280-320 nm) passes through the stratosphere (Chapter 2), UV-B is highly energetic and an important photactivating agent in waters. UV-A (320-400 nm) is less energetic than UV-B but is absorbed less readily and penetrates more deeply into water. The near UV light in the blue portion of the visible spectrum (400-500 nm) has recently been shown to be functionally similar to the adjacent UV-A radiation in many of the important photochemical reactions influenced by UVR and must be considered in any evaluation of composite effects.

Recent measurements in situ have demonstrated great variability in the penetration of UV-B and UV-A, but penetration has been found to be much greater than was believed previously (Chapters 3, 6, [1]). When referenced against pure water, the transmission of radiation is reduced drastically with increasing concentration of naturally occurring chromophoric dissolved organic compounds, particularly humic acids. UV-B attenuation depths (za = 1% of surface irradiance) range from a few centimeters to > 10 m among a number of waters [2-6]. Much (> 90%) of the among-habitat variation in diffuse attenuation coefficients (Kd) could be explained by differences in dissolved organic carbon (DOC) concentrations. Throughout the solar UV-B and UV-A range, Kd was well estimated with a univariate power model based on DOC concentration, particularly in waters of low to moderate phytoplanktonic productivity. The Za is strongly dependent on DOC concentrations when below 2 mg C 1-1....