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7.01 Functioning of Ecosystems at the Land–Ocean Interface CHR Heip, Royal Netherlands Institute of Sea Research, Den Burg, The Netherlands JJ Middelburg, Utrecht University, Utrecht, The Netherlands; The Netherlands Institute of Ecology (NIOO-KNAW), Yerseke, The Netherlands CJM Philippart, Royal Netherlands Institute of Sea Research, Den Burg, The Netherlands © 2011 Elsevier Inc. All rights reserved. 7.01.1 7.01.2 7.01.3 Introduction Functioning of Ecosystems at the Land–Ocean Interface Conclusion 1 2 3 Abstract In this volume, we have focused on a number of new perspectives in trying to understand the responses to changes in land–ocean interfaces. We have compiled a series of chapters that address some key aspects of coastal and estuarine ecosystem functioning. The research field is too large to be covered entirely. We have, therefore, decided to select a few topics that are illustrative of how estuarine and coastal ecosystems function. Although the chapters deal specifically with estuarine and coastal ecosystems, most of the concepts and conclusions apply to other interface ecosystems as well. Many of the tools (long-term monitoring, stable isotopes, and metabolic balances) or concepts (invading species, connectivity, and ecosystem engineering) have value beyond the systems being discussed here. All the chapters in this volume show that estuarine and coastal systems have their own peculiarities and characteristics and are different from the adjacent continental and marine systems. 7.01.1 Introduction Land–ocean interfaces, from an earth system science perspec tive, are very diverse systems for the transfer of matter and energy between the continents and the oceans. These transfers and the ways that organisms adapt to and interact with them are often highly characteristic of specific environments. The interfaces are very heterogeneous and include many different systems at very different spatial scales, varying from river-dominated deltas to macrotidal estuaries, from mangrove-dominated wetlands to macroalgal-dominated rocky shores, and from biofilms in muddy lagoons to dynamic permeable beach sands. Ecosystems at the land–ocean interface also vary tremendously in size (from small pools to large systems such as the Great Barrier Reef), in longevity (from highly dynamic, migrating dunes to geomorphic systems estab lished by geological forces such as fjords), and in openness (from open exposed coasts to sheltered estuaries). Moreover, these interfaces are often short-lived geologically; they are discrete units and change geographic position frequently. Land–ocean interfaces cover the entire Earth from polar to tropical realms and thus differ in the species and biological communities they harbor. Being variable in import or export characteristics, these systems act either as sources or as sinks of material due to biological production or consumption pro cesses or due to physical sedimentation of particles with their associated materials. These processes usually show high tem poral and spatial variability with the consequence that coastal and estuarine ecosystems are highly heterogeneous in terms of their ecological characteristics such as nutrient availabilities and organism distribution patterns. Despite these many differences, there are however also many characteristics that coastal ecosystems share. The land–ocean interface is usually one of steep gradients, between (1) terrestrial and aquatic environments, (2) air and water, (3) freshwater and seawater, (4) eutrophic and oligotrophic systems, and (5) muddy and coarse sandy sediments, to name but a few. Land–ocean inter face ecosystems are often characterized by large gas-exchange rates (e.g., carbon dioxide). They all import and export energy in the form of organic carbon. They may import energy from upstream (the land) or from downstream systems (the sea, due to tides) and they can export energy because of high import or high production within the system. They also import, transport, transform, and export nutrients in all forms: organic and inorganic dissolved forms as well as particulate forms. Land–ocean interface ecosys tems are also affected by the import and export of organisms. This may occur passively by moving currents or actively if animals migrate. Organisms adapting to these gradients and changes are often robust because they require specific physiological adapta tions to survive rapidly changing conditions. Species diversity is often lower than in both the freshwater and the marine end mem ber, but many land–ocean interfaces are areas of high productivity. Understanding the specific requirements for organisms in gradients requires knowledge about both the characteristics of these gradients and the adaptation mechanisms evolved by organisms living there. In order to compare systems that are often very different in physical settings, occur in different geo graphic areas, and have different biological communities, we often describe them in terms of carbon and nitrogen flows, or in terms of production and consumption. Only recently has biodiversity become a major descriptor as well, and this hap pened because we realized that the specific characteristics of species are important to understand the physical and chemical processes, because these processes are shaped by biology as well as by physics, geology and chemistry. This also implies that understanding land–ocean interfaces as biogeochemical 1 2 Functioning of Ecosystems at the Land–Ocean Interface engines or transfer systems is not possible without a good knowledge of the biology and ecology of these interfaces. As species modify habitats or change biogeochemical cycles, their exact nature and characteristics become important attributes to understand the functioning of land–ocean interfaces. For instance, the presence and distribution of coastal plants not only change productivity of coastal systems, but also change the physical structure and carbon storage potential. When coastal plants disappear, for instance, through clearing of man groves for human usage or through changing sediment transport, the capacity of the whole system to perform ecosys tem services changes. 7.01.2 Functioning of Ecosystems at the Land–Ocean Interface Phytoplankton is at the basis of ecosystem productivity and is usually the most critical community to estuarine ecosystem functioning. Lancelot and Muylaert (Chapter 7.02) review the distribution of, and the factors governing, phytoplankton in estuaries. They discuss phytoplankton ecology from the bottom-up (role of hydrodynamics, salinity gradients, and light climate) as well as top-down perspective in terms of grazing by micro- and mesozooplankton and macrobenthic suspension feeders. They emphasize that the high variability of coastal systems requires the use of fully coupled river– estuary–coastal–ocean physical–biological models to advance the field further and to be able to project future trends. An important change in the ecosystem can be the introduc tion of new species. New species can occupy existing niches, compete with and eventually replace the orginal inhabitants without big changes in ecosystem functioning or can occupy new niches and introduce completely new functions in the ecosystem. Examples of this have been summarized by Levin and Crooks (Chapter 7.03), who give a thorough review of what introductions can trigger. Coastal ecosystems are among the systems most sensitive to invading species because of their location at the land–ocean interface and because of high human pressure. Levin and Crooks (Chapter 7.03) present a comprehensive overview of the way ecosystem engineers impact nutrient and biogeochemical processes, trophic interac tions, and energy flow through ecosystems and functioning and services of coastal ecosystems. Moreover, they show that many invading species act as ecosystem engineers and induce new subsystems. Adaptation to changing environmental conditions often involves creating a degree of control on these changes. A great diversity of organisms can modify the physical structure of estuarine and coastal environments. These physical ecosys tem engineers, in particular dune and marsh plants, mangroves, seagrasses, kelps, reef-forming corals and bivalves, burrowing crustaceans, and infauna, often have sub stantial functional impacts over large areas and across distinct geographic regions. By modifying their environment, these species contribute to ecosystem services such as storm protec tion, protection against erosion, and stabilization of sediments. Gutiérrez and colleagues (Chapter 7.04) give a detailed overview of the different communities and species and the ways in which they impact their environment. A surprising richness of mechanisms has evolved in land–ocean interfaces to allow species to acquire some degree of control on their environment and some of these mechanisms provide useful ecosystem services for human use and have sometime great economic value (see also Volume 12). We are only starting to get a good understanding on many of these mechanisms, for instance, of the important impact of biolo gical structures such as tubes, burrows, and leaves on physical processes such as wave attenuation of sedimentation. As land–ocean interfaces are gradients, the flow of energy and material to, from, and through coastal ecosystems is one of the most important characteristics of these ecosystems and their functioning. Whether coastal ecosystems are net auto trophic or net heterotrophic determines their role in the global carbon cycle and as producers or consumers of carbon dioxide. Kemp and Testa (Chapter 7.05) first discuss the var ious approaches for estimating ecosystem metabolism and then systematically treat the factors regulating ecosystem meta bolism. The authors show that the balance between production and consumption provides a first-order understanding and characterization of the functioning of coastal ecosystems. They provide a number of illustrative examples of how pressing environmental issues (hypoxia, food-web support, and carbon uptake) depend on ecosystem metabolism. Not only is the production–consumption pattern within an estuary or coastal system a major determinant of land–ocean interfaces, but there is also an important exchange between the systems. Problems of distribution, such as the problem of getting from one estuary to another, as discussed in the chapter by Gillanders and colleagues (Chapter 7.06), are problems of species adaptation. Connectivity is the linking of subpopula tions among estuaries via genetic or demographic movements. Gillanders and colleagues (Chapter 7.06) review the ways in which marine and terrestrial species connect between estuaries, the factors that influence the strength of connections, the tech niques of assessing links, and the consequences of having connected subpopulations. The establishment and mainte nance of connectivity between estuaries have important implications for ecological interactions between species and habitats, such as ensuring the genetic and population viability of mutualistic and foundation species, which then benefit other species within estuaries. When individuals move between estuaries, their presence can however impact on the abundance and behavior of other species or of individuals of the same species, resulting in inter- and intraspecific competition. Important recent advances in unraveling the energy flow through food webs have been made using stable isotopes or biomarkers such as fatty acids. Research during the last decade has shown that stable isotopes can provide detailed information on energy flow and trophic relationship. The basic premise underlying the use of stable isotopes in food webs is that there are differences in primary food sources available to consumers, that carbon isotopes are consumed with little or very small fractionation (‘you are what you eat’), and that the nitrogen isotope fractionation increases with each trophic transfer. Bouillon and colleagues (Chapter 7.07) have reviewed the lit erature on stable isotope biogeochemistry and ecology in coastal ecosystems and show how they can be used to infer energy flow, food-web relationships, and the migration of animals. Human impacts on land–ocean interfaces are enormous in many areas. A large part of the world population lives close to the coast. Estuaries and bays provide space for towns and Functioning of Ecosystems at the Land–Ocean Interface harbors that are hot spots of industrial activity and the related physical changes, effluents, and emissions (see also Volume 8). Estuaries particularly concentrate the waste products of large surfaces in a narrow zone. Tourism, which often concentrates on land–ocean interfaces, is one of the major industries in both the developed and the developing countries. It is a major source of income determining wealth for local populations but has an increasing impact on coastal ecosystems because the infrastructure it requires and the impacts it produces, for example, on beach ecosystems, coral reefs, and seagrass beds. All such impacts have resulted in a rapid and often massive change of many land–ocean interfaces globally in areas with high human population densities with consequent changes in biological communities and biogeochemical processes. Changing environmental conditions, whether natural or man induced, create selective pressures on species, and species will therefore respond to their changing environment. There is an enormous literature on the effects of chemical and physical pressures on land–ocean interfaces, much of it dating from the later decades of the previous century. This literature has focused on responses at the biochemical, cellular, organismal, and community level and these responses are now well known for the effects of many pollutants, although there are still many new chemical products with unknown impacts being produced every year. The accelerating deterioration of estuarine and coastal waters has led to much national and international legislation and a number of conventions also apply to these ecosystems. Application of legislation has led to an increased demand for data and for monitoring different properties of coastal ecosystems. Carstensen and colleagues (Chapter 7.08) illustrate current problems and future solutions to collect data and synthesize the information derived from coastal monitor ing programs. They advocate that monitoring programs should be based on the concept of a hierarchically organized data– information–knowledge pyramid, with strong interactions between all three levels. Furthermore, they indicate that coastal ecosystems are characterized by large-scale and small-scale var iations in both time and space, which all should be taken into account to describe adequately their status and trends in coastal waters. 7.01.3 Conclusion In this volume, we have focused on a number of new perspec tives in trying to understand the responses to changes in land–ocean interfaces. We have compiled a series of articles that address some key aspects of coastal and estuarine ecosystem 3 functioning. The research field is too large to be covered entirely. We have, therefore, decided to select a few topics that are illus trative of how estuarine and coastal ecosystems function. Although the chapters deal specifically with estuarine and coastal ecosystems, most of the concepts and conclusions apply to other interface ecosystems as well. Many of the tools (long-term monitoring, stable isotopes, and metabolic balances) or concepts (invading species, connectivity, and ecosystem engi neering) have value beyond the systems being discussed here. All the chapters in this volume show that estuarine and coastal systems have their own peculiarities and characteristics and are different from the adjacent continental and marine systems. The existence of strong gradients in nutrients, oxygen and salinity, the so-called ‘species minimum’ that is considered a consequence of the lack of adaptation to these gradients in most estuarine systems, the high productivity of estuaries linked to both low diversity and high nutrient availability, and especially the heavy use of estuaries by humans all justify the large scientific research effort that has been devoted to these systems over the last few decades. Estuaries are highly resilient to many changes but the limit of this resilience has been reached in many estuaries all over the world, triggering regime shifts and requiring very costly restoration projects to recover the original conditions. Understanding estuarine ecology and biogeochemistry is essential to support the adequate manage ment of estuarine ecosystems Organisms, populations, communities, and ecosystems at the land–ocean interface are usually studied by scientists hav ing either terrestrial or oceanographic traditions. Scientific concepts and techniques differ among terrestrial ecologists, limnologists, and marine scientists with the consequence that approaches and techniques are confronted in the coastal realm. This sometimes leads to novel discoveries, but some times also to unnecessary conflicts. For instance, terrestrial scientists often work with biomass or number of organisms, while marine ecologists usually discuss natural ecosystems in terms of fluxes and turnover rates, because of differences in the turnover of primary producers (approximately days in aquatic systems vs. approximately months to years in terres trial systems). Most coastal ecosystems are rather shallow, implying that sedimentary (benthic) and water-column (pelagic) systems are tightly linked and interacting. Open ocean concepts for the photic zone, therefore, cannot be directly applied in coastal systems because light and primary-produced material may reach the bottom. Coastal and estuarine science could therefore profit from a more intense interaction between scientists from various disciplines and different backgrounds.