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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
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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
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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
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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.