Phosphorus in the global environment : transfers, cycles, and management

445 Pages·1996·5.405 MB·English

by  H Tiessen, International Council of Scientific Unions. Scientific Committee on Problems of the Environment., United Nations Environment Programme

SCOPE 54 -Phosphorus in the Global Environment -
Transfers, Cycles and Management
List of Contributors ix
Preface xiv
Acknowledgements xv
1 Introduction And Synthesis 1
Holm Tiessen
2 Global Transfers Of P In Fertilizer Materials And Agricultural Commodities 7
James D. Beaton, Terry L. Roberts, Ed H. Halstead and Lyle E. Cowell
3 Closing The Cycle: Obstacles To Efficient P Management For Improved Global Food Security 27
Artur Runge-Metzger
4 Phosphorus Balance In European Agriculture - Status And Policy Options 43
Erik Sibbesen and Arthur Runge-Metzger
5 Phosphorus Fertility Management In Agroecosystems 59
Michael J. Hedley, John J. Mortvedt, N.S. Bolan and J. Keith Syers
6 Heavy Metal And Radionuclide Contaminants In Phosphate Fertilizers 93
John J. Mortvedt and James D. Beaton
7 Reactions Controlling The Cycling Of P In Soils 107
Emmanuel Frossard, Michel Brossard, Michael J. Hedley and Allister Metherell
8 Strategies Of Phosphorus Acquisition And Conservation By Plant Species And Communities 139
Kate Lajtha and Anthony F. Harrison
9 Rehabilitation Of Anthropic Savanna 149
Helmuth R. von Uexküll and E. Mutert
10 Phosphorus Dynamics In Slash-and-burn And Alley Cropping On Ultisols Of The Humid Tropics 155
Michael Hands, Anthony F. Harrison and T. Bayliss-Smith
11 Phosphorus Transfers From Terrestrial To Aquatic Ecosystems 171
Andrew N. Sharpley, Mike J. Hedley, Erik Sibbesen, Anna Hillbricht-Ilkowska, W. Alan House and Lech
Ryszkowski
12 Phosphorus Transfers And Landscape Structure: Riparian Sites And Diversified Land Use Patterns 201
Anna Hillbricht-Ilkowska, Lech Ryszkowski and Andrew N. Sharpley
13 Phosphate Transport And Retention In A Riparian Meadow - A Case Study 229
Andrzje Kedziora, Lech Ryszkowski and Zbigniew Kundzewicz
14 Influence Of Human Populations On Phosphorus Transfers To Aquatic Systems: A Regional Scale Study
Using Large Rivers 235
Nina F. Caraco
15 Transport And Transformations Of P, Fluvial And Lacustrine Ecosystems 245
John Melack
16 Phosphorus As A Limiting Resource In Inland Waters; Interactions With Nitrogen 255
Johan U. Grobbelaar and W. Alan House
17 The P Cycle In The Balaton Catchment - A Hungarian Case Study 275
Sándor Herodek, Vera Istvánovics, Géza Jolánkai, Péter Csathó, Tamás Németh and G. György Várallyay
18 Nutrient Limitation Of Phytoplankton And Eutrophication Of Inland, Estuarine And Marine Waters 301
Thomas R. Fisher, John M. Melack, Johan U. Grobbelaar and Robert W. Howarth
19 Transport To And Processing Of P In Near-shore and Oceanic Waters 323
Robert W. Howarth, Henning S. Jensen, Roxanne Marino and Henk Postma
20 Phosphorus Transfer From Tropical Terrestrial To Aquatic Systems - Mangroves 347
Ignacio H. Salcedo and Carmen Medeiros
References 363
Index 459

Last updated: 14.09.2002
International Council of Scientific Unions (ICSU)
Scientific Committee on Problems of the Environment (SCOPE)

SCOPE is one of a number of scientific committees established by the non-governmental International
Council of Scientific Unions (ICSU). The membership of ICSU includes representatives from 75 national
academies of science, 20 scientific unions and 29 other associates. To cover interdisciplinary activities of
interest to several unions, ICSU has established 13 scientific committees of which SCOPE is one. SCOPE
was established in 1969 in response to emerging environmental concerns. Currently, representatives of 35
member countries and 21 unions, scientific committees and associates participate in the work of SCOPE,
which directs particular attention to the needs of developing countries. SCOPE’s first task was to prepare a
report on global environmental monitoring (SCOPE 1, 1971) for the UN Stockholm Conference on the
Human Environment. The mandate of SCOPE is to assemble, review and assess the information available
on man-made environmental changes and the effect of these changes on man; to assess and evaluate the
methodologies of measurement of environmental parameters; to examine current research; and, based on
the best available scientific information and constructive thought, to establish itself as a corpus of
informed advice for the benefits of centres of fundamental research, and of organizations and agencies
engaged in the study of the environment.

In 1974 SCOPE launched a major series of projects on biogeochemical cycles. Initially these projects
focused on the study of carbon, nitrogen and sulphur separately. Much of the work on individual cycles
was coordinated by separate SCOPE/UNEP units: carbon, under the direction of the late Prof. Egon
Degens in Hamburg, Germany; nitrogen under the direction of Prof. Thomas Rosswall in Uppsala,
Sweden, sulphur under the direction of Academician Mikhael Ivanov in Moscow, Russia, and phosphorus
under the direction of Dr. C. Vernon Cole of Fort Collins, USA. It was understood that none of these
cycles operates independently and attempts have been made to include the studies of elemental
interactions in each unit’s activities. SCOPE has also sponsored efforts to unravel many of the mechanisms
of natural interactions (SCOPE 17, "Some Perspectives Of The Major Biogeochemical Cycles" edited by
G.E. Likens in 1981, and SCOPE 21 "The Major Biogeochemical Cycles And Their Interactions" edited
by B. Bolin and R.B. Cook in 1983). Various international units have met jointly to explore specific
interactions between two elemental cycles, as between carbon and sulphur (SCOPE 42, edited by Degens
et al., Biogeochemistry of the Major World Rivers).

J.W.B. Stewart, President of SCOPE
V. Plocq-Fichelet, Executive Director
Secretariat: 51 boulevard de Montmorency
75016 Paris, France

Members of the Scientific Advisory Committee for the SCOPE/UNEP
International Phosphorus Project

C.V. Cole (Chairman)
NREL, Colorado State University, Fort Collins, CO 80523 USA
J.D. Beaton
Potash & Phosphate Institute of Canada, CN Tower, Saskatoon,
SK
S7K 1J5 Canada
J. Freney
CSIRO, Division of Plant Industry, GPO Box 1600, Canberra,
ACT 2601 Australia
J. Keith Syers
Department of Agriculture & Environmental Science, The
University of Newcastle, Newcastle Upon Tyne, UK
V. Subramanian
School of Environmental Science, Jawarharlal Nehru University,
New Delhi 110067, India
H. Tiessen
College of Agriculture, University of Saskatchewan, Saskatoon,
SK S7N 5A8 Canada
J.W.B Stewart (ex officio)
College of Agriculture, University of Saskatchewan, Saskatoon,
SK S7N 5A8 Canada

Preface

Phosphorus (P) occupies a key place among nutrients because of its relative scarcity and its essential role
in all life forms. Scientific information is increasingly needed to guide the use of P in order to obtain
maximum benefits without producing undesirable impacts on the environment.
In 1987 SCOPE launched a major study aimed at better understanding the "Phosphorus cycles in terrestrial
and aquatic ecosystems", and the global environmental effects of P. The phosphorus project organised four
regional workshops in Africa, Asia, Europe and South America in which 240 scientists from 51 countries
have synthesised data available in major ecosystems of each region considered; and then integrated the
information at a final synthesis workshop held in Hungary in 1993. The objectives of the project were to:

• critically assess knowledge of the nature, sources, sinks and fluxes of P in the biosphere;
• identify mechanisms of supply of biologically active P in terrestrial and aquatic ecosystems;
• provide the information needed to meet worldwide P requirements for sustainable food and fiber
production while minimising adverse effects on the environment;
• evaluate the environmental effects of current and projected P use in relation to the biogeochemical cycles
of carbon, nitrogen, sulphur and metals;
• evaluate the transfers of P from terrestrial to aquatic and marine environments; and
• assess current and likely future trends in P use.
• explore conceptual or simulation models of phosphorus cycling to help identify gaps in knowledge and
options for careful management of phosphorus resources.

The Scientific Advisory Committee appointed Holm Tiessen as editor of the four workshop proceedings
(published by the University of Saskatchewan and by the International Rice Research Institute) and of the
synthesis SCOPE volume. As with any synthesis of a global cycle with many interactions, this has been a
mammoth task. This final synthesis volume stresses the fact that despite the differences in scientific
approach to terrestrial and aquatic ecosystems, there are many similarities. There should be active
dialogue between these disciplines; in fact it is essential that such dialogue take place if the quality of
environments are to be preserved and we are to maintain food and fibre production, which needs
phosphorus inputs. I commend this volume to you, and would thank the Scientific Advisory Committee
for their sterling efforts.

J.W.B. Stewart
President of SCOPE

Acknowledgements

The SCOPE Phosphorus project, and this book would not have been possible without the contributions by
these donors:
A.W. Mellon Foundation
American Phosphate Foundation
Australian International Development Assistance Bureau (AIDAB)
European Fertilizer Manufacturer Association (EFMA)
Fertilizer Advisory Development and Information Network for Asia & the Pacific
Food and Fertilizer Technology Centre for the Asian and Pacific Region
French Ministry of the Environment
Hungarian Academy of Sciences
Instituto Venezolano de Investigaciones Cientificas
International Atomic Energy Agency
International Council of Scientific Unions
International Development Research Centre
International Fertilizer Development Centre
International Fertilizer Industry Association
International Rice Research Institute
National Academy of Sciences /US Committee for SCOPE
National Science Foundation in the US
Polish Academy of Sciences
Potash and Phosphate Institute
Royal Society of London
Tropical Soil Biology and Fertility Program
United Nations Education, Scientific and Cultural Organization
United Nations Environment Programme
University of Saskatchewan
US AID
World Phosphate Institute

We also gratefully acknowledge the invaluable help of the many volunteers at the Polish Academy of
Sciences, The International Rice Research Institute, The Universidad Central de Venezuela, The United
Nations Environment Programme and the Hungarian Academy of Science, who helped with the
organisation of the workshops.
1 Introduction And Synthesis

Holm Tiessen
College of Agriculture, University of Saskatchewan, Saskatoon,
Canada S7N 5A8

One result of synthesising the information gathered during the SCOPE project on "Phosphorus cycles in
terrestrial and aquatic environments" is the realisation that the term "P cycle" is not appropriate in the
global context. It is estimated that currently 33.106 Mg y-1 of P are discharged into the oceans (Howarth
et al., Ch. 19), while in 1990 16.106 Mg y-1 were applied as mineral fertilizer (FAO, 1992b). The transfer
to the oceans is largely the result of natural processes, which are accelerated by arable agriculture,
concentrated animal husbandry and direct anthropogenic discharges. This volume therefore addresses P
cycling and transformations in specific ecosystems, and also the less well understood transfers across
landscapes and between ecosystems. A crucial problem in understanding the transfers from terrestrial to
aquatic ecosystems is the great sensitivity of aquatic ecosystem function to P: the quantity of P sufficient
to substantially alter the trophic status of a water body is negligible, and often not measurable, in the P
budgets of terrestrial ecosystems in the watershed.
What are the processes that supply the essential nutrient P to the environment; and what is responsible for
the oversupply which is threatening environmental integrity of aquatic ecosystems in some regions? In the
natural environment, P is supplied through the weathering and dissolution of rocks and minerals with very
low solubility. Therefore, P is usually the critical limiting element for plant and animal production, and
throughout the history of natural production and human agriculture, P has been largely in short supply.
Many natural ecosystems and low-input farming systems have adapted to low P supply by recycling P
from litter and soil organic materials. Increases in productivity or exports of commodities require external
nutrient inputs if they are not to cause a decline in fertility. External P inputs have become available on a
large scale with the mining of phosphate deposits and the wide-spread availability of the commodity
"phosphate". This has decoupled patterns of supply, consumption and waste production from natural
nutrient cycles, and has made them dependent on economics. The critical question, when phosphate ore
reserves will be exhausted, is hard to answer, with estimates ranging from <100 y to many hundreds
(Runge-Metzger, Ch. 3). Although all continents show a positive P balance in their trade (excluding mined
P deposits, which do not form part of the land ecosystems) (Beaton et al., Ch. 2), P remains in short
supply over large parts of the globe, where economic and political constraints mean that naturally low P
availabilities are only insufficiently supplemented by imported fertilizers (Runge-Metzger, Ch. 3). In many
regions, where the use of N fertilizer is favoured, this may cause a nutrient imbalance and inefficient N
use.
Meanwhile, elaborate systems of fertility management and a long history of P fertilizer use have built up
soil P levels in some developed countries to such an extent that additional P is currently not needed to
produce food for the resident population. When such countries or regions continue to import both large
amounts of animal feeds and P fertilizers, and where populations are dense enough to produce significant
amounts of waste phosphate, the P over-supply becomes a pollutant. In Europe, intensive animal
production on farms with little land produces manure in excess of the nutrient requirements of crop and
pasture lands (Sibbesen and Runge-Metzger, Ch. 4). The excess P from the animal manure, if it is not
directly dumped into waste waters, accumulates in the surface soils which leads to increased leaching and
erosion losses of P to aquatic environments. Current legislation may not address this problem effectively,
although legislation to reduce the land application of P exists and is being tightened with planned
substantial reductions in allowable P application to soils, and stricter controls on timing and management.
This trend has considerable impact on agricultural land use, and farmers are no longer solely regarded as
producers of food but are also held responsible for the ecological functioning of the landscape (Hedley et
al., Ch. 5). The stewardship required for intensive agricultural production that does not jeopardise the
surrounding environment or the finite resources of rock phosphate (90 y of supply are predicted at current
rock exploration and increasing crop production levels), demands sophisticated methods of soil and
landscape conservation, and nutrient management and monitoring.
Predicting the transformations - and therefore the potential for transfers from production sites to other
ecosystems - of P requires detailed knowledge, not only of chemical equilibria but also of the rates of
transformations of organic and inorganic forms of P in the soil (Frossard et al., Ch. 7), and of the ways in
which P availability and turnover are affected by plant adaptations (Lajtha and Harrison, Ch. 8). Integrated
farming systems, making use of biological processes of nutrient transformation and conservation, and
relying on long-term balancing of nutrient budgets, challenge the current knowledge base and the
understanding of soil processes and management options. Knowledge has been integrated and extrapolated
with computer models which evaluate soil, crop, management and climate information to optimise
fertilizer application and permit the use of low-cost rock phosphates or recycled wastes (Hedley et al., Ch.
5).
A side effect of sustained heavy P fertilizer additions can be the accumulation and introduction into the
human food chain of heavy metal contaminants contained in fertilizers (Mortvedt and Beaton, Ch. 6).
Phosphate fertilizer use has caused small but significant increases soil cadmium levels. These inputs are of
a magnitude similar to those from the atmosphere in industrialised countries. There are still no agreed safe
Cd limits for soils, but it appears that concern about Cd build-up in soils may be warranted only where
several critical factors combine, i.e. on acid soils with low cation exchange and low P fertility, to which
significant P fertilisation is applied as low grade fertilizer or rock phosphate, particularly to Cd
accumulators like leafy vegetable crops, and where the produce is the main source of local food
consumption. This may be the case in truck farming belts around population centres in the tropics.
In developing countries, where 95% of the population increase over the next decades will occur, soil
fertility is often low, and it will be difficult to meet the increased demand for food and fibre. The limited
purchasing power of developing nations has resulted in nutrient mining of some of the better soils with the
result that current productivity trends are downward. The limited use of fertilizer is compounded by the
nearly complete use of high quality lands, which forces additional agricultural production into infertile
acid upland soils. Too little fertilizer P is actually used in those parts of the world where P input could
have a major effect on food production. Phosphorus use may have to double in Asia and quadruple in
Africa. Fertilizer subsidies have been used in attempts to remedy this situation, but they can cause market
distortions, black markets, environmental problems and the misallocation of resources. However, for
reasons of long-term resource conservation and for the reversal of land degradation, fertilizer subsidies
should be considered on P-deficient soils, where environmental costs of the soil resource depletion may
justify the cost of subsidies. Careful monitoring of social and economic side effects would be required
(Runge-Metzger, Ch. 3).
The development of acid upland soils, common in developing countries, requires the use of low-grade
rock phosphate, which is less susceptible to leaching and fixation in the acid environment (Hedley et al.,
Ch. 5; von Uexküll and Mutert, Ch. 9). As a result of logging and accelerated shifting cultivation
accompanied by a rapid deterioration of soil fertility, an estimated 65-80 million ha worldwide have been
degraded into anthropic savanna and scrub (von Uexküll and Mutert, Ch. 9). Reclamation of such
degraded lands can be initiated by rock phosphate application and the establishment of leguminous covers.
Subsequent management will have to be fine-tuned to release P from fertilizer in synchrony with crop
demands.
Natural ecosystems on such acid soils usually conserve P in and recycle P from organic matter and plant
residues. The rates of cycling of P, N and organic C are therefore closely inter-dependent, and must be
viewed together if nutrient cycling and production dynamics are to be understood. Agricultural
management can successfully mimic natural ecosystems, with - for instance - the use of mulches, green
manure crops, legume covers, alley cropping and appropriate crop rotations (Hands et al., Ch. 10). Much
research and education are required to replace or adapt the originally successful regimes of shifting
cultivation with such new technologies appropriate for increased food and fibre demands on a shrinking
land base. Traditional farm communities passed on an understanding of nutrient dynamics, as expressed
for example in the Norfolk rotation or the intercropping patterns of the tropics. Extensive use of
manufactured fertilizer has increased yields but has probably dulled this intuitive skill. Future generations
of farmers will need to rediscover this knowledge to manage limited soil and fertilizer resources, aided by
computer models and scientific knowledge that reduce the risks in farming diverse soils under variable
and changing climatic conditions. Many research results have not yet reached the farm level (e.g. use of
mycorrhiza, the thorough understanding of nutrient cycles). This indicates that the linkage between
scientists and farmers is not well established, training of farmers is insufficient, and feedback from
farmers' problems to research is limited.
The lessons learned in managing a limited P resource under the economic constraints of developing
countries may also benefit developed regions where P "leaking" from production systems poses
environmental threats. Fertilizer use to overcome P deficiency has permitted intensive sedentary
agriculture which feeds large urban populations that now have waste P problems. Negative effects of P use
are mostly associated with redistribution of P into aquatic environments where P limits bioproductivity
and excess P increases the cycling of carbon, nitrogen and sulphur.
While the in situ transformations of P, and their effects on C and N cycling are fairly well understood
(Frossard et al., Ch. 7; Hedley et al., Ch. 5; Hands et al., Ch. 10), the workshops have highlighted the
complexity of P transfers in the landscape, and the lack of quantitative knowledge on landscape
relationships. The P and N status of the world's major rivers reflects land use in their drainage basins
(Caraco, Ch. 14), but the intricate relationships between land use patterns, surface and subsurface transport
of P, and aquatic P loads are only now being explored. Because of their great sensitivity to P inputs, water
bodies are a sensitive diagnostic for monitoring land use practices in the watershed. Based on the impact
of land use on water quality, and using process studies of P transport, some general principles on nutrient
management in landscapes emerge from the chapters by Sharpley et al. (Ch. 11) and Hillbricht-Ilkowska
et al. (Ch. 12). Point sources of P pollution from human population and industry add important P loads to
surface waters, but these point sources are more easily controlled than diffuse sources. Reclamation and
recycling of the P contained in industrial and urban waste waters is an important measure for pollution
control. Although recycling is limited to some extent by heavy metal contamination, the problem is one of
policy and engineering. Point sources of P pollution are therefore not addressed in this volume.
In many aquatic ecosystems, non-point sources of P dominate the eutrophication process. Total P export
rates from a watershed are related to arable land cover, slope, stream network density and rainfall, but the
relationships are not strong enough to predict P exports to aquatic systems. Leaching and groundwater
transport of P usually contribute less than 10% of the total P transport, the remainder being transported
over the surface with runoff and erosion. Leached P, though, consists of soluble compounds with greater
bio-availability and greater effects on eutrophication. Amounts of leached P are positively related to
amounts of organic P and organic matter in the soil, and to P saturation resulting from over-fertilisation. P
losses from soil erosion and surface runoff can range from 0.1 to 10 kg ha-1y-1, or more on steep slopes.
Mixed land-use patterns of arable plots with forest fragments, meadows and pastures can halve total P
export rates from watersheds. Afforestation belts, meadow strips and mid-field marshes, both natural and
planted, are effective land-use modifications effecting P retention.
Land margins of undisturbed vegetation belts or riparian zones bordering aquatic ecosystems act as filters
for P and other nutrients, and also as catches for surface sediment runoff. Retention, particularly of soluble
P is greatly influenced by the water balance of the ecosystem (Kedziora et al., Ch. 13), and retained P may
be re-released under reducing conditions or in organic forms. In situations of high nutrient loads, it may
therefore be necessary to harvest P scavenger plants in riparian strips, or excavate P-saturated sediments
and transport them back to the agricultural source site.
Watersheds cannot be managed only to control P transfers; different management goals such as nutrient
conservation, the accretion and/or conservation of soil organic matter and the maintenance of the
biological diversity of ecosystems must be balanced. Environmental management should be based on a
network of small-scale, "closed" nutrient cycles associated with a variety of patches, spatially and
temporally connected in the landscape by water and nutrient flows. Control and prediction of nutrient
movement may depend on the number, size, shape, sequence and configuration of patches in a landscape.
This is a level of complexity well beyond current landscape-, erosion- and nutrient cycling- models. In
addition to sophisticated models of P transport in landscapes, a simplified approach has therefore been
developed, based on a susceptibility index using soil and site properties and management information
(Sharpley et al., Ch. 11).
Runoff and, to a lesser extent, groundwater move P to surface waters, which thus receive a mix of
dissolved and particulate P. This mix varies greatly with the amount of sediment in runoff, and with storm
intensity and duration. On average, surface waters contain 10 µg l-1 of soluble P, and suspended
sediments carry 1000 µg g-1 (Melak, Ch. 15). The average suspended sediment is therefore P enriched
relative to the source soils, and the solution P concentration is similar to that of average soil solutions, but
higher than rain water (Salcedo and Medeiros, Ch. 20). The resulting P exchanges between the solution
and suspended or bed sediments greatly modify P bioavailability. Uptake of P by algae reduces solution
concentrations, activating further exchanges. Eventually algae die, sediment out and/or re-release P for
further uptake cycles. Algal growth due to P is the main cause for eutrophication of inland waters,
although cells or populations may exhibit N or P limitation, or switch between them (Grobbelaar and
House, Ch. 16). Cellular storage and population changes shift productivity limitations from one element to
another, and introduce micro-scale and temporal variability. Therefore, the composition of surface waters
rarely reaches equilibrium, and is largely determined by the kinetics of processes (Melak, Ch. 15).
The overall result of these processes depends on a mosaic of patches along streams and in lakes with
different substrates and biota forming highly complex systems at micro- and meso-scales. The net result
of increased primary productivity from excess P, though, is relatively simple to gauge: increased biomass
and subsequent decomposition, which can lead to oxygen shortages, faunal death and reductions in water
quality. Many of the factors discussed are exemplified in the basin study of Lake Balaton, Hungary
(Herodek et al., Ch. 17), which receives river inflow, filtered through a constructed marsh, and direct
runoff from the surrounding agricultural and urban landscape.
Fluvial transport of biologically active P adds up to about 2 Tg y-1, all of which eventually reaches
estuaries and coastal seas (Howarth et al., Ch. 19). where eutrophication is an increasingly serious
problem (Fisher et al. Ch. 18). Nitrogen appears to play a more important role in the eutrophication of
coastal waters than P, but spatial variability is great. As freshwaters mix with tidal marine waters, both
geochemical and biological conditions change rapidly, and complex nutrient limitations involving N, P
and Si occur (Fisher et al. Ch. 18). Coastal biota such as corals, sea grasses and algal beds are particularly
sensitive to increased nutrient inputs. Effects are aggravated by long residence time of water masses in
deep channels, and continuous density stratification of the water caused by salinity gradients. The latter
predispose the water mass to oxygen deficiency resulting from excess biological productivity. Because of
the greater volumes over which biogeochemical processes are integrated in marine environments,
laboratory and microcosm studies become difficult to apply. As a result, the reliability of data on nutrient
cycles and on nutrient limitations decreases from fresh water environments to estuaries, coastal seas and
finally open oceans (Fisher et al. Ch. 18; Howarth et al. Ch. 19).
Tropical mangroves serve as an example of the full complexity of P cycles and transfers at the
terrestrial/aquatic interface (Salcedo and Medeiros Ch. 20). Here, fluvial P transport meets soils or
sediments undergoing periodic flooding with both fresh and saline waters. Both terrestrial plants and