Seagrasses: Biology, Ecology and Conservation
Article first published online: 21 NOV 2006
Volume 27, Issue 4, pages 431–432, December 2006
How to Cite
Ogden, J. (2006), Seagrasses: Biology, Ecology and Conservation. Marine Ecology, 27: 431–432. doi: 10.1111/j.1439-0485.2006.00138.x
- Issue published online: 21 NOV 2006
- Article first published online: 21 NOV 2006
‘Elephants eat seagrass.’ This startling, but dubious message from a colleague in Southeast Asia came thumping into my office in 1976 at the West Indies Laboratory in St Croix, U.S. Virgin Islands, on a tabletop-sized teletype machine, intended to be an experiment in international scientific communications. The machine, a considerable amount of research funding, and frequent visits to our laboratory by colleagues from all over the world, were among the benefits of being a co-investigator at a key study site for the Seagrass Ecosystem Study (SES), one of the programs of the International Decade of Ocean Exploration (IDOE) funded by the U.S. National Science Foundation from 1974 to 1979.
The SES was led by Peter McRoy of the University of Alaska, who with others recognized that the seagrasses, while numbering only approximately 60 species, were globally widespread and were virtually unexamined, particularly from an ecological point of view. The SES put seagrasses on the global map, produced a few hundred publications and several books, but more importantly helped to inspire the careers of numerous seagrass biologists and ecologists, many of whom are among the more than 80 authors of the 26 chapters of Seagrasses: Biology, Ecology and Conservation.
This exhaustively researched and superbly produced book begins with taxonomy, evolution and morphology, continues with biology, nutrient dynamics, physiology, community ecology, remote sensing, ecosystem ecology, and case studies. The progress in the field is dramatically shown by comparing it with the book edited by the late Phillips & McRoy (1980), who covered some similar topic areas and opened the door to the 25 years of subsequent research reviewed herein. The final chapters include landscape approaches to seagrass ecology and recommendations for their conservation in view of the increasing human disturbances currently dominating the global coastal zone. In contrast to the earlier book, the decline of seagrasses and the need for science-based management dominates the present concerns of all in the field.
Seagrasses are monocots, with at least four families and about 12 genera. Their taxonomy is still controversial but their diverse and distinctive morphologies are not fundamentally different from freshwater plants. There are strong indications that they evolved along more than one pathway and not from a common ancestor. Connectivity – merging physical oceanography and reproductive and genetic geographic patterns – is being investigated using the modern tools of genetic structure analysis. This approach demonstrates interesting patterns of gene flow and population structure in a few species, which will clearly have an impact on taxonomy, ecology and future management and conservation programs.
Typical of angiosperms, the seagrasses reproduce sexually and produce seeds. While seagrass pollen has fascinating adaptations to the problems of fertilization in a water environment, seagrass seeds bear no special adaptations for dispersal which are so characteristic of land plants. As the details have been worked out in only a few species, additional active and passive dispersal mechanisms may yet be discovered.
The influence of seagrasses on ecosystem processes, their chemical and physical environment, and their dynamics is covered in chapters 6 through 11. Fluid dynamics within and surrounding the densely packed leaves of the seagrass bed influences virtually every aspect of the biology and ecology of the grasses. Seagrass beds alter the chemical and physical environment of the water column and sediments as well as the processes of mineralization and nutrient cycling. The extent of this modification depends upon the species as well as the above- and below-ground biomass. Great progress has been made on modeling seagrass dynamics including clonal growth, shoot demographics and patch and gap dynamics over increasing time and space scales. These studies are critical for management.
The physics of light transmission through the water column and its influence on photosynthesis and remote sensing of seagrasses is covered in three integrated chapters. Seagrass ecosystems are among the most productive in the world. More than half of this production stays within the seagrass bed and much of the rest is exported and grazed. A significant component of seagrass ecosystems productivity is in the great diversity of epiphytes, which are also a primary food source for grazers.
At the start of the SES in the early 1970s, the prevailing view was that detritus was the primary pathway for seagrass production into higher trophic levels. By the end of the program, however, a large number of grazers had been identified and studied but the percentage of seagrass production going to direct grazers was still estimated to be relatively small. Research interest in seagrass herbivores and their importance has grown steadily (there is a large, useful table in Chapter 20), but the global estimated percentage of production grazed is still <30%. Recent historical ecology studies have concluded that the seagrass fauna was dominated by large herbivores, especially green turtles and fishes. This has given rise to an interesting question: have historical declines in the biomass of herbivores made anomalous the conclusion that seagrasses are detrital systems?
Seagrass nutritional quality is not well studied in relation to herbivore food selection, but most marine herbivores, among the notable exception of turtles and sirenids, depend upon high feeding and processing rates. Not to be forgotten is a great diversity of micro-herbivores that crop epiphytes that interfere with seagrass productivity.
The book includes three case study chapters on the biology and ecology of Zostera, Posidonia, and Thalassia respectively, which are all widespread geographically, sometimes in spectacularly disjunct distributions. The authors highlight highly productive coastal seagrass ecosystems as critical in stabilizing the coastal zone, providing substrate and food for a complex community of organisms, and as important indicators of coastal health. Wherever they occur, seagrass ecosystems are under threat from human disturbances, primarily coastal eutrophication, but also over-fishing and consequent changes in trophic structure. Genetics and ecosystem dynamics as well as restoration studies are key research needs. As seagrass research in developing countries is largely done by short-term, developed-country scientists, there is a need for capacity building in research and management.
The final chapters integrate seagrasses into the coastal mosaic of ecosystems, used by a variety of organisms in many ways, and subjected to increasing human disturbance. Mini-case studies of Florida Bay, Chesapeake Bay, and Cockburn Sound are useful. Seagrass systems are apparently particularly sensitive to pollutants that can cause phase shifts to dominance by epiphytic algae. The most successful management involves the catchment and control of sources of point- and non-point runoff of pollutants. The loss of seagrasses is well known to cause a decline in fisheries of organisms directly or indirectly dependent upon them.
What are the key attributes of conservation biology of seagrasses and what scientific input do managers need? Landscape ecology, drawn directly from terrestrial studies, but different in the inherent dynamism of seagrass systems, holds particular hope as a structure which can help to inform management. Landscape studies of seagrasses have examined patch dynamics along with the complex and rapidly changing interactions between organisms and their habitats in both decline and recovery. Mapping and geospatial analysis allow dramatic visualization of seagrasses over time and space, helping to stimulate the political will for conservation and management action. Not to be forgotten are the use- and non-use economic values of the services provided by seagrass systems.
To be expected in a work of this breadth, magnitude and complexity, one can quibble with the lack of integration between chapters as well as their grouping. The Editors acknowledge these difficulties, but in spite of attempts by many of the authors to integrate the chapters by cross-referencing, the structure of the book is still disjointed. This is not a major problem as the book will not be read cover-to-cover but rather by chapters as a reference work for specialists, and as a global tour of the field for aspiring research students. The book is less accessible to conservationists and managers on whose work the future sustainability of seagrass ecosystems depends. Perhaps this is a good project for the immediate future. In the meantime this book will stand for a number of years as the definitive statement on the status of research on seagrasses and seagrass ecosystems at the turn of the 21st century.