A Field Manual Focused on the Biocomplexity on Mangrove Ecosystems

Edited by:

Ilka C. Feller
Smithsonian Environmental Research Center,
Smithsonian Institution

Marsha Sitnik
Department of Systematic Biology
Smithsonian Institution

Editors' preface

Intertidal mangrove swamp communities dominate the world's tropical and subtropical coasts, paralleling the geographical distribution of coral reefs. They survive in substrate salinities ranging from fresh water alongside rivers to hypersaline ponds and mudflats. Mangrove plants are pioneer as well as_ mature-phase species. They not only help form these environments, they create habitats for a diverse and characteristic community, including numerous mangrove-dependent organisms.

The term "mangrove" refers to an ecological rather than a taxonomic assemblage of plants. Worldwide, 34 species or so in nine genera in five families are considered "true mangroves" and another 80 or so species occur as "minor components" and "mangrove associates." True mangroves are ecologically restricted to tidal swamps and form extensive monospecific stands. They are morphologically adapted with aerial roots and vivipary. As halophytes, they are physiologically adapted for either salt exclusion or excretion. Taxonomically, true mangroves are isolated from their nearest terrestrial relatives, at least at the generic level.

Mangroves are critical, not only for sustaining biodiversity in these intertidal swamps, but also for their direct and indirect benefit to human activities. Energy and nutrients are assimilated and stored in leaves of mangrove trees. As a detritus-based ecosystem, leaf litter from these trees provides the basis for adjacent aquatic and terrestrial food webs. Because most energy and nutrients are biotically stored rather than free in the water or substrate, species diversity of these swamps is directly dependent on primary productivity by mangrove plants. Mangrove swamps function as nurseries for most of the sport and commercial fishes found in deeper waters. Mangrove swamps also provide feeding grounds for large reef fishes. As a result, mangrove-assimilated energy and nutrients are exported to surrounding coral reefs.

Besides supporting and renewing coastal fishing stock, mangrove swamps also benefit human economic development by stabilizing shorelines. This is a critical function in tropical coastal areas that are periodically battered by tropical storms and hurricanes. Bangladesh offers examples of the devastation that can happen when deltaic and island mangrove swamps are deforested. "Land" in mangrove swamps is peat, produced primarily by red-mangrove rootlets. This organic substrate is not soil and cannot sustain human activities such as agriculture, buildings, or dredging. Undisturbed, mangrove swamps can buffer the effects of storms and protect property and human life behind these coastal fringes.

Mangrove forests are generally oligotrophic ecosystems. Human-caused enrichment is one of the major global threats to these and other coastal environments. Our experiments show that nutrients are not uniformly distributed among or even within mangrove forests and that soil fertility can switch from conditions of nitrogen to phosphorus limitation across narrow gradients. Likewise, not all ecological processes respond similarly to the same nutrient. Enrichment affects plant growth, metabolism, and tissue quality, which in turn affect primary consumption. It alters litter quality, thereby linking nutrient enrichment to detritivory, decomposition, and below-ground processes. Interactions and feedback among nutrient availability, microbial communities, nutrient cycling, and higher trophic levels have not been explored in mangrove forests. Enrichment with nitrogen and phosphorus may affect microbial communities and activity, nutrient dynamics, photosynthesis, and ultimately, peat accumulation and habitat stability. Furthermore, hydrology and sediment physico-chemical parameters may also influence these interactions and feedbacks.

Despite their ecological and economic significance, mangrove swamps are an imperiled ecosystem. Throughout the tropics, they are threatened directly and indirectly by commercial development. Tropical and subtropical mangroves support adjacent marine communities and ecosystems, including seagrass beds and coral reefs. However, little is known about the intra-wetland processes that regulate those interactions and how they are affected when the coastal zone is altered by eutrophication.

Increased industrial development as well as destruction of inland tropical forests for agricultural development indirectly impact coastal and island mangrove swamps through water pollution from industrial effluent, oil spills, sewer discharge, and agricultural and sediment runoff. Oils and sediments destroy mangroves by coating their stilt roots and pneumatophores; this blocks air passages that ventilate the submerged root systems and enable aerobic respiration to occur. Sewer discharge and agriculture runoff dump large amounts of nitrogen and phosphorus into coastal waters, drastically changing 'nutrient conditions. Although research is ongoing to determine the effects of nutrient over-enrichment on mangrove plant growth and interactions with other organisms, evidence is accumulating that increased nutrient availability to the primary producers results in increased insect populations and increased damage to mangroves by insect herbivores. This modification at the base of both aquatic and terrestrial food webs may impact the entire mangrove system in ways not yet understood or studied.

The objectives of this manual are:

1. Introduce students to the biological complexity of mangrove ecosystems;
2. Provide students with training and experience in field ecology;
3. Offer opportunities for students to formulate and test hypotheses in a field setting, to conduct research in a field setting, and to summarize and present results based on data collected in the field;
4. Encourage and foster self education and discovery;
5. Initiate and stimulate cultural interactions and scientific collaborations among students of different backgrounds;
6. Increase the awareness of the ecological roles, economic importance, and cultural significance of mangrove ecosystems

Ilka C. Feller
Marsha Sitnik
October 2002

Acknowledgements

This manual is based on a Mangrove Ecology course sponsored by the Smithsonian Institution's Caribbean Coral Reef Ecosystems Program in cooperation with Belize's Department of Fisheries, Department of Forestry, and the Institute of Marine Studies of the University of Belize. It is a collaborative effort by ten authors who helped develop and teach this course at Turneffe Atoll, Belize from 1993 to 2002. Material in this manual is based upon work supported by the National Science Foundation under Grant No. 9981535 to Ilka C. Feller. Any opinions, findings, and conclusions or recommendations expressed in this manual are those of the authors and do not necessarily reflect the views of the National Science Foundation. Additional support was provided by the John D. and Catherine T. MacArthur Foundation.

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