The fate of photosynthetic carbon in marine ecosystems dominated by different types of primary producers was examined by compiling published reports on herbivory, autotrophic respiration, decomposition, carbon storage, and export rates as fractions of net primary production (NPP) in ecosystems dominated by different types of autotrophs (i.e. oceanic and coastal phytoplankton, microphytobe.lthos, coral reef algae, macroalgae, seagrasses, marsh plants, and mangroves). A large fraction (>40%) of ,.he NPP of marine ecosystems is decomposed within the system, except for microphytobenthos (decomposition, -25% of NPP). Herbivory tends to be highest for microalgae (planktonic and benthic, >40% of NPP) and macroalgae (33.6+4.9% of NPP) and is somewhat less for higher plants. Microphytobenthos export on average a much higher proportion of their NPP than do other microalgal communities, whereas marine macrophytes, except marsh plants, export a substantial proportion (24.3-43.5% on average) of their NPP. The ?-action of NPP stored in sediments is 4-fold greater for higher plants (-1 O-l 7% of NPP) than for algae (0.4-6% of NPP). On average -90% of the phytoplankton NPP is used to support local heterotrophic metabolism (i.e. grazed or decomposed). This fraction is even higher in oceanic communities. Mangrove forests, and to a lesser extent seagrass meadows and macroalgal beds, produce organic carbon well in excess of the ecosystem requirements, with excess photosynthetic carbon (i.e. export rate plus storage) in these ecosystem; representing -40% of NPP. Extrapolation of these results to the global ocean identifies marine angiosperms, which only contribute 4% of total ocean NPP, as major contributors ofthe NPP stored (30% of total ocean carbon storage) and subsequently buried in marine sediments. Consideration of burial of NPP from marine angiosperms should lead to estimates of total burial of marine NPP that exceed current estimates by 15-50%.

I examine, through an extensive compilation of published reports, the nature and variability of carbon flow (i.e., primary production, herbivory, detrital production, decomposition, export, and biomass and detrital storage) in a range of aquatic and terrestrial plant communities. Communities composed of more nutritional plants (i.e., higher nutrient concentrations) lose higher percentages of production to herbivores, channel lower percentages as detritus, experience faster decomposition rates, and, as a result, store smaller carbon pools. These results suggest plant palatability as a main limiting factor of consumer metabolical and feeding rates across communities. Hence, across communities, plant nutritional quality may be regarded as a descriptor of the importance of herbivore control on plant biomass ("top-down" control), the rapidity of nutrient and energy recycling, and the magnitude of carbon storage. These results contribute to an understanding of how much and why the trophic routes of carbon flow, and their ecological implications, vary across plant communities. They also offer a basis to predict the effects of widespread enhancement of plant nutritional quality due to large-scale anthropogenic eutrophication on carbon balances in ecosystems.

Describing the relative magnitude and controls of herbivory and decomposition is important in understanding the trophic transference, recycling, and storage of carbon and nutrients in diverse ecosystems. We examine the variability in herbivory and decomposition between and within a wide range of aquatic and terrestrial ecosystems. We also analyze how that variability is associated with differences in net primary production and producer nutritional quality. Net primary production and producer nutritional quality are uncorrelated between the two types of system or within either type. Producer nutritional quality is correlated to the percentage of primary production consumed by herbivores or percentage of detrital production decomposed annually, regardless of whether the comparison is made between the two types of systems or within either type of system. Thus, producer nutritional quality stands out as a consistent indicator of the importance of consumers as top-down controls of producer biomass and detritus accumulation and nutrient recycling. However, absolute consumption by herbivores and absolute decomposition (both in g C·m Ϫ2 ·yr Ϫ1 ) are often associated with absolute primary production and independent of producer nutritional quality, because the variability in net primary production across systems largely exceeds that in the percentage consumed or decomposed. Thus, primary production often stands out as an indicator of the absolute flux of producer carbon transferred to consumers and of the potential levels of secondary production maintained in the system. These patterns contribute to our understanding of the variability and control of herbivory and decomposition, and implications on carbon and nutrient cycling, in aquatic and terrestrial ecosystems. Furthermore, in view of their robustness, they may offer a template for global change models seeking to predict anthropogenic effects on carbon and nutrient fluxes.

Plant-herbivore interactions mediate the trophic structure of ecosystems. We use a comprehensive data set extracted from the literature to test the relative explanatory power of two contrasting bodies of ecological theory, the metabolic theory of ecology (MTE) and ecological stoichiometry (ES), for per-capita and population-level rates of herbivory across ecosystems. We found that ambient temperature and herbivore body size (MTE) as well as stoichiometric mismatch (ES) both constrained herbivory, but at different scales of biological organization. Herbivore body size, which varied over 11 orders of magnitude, was the primary factor explaining variation in per-capita rates of herbivory. Stoichiometric mismatch explained more variation in population-level herbivory rates and also in per-capita rates when we examined data from within functionally similar trophic groups (e.g. zooplankton). Thus, predictions from metabolic and stoichiometric theories offer complementary explanations for patterns of herbivory that operate at different scales of biological organization.

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All seagrasses are rhizomatous plants that grow by reiteration of a limited set of modules. Their past growth history can therefore be reconstructed from the scars left by abscised leaves and flowers on the long-lived rhizomes or the seasonal slgnals Imprinted in the frequency and size of their modules. We provide here the basic foundations and assumptions of these reconstruction techniques and the calculations involved in their application. We then show their reliability and potential to quantlfy an array of ecological processes, such as plant demography, leaf and rhlzome production, flowering ~ntensity, and seagrass responses to anthropogenic perturbations, based on our recent studies of Mediterranean, Caribbean and Indo-Pacific seagrass species. Reconstruction techniques have also proven useful in demonstrating the role of seagrasses as tracers of sedlment movement over seagrass beds and the rates of colonisation and expansion of seagrass patches. These reconstruction techniques should provide a powerful tool to improve our knowledge of seagrass species and populations from remote areas based on a single or just a few visits This should, therefore, allow us to sample many seagrass meadows using limited resources, thus generating a strong foundation for the study of comparatlve seagrass ecology and testing of theories previously applied to terrestrial plant populations.

Loss of eelgrass (Zostera marina) habitat from temperate estuaries worldwide often coincides with increased macroalgal accumulations resulting from increased delivery of anthropogenic nitrogen. We conducted macroalgal enclosure/exclosure experiments during summer 1998 within eelgrass populations in two estuaries of Waquoit Bay, Massachusetts, USA, to evaluate how increased macroalgal biomass affects density, recruitment, growth rate, and production of eelgrass. One estuary featured a low nitrogen loading rate and sustained a relatively pristine eelgrass population with a 2 cm high macroalgal canopy. The other estuary had a sixfold higher nitrogen loading rate and a declining eelgrass population with a 9 cm high macroalgal canopy. Experimental units were 1 ϫ 1 m plots of eelgrass fenced within 50 cm high plastic mesh that excluded or included macroalgae at canopy heights ranging from 0 to 25 cm. In both estuaries, rates of eelgrass loss increased, largely a result of decreased recruitment, and growth rates decreased (due to decreased rates of leaf appearance) with increasing macroalgal canopy height. Aboveground summer production in both estuaries decreased exponentially as macroalgal canopy heights increased. We conclude that macroalgal cover is a proximate cause for loss of eelgrass in the higher N estuary since, upon removal of macroalgae, we observed an increase in shoot density, a 55% increase in summer growth, and a 500% increase in summer aboveground net production. Based on summer growth data and density of shoots in our experimental plots the following spring, we suggest that the negative impacts of macroalgal canopies persist, but also that eelgrass recovery upon removal of macroalgae may be possible.To identify the mechanisms by which macroalgae potentially inhibit eelgrass production, we measured changes in nutrient and oxygen concentrations resulting from macroalgal canopies and estimated the relative importance of summer standing stocks of phytoplankton, epiphytes, and macroalgae to potential shading of eelgrass in both estuaries. We document both (1) unfavorable biogeochemical conditions (lowered redox conditions and potentially toxic concentrations of NH 4 ϩ ) imposed by the presence of macroalgal canopies and (2) potential light limitation of eelgrass by standing stocks of producers in the higher N estuary, with estimates of light reduction via macroalgae numerically more important than light sequestration by phytoplankton and epiphytes for newly recruiting shoots. Increased macroalgal biomass associated with increased nitrogen loading to estuaries can lead to eelgrass disappearance, and we identify an approximate 9-12-cm critical macroalgal canopy height at which eelgrass declines.