Just Cebrián

Patterns in the Fate of Production in Plant Communities

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.

MACROALGAL CANOPIES CONTRIBUTE TO EELGRASS (ZOSTERA MARINA) DECLINE IN TEMPERATE ESTUARINE ECOSYSTEMS

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 dur- ing summer 1998 within eelgrass populations in two estuaries of Waquoit Bay, Massachu- setts, 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 X 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 in- creased. 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 NH4+) 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 ma- croalgal 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.

The dependence of herbivory on growth rate in natural plant communities

1. The hypothesis that herbivory pressure (defined as the per cent of the photosynthetic tissue biomass and production consumed daily by herbivores) increases with increasing plant growth rate was tested. The basis for the test was a compilation of 56 published reports on biomass, production and herbivory, from a wide range of aquatic and terrestrial plant communities. 2. Herbivory was independent of ecosystem primary production and the fraction of plant production and biomass daily consumed by herbivores increased as the 0.6 and 1.6 power of plant turnover rate, respectively. 3. These results suggest that the tendency of fast-growing plants to support lower biomass of photosynthetic tissues than slow-growing ones can be accounted for by the tendency of herbivore control of plant biomass and production to increase with increasing plant growth rate

Patterns in leaf herbivory on seagrasses

Abstract We assess leaf herbivory on several populations of four temperate and five tropical seagrass species and examine, along with a compilation of published reports, the extent of herbivory variability and how it affects its general magnitude to help solve the apparent controversy about the importance of herbivory on seagrasses brought out by recent reports. We further test whether herbivory variability is related to differences in leaf specific growth rate (SGR) as a descriptor of leaf nutritional quality for herbivores. The extent of herbivory varied broadly, both within and among species, ranging from negligible values up to 50% of leaf production removed within some species. This variability, along with that shown by other published reports, claim that the classical statement that herbivory represents minor losses of seagrass production, being considered negligible in most cases, can be misleading and lead to the neglect of important seagrass–herbivore interactions. Differences among species in the percentage of leaf production removed were associated with differences in SGR, pointing to herbivore selective feeding upon faster-growing species resulting from their higher nutritional quality. This selection seems to be independent of leaf nutrient concentrations suggesting that, in agreement with past reports, nutrient levels are a poor descriptor of seagrass nutritional quality as most nutrients can be bound to indigestible fibre. No relationship between herbivory intensity and SGR was found among populations of a single species. On the other hand differences among species in the areal flux of production transferred to herbivores seemed related to differences in the level of production attained. These results point out that whereas SGR should be a descriptor of the variability among species in the extent of herbivore pressure (i.e. percentage of production removed), the level of production should be indicative of differences among species in their capacity to support herbivore production (i.e. flux of production channelled).

Role of Salt Marshes as Part of Coastal Landscapes

Salt marshes are located between land and coastal water environments, and nutrient and production dynamics within salt marshes interact with those of adjoining ecosystems. Salt marshes tend to export materials to deeper waters, as shown by mass balance and stable isotopic studies. Salt marshes also intercept land-derived nutrients, and thus modify the potential response of phytoplankton, macroalgae, and seagrasses in the receiving estuarine waters. In particular, the maintenance of eelgrass meadows seems to depend on the ability of fringing salt marshes to intercept land-derived nitrogen. The bulk of the interception of land-derived nitrogen is likely to be the result of relatively high rates of denitrification characteristic of salt marshes. Thus, through exports of energy-rich materials, and interception of limiting nutrients, salt marsh parcels interact in quantitatively important ways with adjoining units of landscape. These interactions are of importance in understanding the basic functions of these mosaics of different coastal systems, as well as provide information needed to manage estuaries, as for example, in conservation of valuable eelgrass meadows.

Plant Growth-Rate Dependence of Detrital Carbon Storage in Ecosystems

Detrital carbon accumulation accounts for most of an ecosystems capacity to store organic carbon because the carbon contained as plant detritus exceeds that stored in living plants by about threefold. A comparative analysis of the mass and turnover of detrital carbon in ecosystems demonstrates that these properties are strongly related to the turnover rate of the dominant primary producers and are poorly related to ecosystem primary production. These results contribute to an understanding of the factors that control carbon storage in ecosystems and the role of carbon storage in the global carbon budget.

OPERATIONALIZING SUSTAINABILITY: MANAGEMENT AND RISK ASSESSMENT OF LAND-DERIVED NITROGEN LOADS TO ESTUARIES

Sustainable coastal management requires that the goals and means of management be made operational and specific. We use Waquoit Bay, Massachusetts, as a case study, to suggest a decision-making process that brings updated scientific results forward while incorporating stakeholder concerns. Land-derived nitrogen loading is the major agent of change for receiving estuaries in the Waquoit Bay estuarine complex, so control of nitrogen loading rates is a principal goal of land management plans. We can establish the relationships of land use pattern to nitrogen loading rates, and of loading rates to mean annual concentrations of nitrogen in the estuaries. The latter, in turn, can be related quantitatively to mean annual production and biomass of phytoplankton, macroalgae, and eelgrass. We propose that phytoplankton, macroalgal, and eelgrass production and biomass are suitable end point measures that can be made meaningful to stakeholders. We define the relationship of agent of change vs. end point measure, and ...

Epiphyte Accrual on Posidonia oceanica (L.) Delile Leaves: Implications for Light Absorption

Abstract We examined the pattern of epiphyte accrual along the life-span of Posidonia oceanica leaves, both for the total epiphyte community and the main epiphyte groups (i. e. red encrusting algae and brown erect algae). Moreover, we document the importance of this epiphyte accrual pattern for evaluating P. oceanica-epiphyte interactions by assessing the dependence of the quantity and quality of light absorbed by epiphytes on their accrual pattern. Epiphyte biomass increased with leaf age following a sigmoidal curve (r2 = 0.90, P < 0.001), both for the epiphyte community and for the two main groups. Total epiphyte biomass increased with leaf age at a rate of about 0.03 day−1 to reach a constant maximum value of 2.60 mg DW cm−2 on leaves older than 200 days. Brown erect algae grew about an order of magnitude faster (0.120 day−1) than red encrusting ones (0.017 day−1). However, the former group of algae reached their maximum biomass (1 mg DW cm−2) on 150 days-old leaves, whereas red encrusting algae continued to grow along the whole leaf life-span to reach a maximum biomass of 1.70 mg DW cm−2. The non-linear increase in epiphyte biomass with leaf age involved a non-linear increase in epiphyte light absorption with leaf age, which reached a maximum constant value of 30% of incident light on 250 days-old leaves. Moreover, because red encrusting algae contribute a higher fraction to total epiphyte biomass on older leaves, we observed a shift in absorbed light quality with increasing leaf-age. Our results indicate the importance of accounting for the pattern of epiphyte accrual with leaf age when assessing seagrass-epiphytes interactions, especially for long-lived seagrass species where epiphytes may differ much in growth and biomass between young and old leaves.

Leaf growth response to simulated herbivory: a comparison among seagrass species

We examined in seven seagrass species the response of the leaf growth rate per shoot (mg DW shoot−1 day−1) to a gradient of herbivory simulated by leaf clipping. The clipping procedure was intended to mimic the removal by herbivores which only consume the leaves of a single shoot at every feeding attack and which do not feed over the same shoots selectively (i.e., most poikilotherm vertebrate and invertebrate herbivores). We tested whether (1) this defoliation procedure does not normally depress shoot leaf growth rates (i.e., the occurrence of compensatory leaf growth), and (2) whether leaf nutrient content, relative leaf growth rate, average distance between consecutive short shoots and rhizome diameter influence the response of the leaf growth rate per shoot to a gradient of defoliation. The leaf growth rate per shoot varied among clipping treatments in nine of the 15 populations treated (ANOVA, p<0.05) and meta-analyses techniques revealed a significant overall variation (χ2 test, p<0.001) when all the populations were considered in concert. The leaf growth rate per shoot was persistently depressed in all the clipping treatments only in one of the 15 populations treated, with only three more populations showing depressed leaf growth under some treatments (Tukey HSD test, p<0.05). The response of the leaf growth rate to clipping intensity, which was analysed on a per shoot basis (i.e. relationship between the leaf growth rate per shoot and clipping intensity on the shoot) was significant only for four populations, although meta-analyses revealed a tendency towards a general significance. None of the seagrass properties considered was related to the response of leaf growth to clipping intensity. Our results stress the remarkable variability seagrass leaf growth may exhibit under single events of defoliation on scattered shoots. Furthermore, because leaf growth rates are rarely depressed, these results suggest that most poikilotherm vertebrate and invertebrate herbivores, which typically remove <30% of leaf production, have a modest impact on the depression of leaf growth rates through removal of photosynthetic tissue.

Herbivory on the seagrass Cymodocea nodosa (Ucria) Ascherson in contrasting Spanish Mediterranean habitats

We assess the magnitude and variability of herbivory (i.e. leaf consumption and sloughing caused by herbivore bites) on the seagrass Cymodocea nodosa along the Spanish Mediterranean coast and test the hypothesis that this is higher in meadows growing in sheltered bays than in exposed, open zones. Total leaf loss by herbivores varied by about three orders of magnitude along the Spanish Mediterranean coast, from < 1 to 130 mg DW shoot−1 yr−1. These differences were paralleled by a great variation in the fraction of leaf production lost by herbivores, which ranged from < 1 to about 50%. Most (75%) of the populations, however, supported modest losses of leaf production (< 10%). A significant fraction (30%) of the variance in herbivory was explained by meadow exposure, the meadows growing in sheltered bays suffering about five times the losses encountered in open sites. These results suggest that the trophic importance of C. nodosa as a food resource for herbivores, and the role of herbivores on the control of the seagrass production, increases notably from exposed to sheltered meadows in the Spanish Mediterranean and point to the importance of considering the degree of exposure when addressing herbivory on other seagrass species.