Importance of within-shoot epiphyte distribution for the carbon budget of seagrasses: the example of Posidonia oceanica

6 páginas, 4 figuras, 1 tablaTo assess the effects of shading by epiphytes on the carbon balance of the seagrass Posidonia oceanica, we examined the patterns of within-shoot epiphyte abundance on leaves, and their variability with season and depth. Epiphyte biomass was found to depend on leaf age (larger epiphyte load on older tissues), leaf side (more epiphytes on the inner face than on the outer) and depth (more epiphytes in deep meadows). Depth differences were maximum in spring and disappeared in late summer. Percent light absorbed (absorptance) by epiphytes was measured; light absorptance followed an exponential- saturated model with epiphyte biomass. Combining these data of percent light absorbed, within-shoot epiphyte biomass distribution and an existing carbon balance model, we conclude that reduction in carbon gains caused by epiphyte shading is relatively small, and greater in deep meadows (8.8% on average, with values up to 14.2% in May) than that in shallow water (4.7% on average with maximum values of 7% in August). This is mainly due to the accumulation of epiphytes on old tissues, which contribute only marginally to the carbon gain of the plant. Using the same procedure, we modelled the effects of a doubling in epiphyte biomass, conserving the observed within-shoot distribution. The result was a very small additional carbon loss. However, using the same biomass but modifying the distribution (shifting the same abundance towards younger leaf age classes), the reduction in carbon gains was dramatic, particularly in deep meadows (between 21% and 41%, depending on the shift used). Therefore, it is the epiphyte growth timing rate rather than the final biomass reached which seems to be a key control for Posidonia oceanica survival, especially near the deep limit of its distribution.Financial support was provided by a grant from CICYT (REN2002-04020).Peer reviewe

Matrix composition and patch edges influence plant-herbivore interactions in marine landscapes

38 páginas, 4 figuras, 3 tablas.1. The functioning of ecosystems can be strongly driven by landscape attributes. Despite its importance, however, our understanding of how landscape influences ecosystem function derives mostly from species richness and abundance patterns, with few studies assessing how these relate to actual functional rates. 2. We examined the influence of landscape attributes on the rates of herbivory in seagrass meadows, where herbivory has been identified as a key process structuring these relatively simple systems. The study was conducted in three representative Posidonia oceanica meadows. The principal herbivores in these meadows are the fish Sarpa salpa and the sea urchin Paracentrotus lividus, and we hypothesised that differences in their interaction with landscape attributes would significantly influence herbivory rates. 3. We measured herbivore abundance, herbivory rates, primary production and plant quality (C:N) in seagrass patches embedded either in rock or in sand (matrix attribute), in patches either near or far from a rocky reef (distance attribute) and at the edges and interior of meadows. 4. Our results show that matrix and meadow edges significantly affected the actual levels of herbivory. Herbivory rates were higher in seagrass patches embedded in a rocky matrix compared to those on sand, and herbivory at the centre of seagrass meadows was higher than at the edges. In contrast, patch distance to rocky reefs did not affect herbivory. Neither herbivore abundance nor food quality explained the patterns across different landscape attributes. This suggests that variation in herbivory across the landscape may be related much more to behavioural differences between species in their evaluation of risk, movement, and food preference in relation to the landscape structure. 5. Our results indicate that richness and abundance patterns may mask critical interactions between landscape attributes and species responses, which result in considerable heterogeneity in the way key functional processes like herbivory are distributed across the ecosystem mosaic.The Spanish Ministry of Science and Innovation funded this research (projects CTM2010-22273-C02-01 and 02). The Spanish Ministry of Education supported JP (scholarship AP2008-01601) and the Spanish National Research Council supported AG (scholarship JAEPre_08_00466).Peer reviewe

Habitat and scale shape the demographic fate of the keystone sea urchin <i>Paracentrotus lividus</i> in Mediterranean macrophyte communities

Demographic processes exert different degrees of control as individuals grow, and in species that span several habitats and spatial scales, this can influence our ability to predict their population at a particular life-history stage given the previous life stage. In particular, when keystone species are involved, this relative coupling between demographic stages can have significant implications for the functioning of ecosystems. We examined benthic and pelagic abundances of the sea urchin Paracentrotus lividus in order to: 1) understand the main life-history bottlenecks by observing the degree of coupling between demographic stages; and 2) explore the processes driving these linkages. P. lividus is the dominant invertebrate herbivore in the Mediterranean Sea, and has been repeatedly observed to overgraze shallow beds of the seagrass Posidonia oceanica and rocky macroalgal communities. We used a hierarchical sampling design at different spatial scales (100 s, 10 s and &lt;1 km) and habitats (seagrass and rocky macroalgae) to describe the spatial patterns in the abundance of different demographic stages (larvae, settlers, recruits and adults). Our results indicate that large-scale factors (potentially currents, nutrients, temperature, etc.) determine larval availability and settlement in the pelagic stages of urchin life history. In rocky macroalgal habitats, benthic processes (like predation) acting at large or medium scales drive adult abundances. In contrast, adult numbers in seagrass meadows are most likely influenced by factors like local migration (from adjoining rocky habitats) functioning at much smaller scales. The complexity of spatial and habitat-dependent processes shaping urchin populations demands a multiplicity of approaches when addressing habitat conservation actions, yet such actions are currently mostly aimed at managing predation processes and fish numbers. We argue that a more holistic ecosystem management also needs to incorporate the landscape and habitat-quality level processes (eutrophication, fragmentation, etc.) that together regulate the populations of this keystone herbivore

Matrix composition and patch edges influence plant-herbivore interactions in marine landscapes

38 páginas, 4 figuras, 3 tablas.1. The functioning of ecosystems can be strongly driven by landscape attributes. Despite its importance, however, our understanding of how landscape influences ecosystem function derives mostly from species richness and abundance patterns, with few studies assessing how these relate to actual functional rates. 2. We examined the influence of landscape attributes on the rates of herbivory in seagrass meadows, where herbivory has been identified as a key process structuring these relatively simple systems. The study was conducted in three representative Posidonia oceanica meadows. The principal herbivores in these meadows are the fish Sarpa salpa and the sea urchin Paracentrotus lividus, and we hypothesised that differences in their interaction with landscape attributes would significantly influence herbivory rates. 3. We measured herbivore abundance, herbivory rates, primary production and plant quality (C:N) in seagrass patches embedded either in rock or in sand (matrix attribute), in patches either near or far from a rocky reef (distance attribute) and at the edges and interior of meadows. 4. Our results show that matrix and meadow edges significantly affected the actual levels of herbivory. Herbivory rates were higher in seagrass patches embedded in a rocky matrix compared to those on sand, and herbivory at the centre of seagrass meadows was higher than at the edges. In contrast, patch distance to rocky reefs did not affect herbivory. Neither herbivore abundance nor food quality explained the patterns across different landscape attributes. This suggests that variation in herbivory across the landscape may be related much more to behavioural differences between species in their evaluation of risk, movement, and food preference in relation to the landscape structure. 5. Our results indicate that richness and abundance patterns may mask critical interactions between landscape attributes and species responses, which result in considerable heterogeneity in the way key functional processes like herbivory are distributed across the ecosystem mosaic.The Spanish Ministry of Science and Innovation funded this research (projects CTM2010-22273-C02-01 and 02). The Spanish Ministry of Education supported JP (scholarship AP2008-01601) and the Spanish National Research Council supported AG (scholarship JAEPre_08_00466).Peer reviewe

Resource allocation and sucrose mobilization in light-limited eelgrass Zostera marina

Este artículo contiene 12 páginas, 9 figuras, 4 tablas.This study evaluated the ability of Zostera marina L. (eelgrass) to balance the daily photosynthetic deficit by mobilization of carbon reserves stored in below-ground tissues during a period of extreme winter light limitation. A quantitative understanding of the mobilization process and its limitations is essential to the development of robust models predicting minimum light levels required to maintain healthy seagrass populations. Plants were grown in running seawater tanks under 2 light regimes. One treatment was provided with 2 h irradiance-saturated photosynthesis (Hsat) to produce severe Light Limitation, while control plants were grown under 7 h Hsat, simulating the typical wintertime condition in Monterey Bay, California, USA. Although plants maintained under 2 h Hsat were more severely carbon limited than plants grown under 7 h Hsat, whole-plant carbon balance calculated from metabolic needs and growth rates was negative for both Hsat treatments. The eelgrass studied here responded to negative carbon balances by suppressing the production of new roots, depleting sucrose reserves, and effecting a gradual decrease in growth rate and an increase in the activity of sucrose synthase (SS, E.C. 2.4.1.13) in sink tissues in the terminal stages of carbon stress. The 7 h Hsat plants survived the 45 d course of the experiment while the plants grown under 2 h Hsat died within 30 d, even though one-third of their carbon reserves remained immobilized in the rhizome. Thus. extreme Light limitation can prevent full mobilization of carbon reserves stored in below-ground tissues, probably through the effects of anoxia on translocation. Metabolic rates, particularly photosynthesis and respiration of the shoot, were unaffected by prolonged carbon limitation in both treatments. The patterns observed here can provide useful indices for assessing the state and fate of seagrass ecosystems in advance of catastrophic declines.Financial support was provided by a CIRIT (Comissionat per a Universitats i Recerca)Trainlng Grant to T.A, and by grant OCE-9223265 from the US National Science Foundation to R.C.Z. and R.Peer reviewe

Resource Allocation and Sucrose Mobilization In Light Limited Eelgrass Zostera marina

This study evaluated the ability of Zostera marina L. (eelgrass) to balance the daily photosynthetic deficit by mobilization of carbon reserves stored in below-ground tissues during a period of extreme winter light limitation. A quantitative understanding of the mobilization process and its limitations is essential to the development of robust models predicting minimum light levels required to maintain healthy seagrass populations. Plants were grown in running seawater tanks under 2 light regimes. One treatment was provided with 2 h irradiance-saturated photosynthesis (Hsat) to produce severe Light Limitation, while control plants were grown under 7 h Hsat, simulating the typical wintertime condition in Monterey Bay, California, USA. Although plants maintained under 2 h Hsat were more severely carbon limited than plants grown under 7 h Hsat, whole-plant carbon balance calculated from metabolic needs and growth rates was negative for both Hsat treatments. The eelgrass studied here responded to negative carbon balances by suppressing the production of new roots, depleting sucrose reserves, and effecting a gradual decrease in growth rate and an increase in the activity of sucrose synthase (SS, E.C. 2.4.1.13) in sink tissues in the terminal stages of carbon stress. The 7 h Hsat plants survived the 45 d course of the experiment while the plants grown under 2 h Hsat died within 30 d, even though one-third of their carbon reserves remained immobilized in the rhizome. Thus. extreme Light limitation can prevent full mobilization of carbon reserves stored in below-ground tissues, probably through the effects of anoxia on translocation. Metabolic rates, particularly photosynthesis and respiration of the shoot, were unaffected by prolonged carbon limitation in both treatments. The patterns observed here can provide useful indices for assessing the state and fate of seagrass ecosystems in advance of catastrophic declines

Pseudovivipary, a new form of asexual reproduction in the seagrass Posidonia oceanica

Short communicationThe seagrass Posidonia oceanica is able to reproduce by asexual formation of plantlets directly in the inflorescence. Pseudoviviparous plantlets were observed during May 2004 in 26% of the inflorescences in a P. oceanica meadow off the island of Formentera (Balearic Islands, Western Mediterranean Sea). We do not know how frequently pseudovivipary occurs in Posidonia oceanica, nor the mechanisms triggering it, but this strategy can contribute significantly to short-distance dispersal and meadow maintenance, which is especially relevant in a species whose meadows can persist for thousands of years and whose colonization of new space occurs very slowly.This work was supported by the Fisheries Department (Direcció General de Pesca) of the Balearic Government.Peer reviewe

Production ecology of Posidonia oceanica (L.) Delile meadows in Nueva Tabarca Marine Reserve: Growth, biomass and nutrient stocks along a bathymetric gradient

The effects of the interaction between depth and nutrients on the primary production of the seagrass Posidonia oceanica in a meadow off Nueva Tabarca island (E Spain) were investigated by assessing C, N and P concentration in the plant, the resource allocation of the different nutrients and the response of the plant to nutrient additions along a bathymetric gradient (1-24 m). The results point to a nutrient limitation in the shallow zone (1- 5 m). The N and P concentrations were low (0.7-1.8 %N; 0.05-0.1%P relative to dry weight) and the N/P atomic ratio high (30-40). Clear tissue enrichment was found after both N and P addition, with lowering of the N/P ratio (to values of 25 by atoms). However, such a possible limitation was not supported by an increment in leaf growth after nutrient addition, although some of the growth features were modified (e.g., relative growth contribution of different leaves to total shoot growth). In a plant with a complex growth mechanism such as Posidonia oceanica, a short-term nutrient addition (1 month) do not suffice to induce a significant growth response. Thus, we conclude that P. oceanica growth off Nueva Tabarca island is probably nutrient limited in shallow areas (1-5 m) in early summer, and that this limitation is due to different concurrent factors: general oligotrophy of the area, massive leaf litter export and leaf losses due to both hydrodynamism and grazing.Ecología de la producción de la fanerógama marina Posidonia oceanica (L.) Delile en la reserva submarina de la isla de Nueva Tabarca: crecimiento, biomasa y nutrientes a lo largo de un transecto batimétrico. En este trabajo se ha analizado el efecto de la interacción entre la profundidad y los nutrientes sobre la producción primaria de Posidonia oceanica en una pradera de la isla de Nueva Tabarca (este de la península Ibérica, España) mediante el estudio de la concentración de C, N y P en los tejidos vegetales, la variación en la asignación de recursos y la respuesta de la planta frente a adiciones de nutrientes a lo largo de un gradiente batimétrico (1-24 m). Los resultados obtenidos indican una limitación por nutrientes en la zona somera (1-5 m). Las concentraciones de N y P en los tejidos son bajas (0,7-1,8 % N; 0,05-0,1 % P respecto a peso seco), el cociente atómico N:P es elevado (30-40), y las hojas de las plantas muestran un enriquecimiento tisular en nutrientes después de la fertilización tanto con N como con P, con una disminución asociada del cociente N:P (hasta valores de 25). Pero esta limitación no fue corroborada por un incremento neto del crecimiento foliar después de la adición de nutrientes, a pesar de que algunas características del crecimiento resultaron modificados (por ejemplo, la contribución relativa de las distintas hojas al crecimiento del haz). La duración de la fertilización (un mes) no fue suficiente para eliminar o mitigar la limitación por nutrientes en una planta con un mecanismo de crecimiento complejo como el que presenta Posidonia oceanica. Concluimos que el crecimiento de P oceanica en los fondos someros (1-5 m) de la isla de Nueva Tabarca está limitado por nutrientes a principios de verano y que esta limitación es debida a varios factores coincidentes: oligotrofia general del área, exportación masiva de hojarasca y pérdidas de material foliar por hidrodinamismo y acción de los herbívoros

The richness of small pockets:Decapod species peak in small seagrass patches where fish predators are absent

Este artículo contiene 6 páginas, 3 figuras, 3 tablas.Patchy landscapes behave differently from continuous ones. Patch size can influence species behaviour, movement, feeding and predation rates, with flow-on consequences for the diversity of species that inhabit these patches. To understand the importance of patchiness on regional species pools, we measured decapod richness and abundance in several seagrass patches with contrasting sizes. Additionally, we evaluated potential drivers of patch-specific species distribution including resource abundance, predator habitat use and the structural complexity of patches. Our results showed a non-random distribution of decapod species: small patches were clear hotspots of diversity and abundance, particularly of larger-bodied epifaunal decapods. Interestingly, these hotspots were characterized by lower nutrient resources, lower canopy height, but also lower predator use. Small fish invertivores such as Coris julis and several species of Symphodus were mostly restricted to large patches. These resident predators may be critical in clumping predation in large patches with consequences for how biodiversity of their prey is distributed across the seascape. Our results highlight the idea that a habitat mosaic with both large and small seagrass patches would potentially bolster biodiversity because preys and predators may seek refuge in patches of different sizes.This research was funded by the Spanish Ministry of Science and Innovation (CTM2017-86695-C3-3-R) and supported J. B. (scholarship BES-2011-043630).Peer reviewe