Esperança Gacia

An approach to measurement of particle flux and sediment retention within seagrass (Posidonia oceanica) meadows

Abstract Seagrass beds have traditionally been considered to act as sinks for particles due to the reduction of flow velocities by the plant canopy. Yet, there is a paucity of measurements to confirm this role. In this work we illustrate changes in flow in the presence and absence of Posidonia oceanica using an ADV, and provide direct measures of particle trapping by the use of sediment traps. We also describe a model to estimate sediment resuspension after measuring particle flux at different distances from the bottom. Measurements of particle flux are conducted parallel to the study of structural parameters of the Posidonia meadow potentially involved in both particle trapping and retention. Data obtained on velocity profiles confirm previous findings that seagrass canopies slow down current velocities with intensities proportional to the canopy height of the plants. The projected surface area of the plants (LAI) significantly correlated with the total amount of particles trapped within the Posidonia meadow, thus indicating seagrass canopy slightly increased particle trapping in the absence of resuspension. The trapping capacity of the canopy was not linearly correlated to LAI but significantly decreased at LAI above four, thus suggesting that other factors such as bending of the leaves and particle attachment to the surface may interfere with particle free sinking within the canopy at high projected surface area. The model proposed to estimate resuspension allowed to measure the retention capacity of the P. oceanica meadow, this being up to 15 times higher compared to a barren bottom during situations of high energy (large eddies reaching the bottom). The results obtained provide direct quantitative support to seagrass beds promoting sediment accretion and demonstrate a promising avenue to provide the needed empirical support for the effect of seagrasses on depositional processes.

Seagrass community metabolism: Assessing the carbon sink capacity of seagrass meadows

[1] The metabolic rates of seagrass communities were synthesized on the basis of a data set on seagrass community metabolism containing 403 individual estimates derived from a total of 155 different sites. Gross primary production (GPP) rates (mean ± SE = 224.9 ± 11.1 mmol O 2 m ―2 d ―1 ) tended to be significantly higher than the corresponding respiration (R) rates (mean ± SE = 187.6 ± 10.1 mmol O 2 m ―2 d ―1 ), indicating that seagrass meadows tend to be autotrophic ecosystems, reflected in a positive mean net community production (NCP 27.2 ± 5.8 mmol O 2 m ―2 d ―1 ) and a mean P/R ratio above 1 (1.55 ± 0.13). Tropical seagrass meadows tended to support higher metabolic rates and somewhat lower NCP than temperate ones. The P/R ratio tended to increase with increasing GPP, exceeding, on average, the value of 1 indicative of metabolic balance for communities supporting a GPP greater than 186 mmol O 2 m ―2 d ―1 , on average. The global NCP of seagrass meadows ranged (95% confidence limits of mean values) from 20.73 to 50.69 Tg C yr ―1 considering a low global seagrass area of 300,000 km and 41.47 to 101.39 Tg C yr ―1 when a high estimate of global seagrass area of 600,000 km 2 was considered. The global loss of 29% of the seagrass area represents, therefore, a major loss of intense natural carbon sinks in the biosphere.

Are seagrass growth and survival constrained by the reducing conditions of the sediment

A literature review of the effects of the reducing conditions of the sediment on seagrass metabolism, growth and survival, and of the morphological and physiological adaptations that seagrasses show to cope with sediment anoxia is presented and major gaps in knowledge are identified. The hypothesis that sediment anoxia controls the survival of seagrasses was tested experimentally by increasing the oxygen demand of the sediment with the addition of sucrose. Experiments were performed in a tropical (Southeast Asia) multispecific seagrass meadow, a Mediterranean Cymodocea nodosa meadow, and a temperate Zostera marina meadow. Sulfide levels in pore water and vertical redox profiles were used to characterise the effects of the sucrose additions on the sediment, while plant responses were quantified through the changes in shoot density and leaf growth. Sulfide levels in pore water increased and sediment redox potential decreased after the addition of sucrose to the sediment of different seagrass meadows. The effect of the addition of sucrose to the sediment of seagrasses was species-specific. Leaf growth was reduced and shoot mortality increased in some of the tropical species (e.g., Thalassia hemprichii), but not in others. Neither mortality nor leaf growth of the Mediterranean species C. nodosa was affected by sucrose additions, and only leaf growth was reduced two months after the addition of sucrose in Z. marina. Our results suggest that increased sediment anoxia might be a factor promoting growth inhibition and mortality in seagrasses, although strong differences have been found among different species and environments.

Seagrass Beds and Coastal Biogeochemistry

Capitulo en: LARKUM, Anthony W.D.; ORTH, Robert J.; DUARTE, Carlos M. (eds.). Seagrasses: Biology, Ecology and Conservation. Repr. with Corrections, 2007. [Dordrecht]: Springer, 2006, p.135-157

Direct Evidence of Imbalanced Seagrass (Posidonia oceanica) Shoot Population Dynamics in the Spanish Mediterranean

Direct census of shoots tagged in permanent plots was used to assess the present (2000–2002)Posidonia oceanica population dynamics in 25 meadows along the Spanish Mediterranean Coast. Shoot density ranged from 154±8 to 1,551±454 shoots m−2, absolute shoot mortality from 5±0 to 249±53 shoots m−2 yr−1, and absolute shoot recruitment from <5 ±1 to 62±42 shoots m−2yr−1. Specific shoot mortality and recruitment rates, which are mathematically and statistically (p>0.05) independent of shoot density, varied from 0.015±0.006 to 0.282±0.138 yr−1 and 0.018±0.005 to 0.302±0.093 yr−1, respectively. Absolute shoot mortality rate was scaled to shoot density (Pearson correlation, r=0.78, p<0.0001), and variability in specific shoot recruitment rate was partially due to differences in the percentage of growing apexes, which produce most of the recruits within the population (Pearson correlation, r=0.50, p<0.001), demonstrating the existence of structural constraints on shoot demography. Shoot half-life was estimated to range from 2.5 to 60.4 yr and meadow turnover times between 6.7 yr and more than a century, provided current estimates of shoot mortality, recruitment rates, and density remain uniform. There were differences in shoot mortality and recruitment at the regional scale, with the meadows developing along the coast of the Spanish mainland experiencing the highest shoot mortality (Tukey test, p<0.05) and tending to exhibit the highest shoot recruitment. The low shoot recruitment did not balance shoot mortality in most (60%) of the meadows, showing a prevalence of declining populations among the 25 meadows studied (Wilcoxon ranked sign test, p<0.0005). This study demonstrates the power of direct census of seagrass shoots in permanent plots to evaluate the present status of seagrass meadows, to detect on-going population decline, and to provide some insight onto the possible factors involved. The incorporation of direct census of seagrass meadows to monitoring programs will help provide the early-warning signals necessary to support management decisions to conserve seagrass meadows.

Chemical composition of disturbed and undisturbed high-mountain lakes in the Pyrenees: A reference for acidified sites

102 lakes distributed along the Catalan Pyrenees (from 0°41′ E to 1°48′ E) were sampled during summer in 1987. The alkalinity of the lakes was low (<300 μequiv l−1) but there was no regional acidification. Most chemical variance was related to the nature of the bedrock. Lakes on Devonian, Cambro-Ordovician, Silurian or granodioritic batholites were easily distinguished. Even between different batholites, there were significant differences. There was no clear distinction either in the chemistry or in trophic status between disturbed and undisturbed lakes, except for certain extreme cases. Pasture is the most important source of eutrophication, but is restricted to small, very shallow and usually endorreic lakes.

Differential accumulation of mercury and other trace metals in the food web components of a reservoir impacted by a chlor-alkali plant (Flix, Ebro River, Spain): Implications for biomonitoring

Comparative studies of biomonitors of trace metal contamination are relatively scarce. We took advantage of a point source pollution in a reservoir (Flix, Spain) to compare trace metal (Hg, Pb, Cd, Se, As, Zn, Cu, Cr) bioaccumulation patterns among 16 food web components. Our results indicate that most organisms are suitable for Hg biomonitoring, whereas other metals are better monitored by only some of them. Biofilms and zebra mussel were the organisms with larger and more diverse biomonitoring capacity. However, we show that using groups of biomonitors increase the scope and strengths of the conclusions and specific goals can be better addressed. We conclude providing an overview of the strengths and weaknesses of the main organisms considered for biomonitoring trace metals in rivers and reservoirs.

Lack of severe nutrient limitation in Caulerpa taxifolia (Vahl) C. Agardh, an introduced seaweed spreading over the oligotrophic Northwestern Mediterranean

We performed a seasonal study of the importance of nitrogen (N) and phosphorus (P) as limitation to the productivity of Caulerpa taxifolia (Vahl) C. Agardh, a seaweed invading the western Mediterranean coasts. The study presented here indicates that Caulerpa taxifolia did not respond to nutrient enrichment, except for plants collected in April and September 1993, where productivity and alkaline phosphatase activity (APA) of enriched plants varied 0.5- to 3-fold relative to those of control plants. Rates of APA ranged from 9 to 52 mu M PO4 odw h(-1), close to those characteristic of P-sufficient macroalgae. Tissue nutrient concentrations were frequently above typical critical levels for species of macroalgae ranging from 1.36 to 3.13% dw, and 0.16 to 0.21% dw, for N and P, respectively. Seasonal variation was moderate and not consistent. The lack of severe nutrient limitation in Mediterranean populations of Caulerpa taxifolia could be an important factor enabling it to outcompete native macrophytes

Environmental filtering determines community patterns in temporary wetlands: A multi-taxon approach

Climate characteristics appear to play a key role in filtering organisms based on their biological traits. If this trait filtering by climate indeed occurs, it should have effects on the composition, dynamics, taxonomic relatedness and co-occurrence patterns of local assemblages, regardless of the taxonomic group considered. This preliminary study aimed to assess the extent to which environmental variables might determine these patterns in local communities and to evaluate whether the ultimate cross-taxon congruence relationships are consistent across, or dependent on, the selected region. To this end, we studied the bryophyte, macrophyte, macroinvertebrate, and amphibian communities in two clusters of temporary wetlands on the NE Iberian Peninsula under mesothermal and semiarid climates. We observed effects of environmental filtering, with the communities differing between the climatic regions not only in their compositions but also in their dynamics and taxonomic relatedness patterns. Although the cross-taxon congruence in terms of species richness was high in the mesothermal climate, most of the congruent relationships were disrupted in the semiarid environment. Overall, because climate-dependent patterns appear to prevail over climate-consistent ones, we suggest that the use of surrogate taxa may be of limited value when aiming to assess wetland biodiversity across large areas.

Variability in δ15N natural abundance of basal resources in fluvial ecosystems: a meta-analysis

Abstract Variation in N stable isotope (&dgr;15N) signatures of basal resources can influence interpretation of trophic relationships in ecosystems, and significant variation in &dgr;15N signatures has been reported in streams and rivers. However, a comprehensive understanding of the main factors driving &dgr;15N variability is lacking, and this variability confounds the consumers trophic-level position during &dgr;15N analysis. We conducted a meta-analysis to examine the variability in &dgr;15N natural abundance of basal resources and dissolved inorganic N (DIN) in streams and rivers in relation to the environmental factors that may drive this variability. The meta-analysis was based on a literature review over the last 20 y (1989–2009) and contained signatures of &dgr;15N-DIN (&dgr;15N-NO3 and &dgr;15N-NH4) and &dgr;15N-basal resources (&dgr;15N-detrital compartments, &dgr;15N-biofilm, &dgr;15N-algae, and &dgr;15N-macrophytes) from >100 rivers or streams. Signatures of &dgr;15N-DIN varied widely (−8.4–19.4‰), and we found fewer values for &dgr;15N-NH4 than &dgr;15N-NO3, even though NH4+ is assimilated rapidly by basal resources. The range of &dgr;15N-basal resources was also broad (−4–16‰) within and among compartments. Human land use was the most significant factor explaining variability in &dgr;15N-DIN and &dgr;15N-basal resource signatures. We found significant differences between &dgr;15N signatures of photoautotrophic (i.e., autochthonous) and detrital (i.e., allochthonous) basal resources. Our results point out the difficulty in defining a baseline &dgr;15N signature of the food web, and provide a basis to explain confounding results in studies using &dgr;15N analysis to identify trophic linkages in fluvial food webs.