Nona S. R. Agawin

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.

Evidence of Direct Particle Trapping by a Tropical Seagrass Meadow

The capacity of seagrass canopies to directly retain sestonic particles was tested by quantifying the rate at which suspended fluorescent tracer particles were retained within a tropical Philippine seagrass meadow and by examining whether the test particles lost from the water column were later bound to seagrass leaves or inside epibionts. The particle loss rates in the presence of seagrass canopies were up to 4 times higher than those in unvegetated and plankton controls. The seagrass canopies trapped particles with a maximum rate of 0.73 (±0.24) h−1. As much as 5% of the particles trapped by the seagrass leaves were physically adhered to the leaf surfaces following rinsing. Particles were also observed to be ingested by protozoa (ciliates and amoeba-like organisms), residing on the surface of the leaves, and may be the dominant particle trapping mechanism by seagrass leaves. These processes should provide an efficient mechanism for the transfer of planktonic production to the benthos, adding to the high organic carbon input maintained by the high production of the seagrass themselves.

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.

Temporal changes in the abundance, leaf growth and photosynthesis of three co-occurring Philippine seagrasses

The analysis of the temporal changes in shoot density, areal leaf biomass, leaf growth and parameters of the photosynthesis-irradiance relationship of three tropical seagrass species (Enhalus acoroides, Thalassia hemprichii and Cymodocea rotundata), co-existing in a shallow subtidal meadow in Cape Bolinao, Philippines, shows that species-specific traits are significant sources of temporal variability, and indicates that these seagrass species respond differently to a common environmental forcing. Species-specific differences are much less important as source of variability of the temporal change in chlorophyll concentration of seagrass leaves. The results indicate that the temporal changes in photosynthetic performance of these seagrasses were driven by environmental forcing and their specific responses to it mostly, but the temporal change in their abundance and leaf growth was also controlled by other factors. The significant contribution of species-specific factors in the temporal changes of biomass, growth and photosynthetic performance of co-occurring seagrass species in Cape Bolinao should contribute to the maintenance of the multispecific, highly productive meadows characteristic of pristine coastal ecosystems in Southeast (SE) Asia.

Flowering Frequency of Philippine Seagrasses

The flowering frequency of the Philippine seagrasses Thalassia hemprichii (Ehrenb.) Aschers., Cymodocea rotundata Ehrenb. et Hempr. ex Aschers., and Enhalus acoroides (L.f.) Royle, growing on a reef flat in Bolinao (Pangasinan Province, The Philippines) was examined based on examination of flowering scars on the seagrass shoots. The flowering frequency of C. rotundata and T.: hemprichii was low (0.064 flowers shoot(-1) yr(-1) and 0.125 flowers shoot(-1) yr(-1), respectively), indicating that only a fraction of the shoots of these species will flower during their life spans. Shoots of these species required a maturation period of between half a year and one year before flowering. In contrast, most of the E. acoroides shoots examined had flowered several times, producing, on average, 2.8 flowers shoot(-1) yr(-1). Examination of the past flowering of E. acoroides revealed substantial interannual differences in flowering frequency in the period 1985-1992, with a maximum in 1987. Because of the large flowers of this species, the estimated biomass allocated to flowering was orders of magnitude greater for E. acoroides (35.8 g dw m(-2) yr(-1)) than for C. rotundata (0.021 g dw m(-2) yr(-1)) and T. hemprichii (3.56 g dw m(-2) yr(-1)). These results indicate that sexual reproduction could be a minor sink of resources for C. rotundata and I: hemprichii (<1% of the annual above-ground production), while it may represent a dominant source of losses of resources acquired by E. acoroides (up to 50% of the annual above-ground production). The implications of these contrasting strategies in the flowering effort of the seagrass species examined are, however, unclear, but the large output of sexual propagules of E. acoroides, compared to the other two species, should confer this species a greater capacity to recover after disturbance. [KEYWORDS: Cymodocea-nodosa; growth; dynamics]

Nutrient limitation of the tropical seagrass Enhalus acoroides (L.) Royle in Cape Bolinao, NW Philippines

Abstract Experimental additions of nutrients to the sediment of Enhalus acoroides stands were performed at four sites and three times along the year in Cape Bolinao, NW Philippines to test the hypothesis that seagrass growth in tropical environments is limited by the availability of nutrients. Both the nitrogen content (as % DW) and the nitrogen incorporation of E. acoroides leaves increased after the addition of nutrients. The size (g DW per shoot) and the leaf growth rates (g DW per shoot d −1 ) of E. acoroides shoots also increased after the addition of nutrients. Nitrogen rather than phosphorus was the nutrient limiting shoot size and leaf growth of E. acoroides in the area. The extent of nutrient limitation of E. acoroides showed high variability both in space and time which cannot be directly linked with differences in light or nutrient availability among the experimental sites.

Prokaryotic picoplankton spatial distribution during summer in a haline front in the Balearic Sea, Western Mediterranean

In oligotrophic regions, picophytoplankton can play a key role in total carbon production and energy transfer. Since the mesoscale hydrographic variability can influence the resource availability and therefore the biological communities, here we studied the linkage between hydrography, resource supply, abundance, and biomass contribution of prokaryotic picoplankton in the south Balearic Sea during the stratified season. The sampling area covered the confluence of two different water masses, the fresher new Atlantic water, and the saltier resident Atlantic water. Our results showed higher Synechococcus abundances in the more oligotrophic new Atlantic water mass and suggest that the spatial patterns of prokaryotic picophytoplankton are dictated by the mesoscale processes in this region. The summer stratification condition separated clearly the surface mixed layer (ML) from the deep layer (DL); our results support different limiting factors for picophytoplankton in the two layers: nutrient and light availability in the ML and DL, respectively. We also obtained no significant difference in the Synechococcus biomass contribution to total autotrophic biomass within the water column, but higher contribution in the new Atlantic water mass. These results demonstrate the general importance of picophytoplankton as carbon producers in oligotrophic waters and particularly their variability as biomass source at the mesoscale.