Miguel D. Fortes

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.

The role of epiphytic periphyton and macroinvertebrate grazers in the trophic flux of a tropical seagrass community

Abstract Biomass and production of epiphytic periphyton, and the abundance, distribution and grazing rate of epifauna were measured in tropical seagrass beds in the Philippines. Periphyton comprised mainly detritus, diatoms and filamentous algae (Polysiphonia sp. 1, Centroceras clavulatum (C. Agardh) Montagne, Ceramium gracillimum Harvey and Cladophora sp.). Mean biomass of periphyton was 0.16 mg ash-free dry weight (AFDW) cm−2 frond of Enhalus acoroides (L.f.) Royle and 0.24 mg AFDW cm−2 frond of Cymodocea serrulata (R. Br.) Aschers. and Magnus. Total periphyton biomass per unit area (m2) of seagrass bed varied between habitats because of differing densities of seagrass, and ranged from 598 to 1061 mg AFDW or (24–646 mg C). Maximum (midday, summer) in situ rates of photosynthesis and respiration by epiphytes colonising artificial seagrass material averaged 11.6 μg O2 cm−2 h−1 and 2.0 μg O2 cm−2 h−1, respectively. Daily net productivity was 14 μg C cm−2 frond. Productivity of epiphytes per area of seagrass bed varied with site (36–77 mg C m−2 day−1). Relative to biomass, these data show that epiphytes are highly productive, with turnover times of 6–8 days, compared with known values of 30–100 + days for tropical seagrass fronds. The epifaunal grazer community was dominated by a few species of gastropod molluscs (especially Strombus mutabilis Swainson and Cerithium tenellum (Sowerby)). Within habitats, numbers of grazers on particular seagrass species were directly related to their available surface. Three groups of grazers were identified: those occurring on fronds day and night (e.g. S. mutabilis); those foraging over sediment during the day and fronds at night (e.g. Cerithium tenellum); those mainly confined to sediments (e.g. Strombus urceus L.). All epifaunal grazers exhibited upward movement into the seagrass canopy at night. Grazing was non-selective, removing the periphyton, except for the unutilised encrusting coralline algae, in proportion to abundance. Epifaunal grazers consumed between 20 and 62% of periphyton net production and, as in temperate systems, must therefore play a major role in the trophic flux of this tropical seagrass community.

Impacts of milkfish (Chanos chanos) aquaculture on carbon and nutrient fluxes in the Bolinao area, Philippines

Sediment oxygen consumption, TCO2 production and nutrient fluxes across the sediment-water interface were measured in sediments within and along a transect from four fish pens with production of milkfish (Chanos chanos) in the Bolinao area, The Philippines. The four fish pens were each representing a specific period in the production cycling. There was a positive linear relationship between the rates of sedimentation inside the fish pens and the sediment oxygen consumption indicating that the benthic processes were controlled by the input of organic matter from fish production. The nutrient fluxes were generally higher inside the fish pens, and nitrate was taken up (1.7-5.8 mmol m(-2) d(-1)) whereas ammonium (1-22 mmol m(-2) d(-1)) and phosphate (0.2-4.7 mmol m(-2) d(-1)) were released from the sediments. The sediments were enriched in organic matter with up to a factor 4 compared to outside. A mass balance for one crop of milkfish was constructed based on production data and on measured fluxes of nutrients in the fish pens to assess the loss of carbon and nutrients to the environment. There was a loss to the surroundings of carbon and nitrogen of 51-68% of the total input, whereas phosphorus was buried in the sediments inside the fish pens which acted as net sinks of phosphorus. The results obtained suggest that fish pen culture as practiced in the Bolinao area, leads to even greater impacts on benthic carbon and nutrient cycling than those found in suspended cage cultures.

Feeding ecology and trophic role of sea urchins in a tropical seagrass community

The grazing impact of urchins on seagrass and algal resources, and the relative importance of this to the lower-level trophic flux of a tropical seagrass community were investigated. Thalassia hemprichii (Ehrenb.) Aschers. accounted for 80–93% of seagrass frond biomass at Bolinao in the Philippines. Growth rate of seagrass was 6.6 mm per shoot day−1, or 2.3 mg AFDW per shoot day−1. Production of seagrass fronds per unit area of seagrass bed varied with location from 870 to 1850 mg AFDW m−2 day−1. Urchin density ranged from 0.9 to 4.2 m−2, with Tripneustes gratilla (L.) and Salmacis sphaeroides (L.) being the most common species. Tripneustes gratilla fed mostly on attached seagrass fronds (77–89% of diet), especially Thalassia hemprichii, whereas S. sphaeroides was a generalist, consuming Thalassia hemprichii fronds (13–65%), detached seagrass debris (5–39%), the red alga Amphiroa fragilissima (L.) Lamour. (0–30%), algal-coated sediment and rubble (0–51%) in proportions that varied with the availability of preferred food types. Live Thalassia hemprichii fronds were clearly preferred over macroalgae or dead seagrass fronds by Tripneustes gratilla, but S. sphaeroides consumed all three food types without preference. Both urchins avoided the common brown alga, Sargassum crassifolium J. Agardh. Urchins absorbed 73–76% of organic matter in seagrass fronds with epiphytes (75% of DW), and 55% of that in epiphyte-free fronds. Seagrass debris and the macroalgae A. fragilissima were of lower food quality as they were lower in organic matter, and this matter was absorbed less efficiently by urchins. Rates of ingestion (IR in g WW per urchin day−1) were proportional to body weight (W in g WW) according to the functions: IR = 0.56W0.34 (T. gratilla) and IR = 0.17W0.53 (Salmacis sphaeroides). Predicted grazing impact of urchins on seagrass resources varied spatially and temporally. Estimated annual grazing rate at the main study site was 158 g AFDW m−2, equivalent to 24% of annual seagrass production, but owing to large changes in urchin population structure and density, grazing impact is expected to vary from 100% at different times of year. A synthesis of knowledge on the lower-level trophic pathways in this system indicates that seagrass-urchin and periphyton-epifauna grazing interactions are both important in their contribution to overall trophic flux.

Recolonization dynamics in a mixed seagrass meadow: The role of clonal versus sexual processes

Recolonization dynamics from disturbance on a Philippine mixed seagrass meadow, containing species spanning more than 10-fold in rhizome elongation rates and reproductive effort, was examined by following the recovery of a 1,200 m2 gap over 2.5 yr. The objective was to assess the contribution of contrasting species to the recovery process and to evaluate the importance of sexual versus vegetative colonization. Large, slow-growing species,Thalassia hemprichii andEnhalus acoroides, that produce large, broadly-dispersed seeds dominated sexual colonization with a total of 2,643 and 210 seedlings, respectively, recruiting to the area. Despite very rapid turnover of sexual recruits, the high frequency of seedling establishment ensured successful development of new patches in areas devoid of vegetation, leading to a scattered and evenly distributed presence of vegetation inside the gap. The small seagrass speciesCymodocea rotundata andHalodule uninervis, characterized by fast rhizome elongation rates but low reproductive output and limited seed dispersal, were the major contributors to the overall 450 m2 increase in vegetation cover through fast lateral extension (144±6 cm yr−1) from meadow edge and surviving patches, forming a compact vegetation cover in one edge of the denuded area. We conclude that contrasting recruitment strategies in the mixed-species seagrass community examined have implications for colonization potential at different spatial scales. Fast clonal growth is only an efficient mechanism for colonization of disturbances within established meadows (small gaps), whereas the large species, which combined high reproductive output with high seed dispersal capacity, may act to accelerate the colonization process in large gaps or distant from established meadows.

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.

Contrasting Recolonization Strategies in Multi-Species Seagrass Meadows

Abstract This study shows that in a multi-species seagrass meadow in a shallow and clear-water site, all the former seagrass species were able to recolonize in the artificially created gaps of 0.25 m 2 in size within ca. 2 yr. Extrapolation of the recolonization curves of the different species predicted a full recovery within 10 yr post-disturbance. Fitted curves for the dominant species Enhalus acoroides and Thalassia hemprichii showed contrasting strategies, the latter having a comparatively high intrinsic rate, achieving full recovery within ca. 2 yr post-disturbance. E. acoroides was the latest species to establish and the projected full-recovery time was among the longest (ca. 10 yr). The effect of timing of gap creation was generally not significant (except for Syringodium isoetifolium ) neither was the temporal variation in density of most species outside the gaps. As recolonization by sexual propagules was found to be low, increasing the gap size would most probably require a much longer recovery period. A crude estimate for E. acoroides would be >10 yr for 1 m 2 of gap. Further, since the densities of most seagrass species vary significantly between sites, and colonization rates depend on adjacent seagrass densities, the recovery curves would also be different across sites.

Experimental evaluation of the effects of siltation-derived changes in sediment conditions on the Philippine seagrass Cymodocea rotundata

This study investigated if siltation-associated changes in the sediments are detrimental to seagrasses. We chose Cymodocea rotundata as the test species because it is considered one of the Southeast Asian seagrass species most sensitive to siltation. The approach included the (1) evaluation of the effects of silted sediments on plant growth, evaluation of the effects of in situ sulfide additions to the sediment on (2) the production of shoots, rhizomes and roots, and on the elongation rate of the horizontal rhizomes of plants located at the edge of a meadow, and on (3) leaf growth, mass allocation patterns and shoot density in a well-developed seagrass meadow. The results showed that under high light availability, major changes in sediment conditions associated with siltation did not negatively affect the plants but enhanced their growth likely by increasing the availability of nutrients. Pore water sulfide concentrations of 1 mM reduced by more than half the production of shoots, rhizome and roots, and the elongation rates of horizontal rhizomes of C. rotundata plants at the edge of the meadow, but had no effects on leaf growth and shoot density in a well-developed C. rotundata meadow.

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.

Habitat Use by Fishes in Coral Reefs, Seagrass Beds and Mangrove Habitats in the Philippines

Understanding the interconnectivity of organisms among different habitats is a key requirement for generating effective management plans in coastal ecosystems, particularly when determining component habitat structures in marine protected areas. To elucidate the patterns of habitat use by fishes among coral, seagrass, and mangrove habitats, and between natural and transplanted mangroves, visual censuses were conducted semiannually at two sites in the Philippines during September and March 2010–2012. In total, 265 species and 15,930 individuals were recorded. Species richness and abundance of fishes were significantly higher in coral reefs (234 species, 12,306 individuals) than in seagrass (38 species, 1,198 individuals) and mangrove (47 species, 2,426 individuals) habitats. Similarity tests revealed a highly significant difference among the three habitats. Fishes exhibited two different strategies for habitat use, inhabiting either a single (85.6% of recorded species) or several habitats (14.4%). Some fish that utilized multiple habitats, such as Lutjanus monostigma and Parupeneus barberinus, showed possible ontogenetic habitat shifts from mangroves and/or seagrass habitats to coral reefs. Moreover, over 20% of commercial fish species used multiple habitats, highlighting the importance of including different habitat types within marine protected areas to achieve efficient and effective resource management. Neither species richness nor abundance of fishes significantly differed between natural and transplanted mangroves. In addition, 14 fish species were recorded in a 20-year-old transplanted mangrove area, and over 90% of these species used multiple habitats, further demonstrating the key role of transplanted mangroves as a reef fish habitat in this region.