Seagrass Above-ground Biomass Research Articles

Inter-seasonal comparison of acoustic propagation in a Thalassia testudinum seagrass meadow in a shallow sub-tropical lagoon.

Acoustic propagation measurements were collected in a seagrass meadow in a shallow lagoon for periods of over 65 h in winter and 93 h in summer. A bottom-deployed sound source transmitted chirps (0.1-100 kHz) every 10 min that were received on a four-receiver horizontal hydrophone array. Oceanographic probes measured various environmental parameters. Daytime broadband acoustic attenuation was 2.4 dB greater in summer than winter, and the median received acoustic energy levels were 8.4 dB lower in summer compared to winter. These differences were attributed in part to seasonal changes in photosynthesis bubble production and above-ground seagrass biomass.

Appraisal of seagrass aboveground biomass changes using satellite data within the tropical coastline of Peninsular Malaysia

Seagrass habitats are crucial marine ecosystem component that affects the biophysical and chemical environment, providing nursery beds to various invertebrates, juvenile fish, and dugongs. This study focuses on mapping and monitoring changes in seagrass total above-ground biomass (STAGB) within the coastline of Peninsula Malaysia. The retrieval of the substrate leaving radiance was realized via was realized via Bottom Reflected Index (BRI) on Landsat ETM + visible bands. A strong relationship was established between BRI and seagrass biomass from in-situ samples (R2 > 0.8, p < 0.001), hence enable mapping and detection of changes in STAGB. Other related coastal changes caused by natural and human activities, namely total fish landing and erosion were also correlated with STAGB, to demonstrate the impacts of these variables on the habitat. The results offer inputs for coastal zone management, apart shall fast-track the achievements of target 14.2 of SDG 14.

The use of radar and optical satellite imagery combined with advanced machine learning and metaheuristic optimization techniques to detect and quantify above ground biomass of intertidal seagrass in a New Zealand estuary

ABSTRACT Seagrass provides numerous valuable ecosystem services across a wide range of climatic regions. However, in terms of area and habitat, this resource is in decline globally and there is an urgent need for accurate mapping of extant meadows and biomass to support sustainable seagrass blue carbon conservation and management. This study develops a novel method for a binary mapping of seagrass distribution and estimating seagrass above-ground biomass (AGB) by applying a suite of advanced machine learning (ML) algorithms combined with and without a metaheuristic optimization approach (particle swarm optimization – PSO) to various combinations of multispectral (Sentinel-2) and synthetic aperture radar (Sentinel-1) remote sensing data. Our results reveal that the Sentinel-1 data has potential for the binary mapping of seagrass meadows using an extreme gradient boosting (XGB) model (scores of precision (P) = 0.82, recall (R) = 0.90, and F 1 = 0.86) but is less effective at estimating AGB. The optimal method for estimation of AGB used both Sentinel-1 and Sentinel-2 imagery, the XGB model, and PSO optimization (coefficient of determination (R 2) = 0.75, root mean squared error (RMSE) = 0.35, Akaike information criteria (AIC) = 24.80, Bayesian information criteria (BIC) = 44.70). Our findings contribute novel and advanced methods for seagrass detection and improvement of AGB estimation, which are fast and reliable, use open-source data and software and should be easily applicable to intertidal zones across many regions of the world.

First Field-Based Evidence That the Seagrass-Lucinid Mutualism Can Mitigate Sulfide Stress in Seagrasses

Seagrass meadows form vital ecological components of coastal zones worldwide, but are rapidly declining. Large-scale seagrass diebacks have been related to accumulation of toxic sulfide in the sediment, a phenomenon predicted to occur more frequently in the near future due to ongoing global warming and increasing organic loading of coastal systems worldwide. Recently, a facultative mutualism between seagrasses and lucinid bivalves with endosymbiotic sulfide-consuming gill bacteria was discovered that may prevent toxic sulfide accumulation in seagrass sediments. Yet, direct field-based evidence for the importance of this mutualism in alleviating sulfide stress in seagrasses is currently lacking, as well as how its role may change when sediment sulfide levels increase due to environmental change. Here, we investigated the sulfide detoxification function of this seagrass-lucinid mutualism and its resilience to organic-loading induced sulfide stress in a temperate lagoon system (Thau lagoon, France), using a correlative field survey and a full factorial field experiment. The field survey revealed a strong positive correlation between seagrass above-ground biomass and lucinid densities, and pore water sulfide concentrations close to zero at all sites. Furthermore, the field experiment revealed that addition of organic matter (starch mixed with sucrose) increased sedimentary sulfide intrusion into seagrass (Zostera noltei) leaves (a proxy for sulfide stress), while experimentally enhanced lucinid densities reduced sulfide intrusion, regardless of addition of organic matter. Moreover, addition of organic matter reduced seagrass rhizome biomass and increased pore water sulfide levels, lucinid tissue sulfur content, lucinid condition (expressed as flesh/shell dry weight ratio), and total lucinid biomass, while enhancement of lucinid densities reduced lucinid condition. These results provide the first field-based evidence that lucinid bivalves and their sulfide-oxidizing gill symbionts mitigate sulfide stress in seagrasses, and suggests that the dependence of seagrass on this seagrass-lucinid mutualism will increase under conditions of enhanced sediment sulfide production, as predicted for the near future. Therefore, we suggest that awareness of the ecological importance of the seagrass-lucinid mutualism may be instrumental for designing new measures for improving long-term restoration success and seagrass resilience to global change.

Plant and sediment properties in seagrass meadows from two Mediterranean CO2 vents: Implications for carbon storage capacity of acidified oceans

Assessing the status of important carbon sinks such as seagrass meadows is of primary importance when dealing with potential climate change mitigation strategies. This study examined plant and sediment properties in seagrass meadows (Cymodocea nodosa (Ucria) Asch.) from two high pCO2–low pH Mediterranean vent systems, located at Milos (Greece) and Vulcano (Italy) Islands, providing insights on carbon storage potential in future acidified oceans. Contrary to what has been suggested, carbon content (both inorganic and organic) and its surficial accumulation decreased at high pCO2–low pH in comparison with controls. The decrease in inorganic carbon may result from the higher solubility of carbonates due to the more acidic conditions. At Vulcano, the seagrass properties (e.g., leaf area, biomass) appeared negatively affected by environmental conditions at high pCO2–low pH conditions and this may have had a detrimental effect on the organic carbon content and accumulation. At Milos, organic carbon decreased at high pCO2–low pH conditions, despite the increase in seagrass aboveground biomass, leaf length and area, probably as a consequence of site-specific features, which need further investigation and may include both biotic and abiotic factors (e.g., oligotrophic conditions, decreased sedimentation rate and input of allochthonous material). Results suggest that, in contrast to previous predictions based exclusively on the expected positive response of seagrasses to ocean acidification, carbon storage capacity of the seagrass C. nodosa may not increase at high pCO2-low pH conditions. This study emphasizes the need to investigate further the potential alteration in the climate mitigation service delivered by seagrass meadows in acidified oceans.

Effects of green turtle grazing on seagrass and macroalgae diversity vary spatially among seagrass meadows

Seagrass meadows are often comprised of diverse assemblages of seagrasses and algae. Green turtles (Chelonia mydas) are a prominent megaherbivore in seagrass meadows—capable of consuming large amounts of aboveground seagrass biomass and driving shifts in seagrass species dominance. Previous green turtle grazing studies have focused on diversity and dynamics of seagrasses, but there has been little focus thus far on diversity of other primary producers within meadow communities. We investigated the effects of grazing on seagrasses and benthic macroalgae composition by surveying a series of seagrass meadows across two separate bays in Little Cayman, Cayman Islands, in which green turtles had established foraging areas. Effects of grazing on diversity were spatially variable among meadows. Diversity of the seagrass and macroalgae community (Simpson’s Index) was not significantly different between areas grazed by turtles compared to ungrazed areas in one surveyed bay but was significantly lower among grazed areas in the other bay. Lower diversity was the result of lower densities of all seagrass and macroalgae species other than Thalassia testudinum among grazed areas in one bay. Diversity was also positively correlated with densities of total seagrass and total macroalgae across all meadows. Density of the dominant seagrass, T. testudinum, did not differ between the two bays or between grazed and ungrazed areas. As green turtle abundance increases and more seagrass meadows return to a naturally grazed state, it is important to understand the effects grazing has in these ecosystems and how effects may vary among locations.

Contributions of the direct supply of belowground seagrass detritus and trapping of suspended organic matter to the sedimentary organic carbon stock in seagrass meadows

Abstract. Carbon captured by marine living organisms is called “blue carbon”, and seagrass meadows are a dominant blue carbon sink. However, our knowledge of how seagrass increases sedimentary organic carbon (OC) stocks is limited. We investigated two pathways of OC accumulation: trapping of organic matter in the water column and the direct supply of belowground seagrass detritus. We developed a new type of box corer to facilitate the retrieval of intact cores that preserve the structures of both sediments (including coarse sediments and dead plant structures) and live seagrasses. We measured seagrass density, total OC mass (OCtotal) (live seagrass OC biomass (OCbio) + sedimentary OC mass (OCsed)), and the stable carbon isotope ratio (δ13C) of OCsed and its potential OC sources at Thalassia hemprichii dominated back-reef and Enhalus acoroides dominated estuarine sites in the tropical Indo-Pacific region. At points with vegetation, OCbio accounted for 25 % and OCsed for 75 % of OCtotal; this contribution of OCbio to OCtotal is higher than in globally compiled data. Belowground detritus accounted for ∼ 90 % of the OC mass of dead plant structures (&gt; 2 mm in size) (OCdead). At the back-reef site, belowground seagrass biomass, OCdead, and δ13C of OCsed (δ13Csed) were positively correlated with OCsed, indicating that the direct supply of belowground seagrass detritus is a major mechanism of OCsed accumulation. At the estuarine site, aboveground seagrass biomass was positively correlated with OCsed but δ13Csed did not correlate with OCsed, indicating that trapping of suspended OC by seagrass leaves is a major mechanism of OCsed accumulation there. We inferred that the relative importance of these two pathways may depend on the supply (productivity) of belowground biomass. Our results indicate that belowground biomass productivity of seagrass meadows, in addition to their aboveground morphological complexity, is an important factor controlling their OC stock. Consideration of this factor will improve global blue carbon estimates.

Contrasting impacts of light reduction on sediment biogeochemistry in deep- and shallow-water tropical seagrass assemblages (Green Island, Great Barrier Reef)

Seagrass meadows increasingly face reduced light availability as a consequence of coastal development, eutrophication, and climate-driven increases in rainfall leading to turbidity plumes. We examined the impact of reduced light on above-ground seagrass biomass and sediment biogeochemistry in tropical shallow- (∼2 m) and deep-water (∼17 m) seagrass meadows (Green Island, Australia). Artificial shading (transmitting ∼10-25% of incident solar irradiance) was applied to the shallow- and deep-water sites for up to two weeks. While above-ground biomass was unchanged, higher diffusive O2 uptake (DOU) rates, lower O2 penetration depths, and higher volume-specific O2 consumption (R) rates were found in seagrass-vegetated sediments as compared to adjacent bare sand (control) areas at the shallow-water sites. In contrast, deep-water sediment characteristics did not differ between bare sand and vegetated sites. At the vegetated shallow-water site, shading resulted in significantly lower hydrogen sulphide (H2S) levels in the sediment. No shading effects were found on sediment biogeochemistry at the deep-water site. Overall, our results show that the sediment biogeochemistry of shallow-water (Halodule uninervis, Syringodium isoetifolium, Cymodocea rotundata and C. serrulata) and deep-water (Halophila decipiens) seagrass meadows with different species differ in response to reduced light. The light-driven dynamics of the sediment biogeochemistry at the shallow-water site could suggest the presence of a microbial consortium, which might be stimulated by photosynthetically produced exudates from the seagrass, which becomes limited due to lower seagrass photosynthesis under shaded conditions.

Does Nutrient Availability Regulate Seagrass Response to Elevated CO2?

Future increases in oceanic carbon dioxide concentrations (CO2(aq)) may provide a benefit to submerged plants by alleviating photosynthetic carbon limitation. However, other environmental factors (for example, nutrient availability) may alter how seagrasses respond to CO2(aq) by regulating the supply of additional resources required to support growth. Thus, questions remain in regard to how other factors influence CO2(aq) effects on submerged vegetation. This study factorially manipulated CO2(aq) and nutrient availability, in situ, within a subtropical seagrass bed for 350 days, and examined treatment effects on leaf productivity, shoot density, above- and belowground biomass, nutrient content, carbohydrate storage, and sediment organic carbon (Corg). Clear, open-top chambers were used to replicate CO2(aq) forecasts for the year 2100, whereas nutrient availability was manipulated via sediment amendments of nitrogen (N) and phosphorus (P) fertilizer. We provide modest evidence of a CO2 effect, which increased seagrass aboveground biomass. CO2(aq) enrichment had no effect on nutrient content, carbohydrate storage, or sediment Corg content. Nutrient addition increased leaf productivity and leaf N content, however did not alter above- or belowground biomass, shoot density, carbohydrate storage, or Corg content. Treatment interactions were not significant, and thus NP availability did not influence seagrass responses to elevated CO2(aq). This study demonstrates that long-term carbon enrichment may alter the structure of shallow seagrass meadows, even in relatively nutrient-poor, oligotrophic systems.

Morphological plasticity in Zostera muelleri across light, sediment, and nutrient gradients in Australian temperate coastal lakes

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 556:91-104 (2016) - DOI: https://doi.org/10.3354/meps11830 Morphological plasticity in Zostera muelleri across light, sediment, and nutrient gradients in Australian temperate coastal lakes Angus J. P. Ferguson1,*, Renee K. Gruber1,2, Michael Orr1, Peter Scanes1 1New South Wales Office of Environment and Heritage, Sydney, NSW 2000, Australia 2Present address: University of Western Australia, Perth, WA 6009, Australia *‑Corresponding author: angus.ferguson@environment.nsw.gov.au ABSTRACT: Biomass and morphometrics of Zostera muelleri were monitored across depth, sediment type, and nutrient gradients in 2 coastal lakes (Tuggerah Lakes and Lake Macquarie) on the east coast of Australia. Tuggerah Lakes had significantly higher nutrient, chlorophyll a, and suspended sediment concentrations in the water column and significantly higher fine sediment fraction and sediment organic matter content. Seagrass above-ground biomass (AGB) was significantly greater in the mesotrophic Tuggerah Lakes, while below-ground biomass (BGB) was significantly greater in the oligotrophic Lake Macquarie, most likely reflecting the different nutrient status of the lakes. Light gradients were the primary control over total biomass, BGB, and shoot density across the study area. Although no general trends between light and AGB were found in this study, lake- and site-specific relationships between light, AGB:BGB ratios, and leaf area index were seen to vary along gradients in nutrient status and sediment quality. These trends are thought to be driven by morphological acclimation that allows seagrass to maintain favourable plant carbon and net community metabolism balances while minimising sulphide exposure. Seagrass depth limits were best predicted by a multilinear model including Secchi depth, fine sediment fraction, and organic matter content, suggesting that negative feedbacks associated with sulphide exposure in the rhizosphere increased the minimum light requirements of this species. Our results support an emerging view that sediment quality and nutrient status are important controls over minimum light requirements in seagrasses. Morphological plasticity can moderate but not completely compensate for the negative impacts of sediment properties on minimum light requirements. KEY WORDS: Seagrass · Morphological plasticity · Depth gradient · Sediment quality · Nutrients Full text in pdf format PreviousNextCite this article as: Ferguson AJP, Gruber RK, Orr M, Scanes P (2016) Morphological plasticity in Zostera muelleri across light, sediment, and nutrient gradients in Australian temperate coastal lakes. Mar Ecol Prog Ser 556:91-104. https://doi.org/10.3354/meps11830 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 556. Online publication date: September 08, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Carbon storage in seagrass soils: long-term nutrient history exceeds the effects of near-term nutrient enrichment

Abstract. The carbon sequestration potential in coastal soils is linked to aboveground and belowground plant productivity and biomass, which in turn, is directly and indirectly influenced by nutrient input. We evaluated the influence of long-term and near-term nutrient input on aboveground and belowground carbon accumulation in seagrass beds, using a nutrient enrichment (nitrogen and phosphorus) experiment embedded within a naturally occurring, long-term gradient of phosphorus availability within Florida Bay (USA). We measured organic carbon stocks in soils and above- and belowground seagrass biomass after 17 months of experimental nutrient addition. At the nutrient-limited sites, phosphorus addition increased the carbon stock in aboveground seagrass biomass by more than 300 %; belowground seagrass carbon stock increased by 50–100 %. Soil carbon content slightly decreased ( ∼ 10 %) in response to phosphorus addition. There was a strong but non-linear relationship between soil carbon and Thalassia testudinum leaf nitrogen : phosphorus (N : P) or belowground seagrass carbon stock. When seagrass leaf N : P exceeded an approximate threshold of 75 : 1, or when belowground seagrass carbon stock was less than 100 g m−2, there was less than 3 % organic carbon in the sediment. Despite the marked difference in soil carbon between phosphorus-limited and phosphorus-replete areas of Florida Bay, all areas of the bay had relatively high soil carbon stocks near or above the global median of 1.8 % organic carbon. The relatively high carbon content in the soils indicates that seagrass beds have extremely high carbon storage potential, even in nutrient-limited areas with low biomass or productivity.

Benthic macrofaunal assemblages in multispecific seagrass meadows of the southern Philippines: Variation among vegetation dominated by different seagrass species

Although the influence of seagrasses on the diversity and abundance of associated fauna is generally well understood, the effects of seagrass specific identity are not. To determine whether benthic epifaunal and infaunal assemblage patterns vary by the identity of dominant seagrass species in shallow water meadows, we compared quantitative measures of faunal species richness, abundance, and assemblage structure among beds dominated by Cymodocea rotundata, Enhalus acoroides, and Thalassia hemprichii; the study site was at Lopez Jaena, Misamis Occidental Province in the southern Philippines. The Cymodocea-dominated vegetation had higher seagrass shoot density and lower seagrass biomass than those dominated by Enhalus and Thalassia. Across vegetation types, we encountered 30 and 15 species of epifauna and infauna at average densities of 1.73 and 0.82 animals/core (0.0314m2), respectively. Neither densities and species richnesses of epifauna and infauna nor species composition varied significantly by vegetation type. Multivariate analyses of macrofaunal assemblage structure and abiotic/biotic environmental factors demonstrated that seagrass aboveground biomass explained a significant proportion of the multivariate variation in epifaunal species composition (39%); none of the potential explanatory variables was related to variation in ecological parameters of the infauna. Thus, seagrass specific identity is not a good predictor of either macrofaunal abundance or diversity patterns. Although the ecological parameters of the benthic macrofauna may be influenced by seagrass biomass and structure, responses differ between epifauna and infauna.

Effect of Sediment Nutrient Enrichment and Grazing on Turtle Grass Thalassia testudinum in Jobos Bay, Puerto Rico

We examined the effect of nutrients and grazers on Thalassia testudinum in Jobos Bay, Puerto Rico by fertilizing sediment and manipulating grazer abundances. Bottom-up effects were variable: Added nutrients did not increase seagrass aboveground biomass, but decreased belowground biomass—perhaps as a result of less biomass being allocated to belowground structures in response to greater nutrient supply in porewater. Experimental fencing of 1.5 × 1.5 m plots provided shelter that attracted large aggregations of fish, including seagrass herbivores. Seagrass biomass and shoot density decreased with increasing abundance of herbivorous fish, indicating a significant top-down effect. There were interactions between nutrient supply, provision of shelter, and grazing pressure. Fertilization enhanced seagrass %N; however, %N also increased in unfertilized plots that were fenced, most likely due to uptake of N excreted from the large numbers of fish associated with the fences. Only plots where shelter was provided and fertilizer was applied to sediments exhibited evidence of heavy grazing, reducing both seagrass cover and aboveground biomass. In the unfertilized fenced plots, signs of grazing were fewer despite large abundances of fish and enhanced nutritional quality of seagrass leaves. This suggests the possibility that high nutrient availability in sediments lowered concentrations of chemical defense compounds in the seagrass and that cues other than %N may have been involved in stimulating grazing. This study highlights the complexity of bottom-up and top-down interactions in seagrass systems and the important role of refuge availability in shaping the relative strengths of these controls.

Productivity, carbon assimilation and intra-annual change in tropical reef platform seagrass communities of the Torres Strait, north-eastern Australia

Detailed data on seagrass distribution, abundance, growth rates and community structure information were collected at Orman Reefs in March 2004 to estimate the above-ground productivity and carbon assimilated by seagrass meadows. Seagrass meadows were re-examined in November 2004 for comparison at the seasonal extremes of seagrass abundance. Ten seagrass species were identified in the meadows on Orman Reefs. Extensive seagrass coverage was found in March (18,700 ha) and November (21,600 ha), with seagrass covering the majority of the intertidal reef-top areas and a large proportion of the subtidal areas examined. There were marked differences in seagrass above-ground biomass, distribution and species composition between the two surveys. Major changes between March and November included a substantial decline in biomass for intertidal meadows and an expansion in area of subtidal meadows. Changes were most likely a result of greater tidal exposure of intertidal meadows prior to November leading to desiccation and temperature-related stress. The Orman Reef seagrass meadows had a total above-ground productivity of 259.8 t DW day −1 and estimated carbon assimilation of 89.4 t C day −1 in March. The majority of this production came from the intertidal meadows which accounted for 81% of the total production. Intra-annual changes in seagrass species composition, shoot density and size of meadows measured in this study were likely to have a strong influence on the total above-ground production during the year. The net estimated above-ground productivity of Orman Reefs meadows in March 2004 (1.19 g C m −2 day −1) was high compared with other tropical seagrass areas that have been studied and also higher than many other marine, estuarine and terrestrial plant communities.

HABITAT FRAGMENTATION INFLUENCES SURVIVAL AND GROWTH OF TRANSPLANTED NORTHERN QUAHOG (MERCENARIA MERCENARIA LINNÉ) IN BIG LAGOON, FLORIDA

Abstract To test the effects of habitat fragmentation per se on the survival and growth of the northern quahog (Mercenaria mercenaria Linne), we stocked juvenile quahogs in 10 seagrass patches (Halodule wrightii Ascherson) ranging in size from 0.5–1,750 m2 for two consecutive months during the summer of 2004. As a control, additional quahogs were stocked in cages located in seagrass patches that spanned the same size range. We tested for changes in shell growth, somatic growth, and mortality between caged and non-caged treatments and we estimated the strength of linear relationships between all three measures of growth and mortality with patch area, perimeter, perimeter: area ratio, seagrass aboveground biomass, and epiphyte biomass. Shell and somatic growth were each significantly greater in cages, whereas mortality was significantly less than in uncaged treatments. There were no significant relationships between clam mortality and either patch perimeter, seagrass above ground biomass, or epiphyte biomas...

Effects of tropical open-water seaweed farming on seagrass ecosystem structure and function

Seaweed farming is often depicted as a sustainable form of aquaculture, but suspected habitat alterations and spread of algae outside farms have rendered speculations on the actual degree of sustainability. We conducted an experimental field study on Unguja Island (Zanzibar, Tan- zania) to investigate the effects of off-bottom seaweed farming on a tropical seagrass ecosystem, using 1.5 × 2.5 m experimental farm plots. After 11 wk, above-ground seagrass biomass was 40% lower than in control plots, owing to a combination of lower shoot density, shoot length and leaf growth rate. Since the biomass was constant between Day 15 and 75 in the farm (F) treatment, but increased by 67 vs. 48% in the 2 controls (control treatment (C) and stick-and-line control treatment (CSL)), the effect exerted by the farm was a lack of potential biomass increase rather than an actual decrease. The effect was transplanted to associated organisms both in terms of lower seagrass epi- phyte cover and changes in the abundance of 2 dominating epifauna taxa (>1 cm): sea urchins and sponges. Furthermore, the F treatment caused an accumulation of seagrass leaf litter, but did not affect sediment organic matter (SOM) content. The mechanisms behind these effects were not explic- itly tested, but algal shading, emergence stress and mechanical abrasion were identified as likely contributors. Interestingly, the effects were largely restricted to 1 of the 2 seagrass species present, Enhalus acoroides, while the other, Thalassia hemprichii, remained more or less unaffected. This may be due to reduced interspecific competition or species-specific differences in morphology and stress tolerance, and could in the long-term have implications for (amongst others) associated fish commu- nities. Although seaweed farming at the current level is less detrimental than, for example, intensive shrimp farming, and therefore should be seen as a strong option for future aquaculture develop- ments, intensive farming on seagrasses should be avoided or at least minimized by, for example, implementing other farming methods. The risk of ecosystem-level changes in large-scale and uncon- trolled farm enterprises warrants a holistic and integrated coastal management approach which considers all aspects of the tropical seascape including human societies and natural resource use.

A visual assessment technique for estimating seagrass standing crop

1. Ground-truthing techniques for measuring seagrass standing crop need to be simple, precise, non-destructive and quick. 2. Mellors (1991) designed a visual assessment technique for estimating above-ground seagrass biomass. This paper builds on Mellors' work and presents a modified method for estimating standing crop which uses a six-point ordinal scale (excluding zero). 3. Being non-destructive, this technique is ideally suited for resource assessment in environmentally sensitive areas and, being quick, is potentially useful when threats are imminent. 4. Statistical methods and data presented in this study will allow sample size/statistical power relationships to be estimated for any comparable study and therefore negate the need to conduct pilot surveys. © 1997 John Wiley & Sons, Ltd.

Seasonal abundance and distribution of drift algae and seagrasses in the mid-Indian river lagoon, Florida

The distribution of seagrasses in a 15-ha area in the mid-Indian River lagoon on Florida's central east coast was mapped. Halodule wrightii Aschers. dominated in shallow (< 0.4 m) and Syringodium filiforme Kutz. in deeper water (> 0.5 m). Thalassia testudinum Banks ex König occurred as scattered patches. Areal coverage of monospecific stands of the three major seagrasses was: Syringodium 35%, Halodule 14%, Thalassia 6% and bare sand 21%. Mixed species stands, mostly Syringodium with Hallodule, covered 25% of the total study area. Above-ground seagrass biomass was maximum in summer (June–July) and minimum in late winter (February–March). Summer maxima ranged from 60 g dry wt. m −2 for Syringodium to ∼ 300 g dry wt. m −2 for Thalassia, with Halodule intermediate at 160 g dry wt. m −2. Because distribution of unattached benthic macroalgae (“drift algae”), primarily Gracilaria spp., was highly aggregated, aggregations were first mapped, followed by stratified quadrat sampling in order to estimate total drift algal abundance. In April 1982, high-density patches covering a few hectares averaged 409 g dry wt. m −2. At maximum abundance, averaged over the entire 15-ha mapped area, drift algal biomass was 164 g dry wt. m −2; mean above-ground seagrass biomass was only 49 g dry wt. m −2. Other large expanses of the lagoon had similar accumulations of drift algae; densities of some accumulations exceeded 15 000 g dry wt. m −2. Year-to-year variability of seagrass and drift algal abundance was high and may be related to variations in light levels. Drift algae harbor high densities of animals and at times may be quantitatively more important locally than seagrasses in terms of habitat, nutrient dynamics and primary production.