Seagrass Species Composition Research Articles

Multi-temporal mapping of seagrass cover, species and biomass: A semi-automated object based image analysis approach

The spatial and temporal dynamics of seagrasses have been studied from the leaf to patch (100m2) scales. However, landscape scale (>100km2) seagrass population dynamics are unresolved in seagrass ecology. Previous remote sensing approaches have lacked the temporal or spatial resolution, or ecologically appropriate mapping, to fully address this issue. This paper presents a robust, semi-automated object-based image analysis approach for mapping dominant seagrass species, percentage cover and above ground biomass using a time series of field data and coincident high spatial resolution satellite imagery. The study area was a 142km2 shallow, clear water seagrass habitat (the Eastern Banks, Moreton Bay, Australia). Nine data sets acquired between 2004 and 2013 were used to create seagrass species and percentage cover maps through the integration of seagrass photo transect field data, and atmospherically and geometrically corrected high spatial resolution satellite image data (WorldView-2, IKONOS and Quickbird-2) using an object based image analysis approach. Biomass maps were derived using empirical models trained with in-situ above ground biomass data per seagrass species. Maps and summary plots identified inter- and intra-annual variation of seagrass species composition, percentage cover level and above ground biomass. The methods provide a rigorous approach for field and image data collection and pre-processing, a semi-automated approach to extract seagrass species and cover maps and assess accuracy, and the subsequent empirical modelling of seagrass biomass. The resultant maps provide a fundamental data set for understanding landscape scale seagrass dynamics in a shallow water environment. Our findings provide proof of concept for the use of time-series analysis of remotely sensed seagrass products for use in seagrass ecology and management.

Juvenile Spotted Seatrout (Cynoscion nebulosus) Habitat Use in an Eastern Gulf of Mexico Estuary: The Effects of Seagrass Bed Architecture, Seagrass Species Composition, and Varying Degrees of Freshwater Influence

Spotted seatrout (Cynoscion nebulosus) are estuarine residents that have been extensively studied primarily due to their economic importance. Although spotted seatrout have a documented affinity for seagrass beds, there is little information on how seagrass bed architecture, seagrass species composition, and freshwater inflow affect this relationship. The objective of this study was to document patterns in spatiotemporal distribution and abundance of juvenile spotted seatrout (≤100 mm SL) in estuarine seagrass beds. We analyzed long-term, fisheries-independent monitoring data (1996–2009) from Tampa Bay, Florida, and also conducted a short-term, synoptic study specifically targeting seagrass beds with varying vegetative characteristics and degrees of freshwater influence. Juvenile spotted seatrout were most abundant in seagrass beds influenced by freshwater and dominated by shoal grass, Halodule wrightii. Annual abundance varied over the sampling period, with some peaks in juvenile abundance corresponding with high levels of freshwater inflow. Although analysis of long-term data indicated that juvenile spotted seatrout abundance was positively related to estimated percentage cover of seagrass, the canopy cover of seagrass beds measured quantitatively during the synoptic study was less in areas influenced by freshwater. The value of seagrass as spotted seatrout habitat transcends the simple presence or absence of vegetation and can differ based on factors such as location within the bay, seagrass species composition, and amount of freshwater influence, and management decisions regarding this species and the restoration of particular seagrass beds need to be made with full consideration of these complex relationships.

Community Structure of Seagrass Meadows In Mantehage Island Waters, North Sulawesi

Research on the structure of seagrass communities in the waters of the island Mantehage was conducted in September 2010. The purpose of this study is to collect data and information on the structure of seagrass communities. Observations includes the identification seagrass species, measuring the number of individual/stand, cover percentage of each type/species on the transect. 6 types of seagrass found, i.e. Enhalus acoroides, Halophila ovalis, Thalassia hemprichii, Thalassodendron ciliatum, Cymodocea rotundata and Syringodium isoetifolium. Seagrass species composition and distribution varies at each location and is dominated by the presence of Thalassia hemprichii (48.14%) with coverage percentage ranges between 15.91% to 35.11%. Diversity index values ranged from 0.79 to 1.69 ; evenness index between 0.57 to 0.94 and the index of dominance between 0.20 to 0.56. In conclusion, the condition of seagrass meadows in Mantehage island is relatively good and can support marine life adequately.Keywords : Structure communities, seagrass, Mantehage Island

The influence of canopy structure and tidal level on fish assemblages in tropical Southeast Asian seagrass meadows

Seagrass meadows support abundant and diverse fish assemblages, but there are very few studies on the relation between seagrass beds with distinctly different plant canopies and their associated fish fauna. In the present study, fish assemblages were investigated by underwater visual census at intertidal and subtidal sites with varying seagrass species composition, shoot density, biomass, and leaf area index (LAI) on two small coral islands in the Spermonde Archipelago, Indonesia. We investigated (1) whether fish assemblages in distinctly different seagrass beds differ regarding community parameters, and (2) whether seagrass parameters affect fish abundances. Overall, more than 120 fish taxa were found. Bray–Curtis cluster analysis and non-Metric Multidimensional Scaling ordination (nMDS) showed site-specific similarities for fish assemblage structure with a distinct separation into subtidal and intertidal sites. Species accumulation curves for gamma diversity, single study sites, and the two most diverse fish families (Labridae, Pomacentridae), suggesting that species numbers are likely to increase with diel sampling. Total fish abundance and abundance for six out of the nine most common species varied distinctly among the study sites. The study indicates that seagrass beds with differing canopy parameters support distinct fish assemblages that differ with respect to species richness, dominant species, and the abundance of total and most common fish species. These differences are likely due to different canopy structures and water depth.

Integrating Quickbird Multi-Spectral Satellite and Field Data: Mapping Bathymetry, Seagrass Cover, Seagrass Species and Change in Moreton Bay, Australia in 2004 and 2007

Shallow coastal ecosystems are the interface between the terrestrial and marine environment. The physical and biological composition and distribution of benthic habitats within these ecosystems determines their contribution to ecosystem services and biodiversity as well as their connections to neighbouring terrestrial and marine ecosystem processes. The capacity to accurately and consistently map and monitor these benthic habitats is critical to developing and implementing management applications. This paper presents a method for integrating field survey data and high spatial resolution, multi-spectral satellite image data to map bathymetry and seagrass in shallow coastal waters. Using Quickbird 2 satellite images from 2004 and 2007, acoustic field survey data were used to map bathymetry using a linear and ratio algorithm method; benthic survey field data were used to calibrate and validate classifications of seagrass percentage cover and seagrass species composition; and a change detection analysis of seagrass cover was performed. The bathymetry mapping showed that only the linear algorithm could effectively and accurately predict water depth; overall benthic map accuracies ranged from 57–95%; and the change detection produced a reliable change map and showed a net decrease in seagrass cover levels, but the majority of the study area showed no change in seagrass cover level. This study demonstrates that multiple spatial products (bathymetry, seagrass and change maps) can be produced from single satellite images and a concurrent field survey dataset. Moreover, the products were produced at higher spatial resolution and accuracy levels than previous studies in Moreton Bay. The methods are developed from previous work in the study area and are continuing to be implemented, as well as being developed to be repeatable in similar shallow coastal water environments.

Growth Patterns of Shoal Grass Halodule wrightii and Manatee Grass Syringodium filiforme in the Western Gulf of Mexico

IntroductIon Seagrass beds are a valuable resource because of the numerous roles they play in coastal systems. Seagrass blades provide habitat for abundant micro– and macro–algal communities, which in conjunction with the seagrass, contribute substantially to primary productivity in estuarine systems (Heck and Valentine 2006). The blades and roots stabilize sediments, which improves water clarity and increases light penetration, further increasing primary production (Zieman 1982, Gacia and Duarte 2001). Seagrass beds may also provide increased growth rates, critical habitat, and predation refuges for a variety of organisms (e.g., Irlandi and Peterson 1991, Hemminga and Duarte 2000) and have an increased abundance and diversity of juvenile and adult fish and other epibenthic organisms (Summerson and Peterson 1984, Heck et al. 1995). In Texas, like most of the western Gulf of Mexico (GOM), shoal grass (Halodule wrightii), manatee grass (Syringodium filiforme), and turtle grass (Thalassia testudinum) are the most common seagrasses (Zieman 1982, Quammen and Onuf 1993, Withers 2002). Eighty percent of seagrass beds in Texas currently occur in the Laguna Madre (hereafter LM), a hypersaline lagoon that separates a coastal barrier island (Padre Island) from the Texas mainland (Pulich 1998, Tunnell and Judd 2002). Seagrasses were once common in many Texas bays, but have disappeared or declined in coverage in many areas due to anthropogenic causes (Pulich and Onuf 2007). Fortunately, the loss of seagrasses in Texas bays was offset by the increase in seagrass abundance in the LM due to the moderation of salinity after dredging of the Gulf Intracoastal Waterway (Quammen and Onuf 1993). Shoal grass can survive and grow in salinities from 5–80 (McMillan and Moseley 1967) and this seagrass dominated the LM for decades because of its ability to withstand these extreme salinities (Withers 2002). Shoal grass harbors a diverse resident fauna (Tolan et al. 1997) and the migratory redhead duck (Aythya americana) population depends on shoal grass in the LM for food during the winter season (Cornelius 1977). Shoal and manatee grass are currently present in the upper LM, with shoal grass historically being dominant (Quammen and Onuf 1993) and extensively studied (e.g., Dunton 1994, 1996). However, manatee grass is increasing in coverage in much of the LM and is steadily replacing shoal grass in this system (Quammen and Onuf 1993, Pulich and Onuf 2007). In other areas such as Corpus Christi Bay, shoal, manatee, and turtle grass have coexisted for the past 20 yrs (Czerny and Dunton 1995, K. Dunton pers. comm.). Although manatee grass is becoming increasingly abundant in Texas bays, its growth characteristics have not been measured in the western GOM. Changes in seagrass species composition can have significant community effects (Micheli et al. 2008), but the effects of a transition from shoal to manatee grass in the LM have not been extensively studied (but see Tolan et al. 1997). The goals of our study were to measure growth patterns of these two seagrass species in two locations in the western GOM that vary in salinity, epiphyte loads, and nutrient inputs.

Mapping seagrass species, cover and biomass in shallow waters: An assessment of satellite multi-spectral and airborne hyper-spectral imaging systems in Moreton Bay (Australia)

Understanding, monitoring and modelling attributes of seagrass biodiversity, such as species composition, richness, abundance, spatial patterns, and disturbance dynamics, requires spatial information. This work assessed the accuracy of commonly available airborne hyper-spectral and satellite multi-spectral image data sets for mapping seagrass species composition, horizontal horizontal-projected foliage cover and above-ground dry-weight biomass. The work was carried out on the Eastern Banks in Moreton Bay, Australia, an area of shallow and clear coastal waters, containing a range of seagrass species, cover and biomass levels. Two types of satellite image data were used: Quickbird-2 multi-spectral and Landsat-5 Thematic Mapper multi-spectral. Airborne hyper-spectral image data were acquired from a CASI-2 sensor using a pixel size of 4.0 m. The mapping was constrained to depths shallower than 3.0 m, based on past modelling of the separability of seagrass reflectance signatures at increasing water depths. Our results demonstrated that mapping of seagrass cover, species and biomass to high accuracy levels (> 80%) was not possible across all image types. For each parameter mapped, airborne hyper-spectral data produced the highest overall accuracies (46%), followed by Quickbird-2 and then Landsat-5 Thematic Mapper. The low accuracy levels were attributed to the mapping methods and difficulties in matching locations on image and field data sets. Accurate mapping of seagrass cover, species composition and biomass, using simple approaches, requires further work using high-spatial resolution (< 5 m) and/or hyper-spectral image data. Further work is required to determine if and how the seagrass maps produced in this work are suitable for measuring attributes of seagrass biodiversity, and using these data for modelling floral and fauna biodiversity properties of seagrass environments, and for scaling-up seagrass ecosystem models.

ALTERATION OF SEAGRASS SPECIES COMPOSITION AND FUNCTION OVER TWO DECADES

Changes in the species composition and structural characteristics of marine vegetated habitats in response to climate change or local anthropogenic impacts may alter their quality as habitat for associated fish and invertebrates. Summer densities and biomass of the eelgrass, Zostera marina, declined significantly between 1985 and 2004 in Bogue Sound, North Carolina, USA, within the present‐day zone of biogeographic overlap in the distribution of this subtidal temperate species and the intertidal subtropical seagrass, Halodule wrightii. Zostera decline was associated with increased spring water temperatures and water nutrient concentrations. In contrast, Halodule did not exhibit a consistent trend of temporal change. Experimental seagrass transplants indicated that Halodule has the capacity to grow at depths greater than it currently occupies, suggesting that Halodule might, over time, replace Zostera. The abundance and diversity of infaunal invertebrates were lower in seagrass beds dominated by Halodule than in those dominated by Zostera or mixtures of the two species, suggesting that changing seagrass species composition affects associated faunal assemblages. Experimental deployment of artificial seagrass patches mimicking the structure of the two species showed that both depth and structural characteristics of seagrass explain differences in faunal assemblages. Epifaunal community structure differed significantly between structurally identical seagrass mimics deployed in intertidal and subtidal habitat, and invertebrates and fishes were significantly more abundant in artificial Zostera than Halodule patches at one of the two study sites. Synthesis of these results suggests that, in the event of continued Zostera loss, with or without replacement by Halodule, important habitat functions will be lost, and secondary productivity of these lagoonal ecosystems may significantly decrease.

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.

A new method to describe seagrass habitat sampled during fisheries-independent monitoring

We explain a new method of quantifying seagrass cover and describing seagrass species composition during fisheries-independent monitoring. This new method is similar to a point-intercept method developed to estimate arboreal crown cover, but it uses an aquascope designed for shallow water. The method does not require a diver. Seagrass cover (cover ratio) distinguished different percentage cover categories in 0.25-m2 seagrass plots. Estimates of species composition determined by using the new method were most similar to those obtained by using estimates of aboveground biomass. Within each 141-m2 area sampled with a 21.3-m fish seine, we accurately estimated seagrass cover ratio and species composition with six observations that typically required less than 6 total minutes. Within such areas, 42 trials were conducted to evaluate the precision with which different observers estimated seagrass cover ratio and species composition. In 98% of the trials, observers attained statistically similar estimates of cover ratio, and in 100% of the trials in areas with multiple seagrass species, observers attained statistically similar estimates of species composition. We conclude that the new method provided efficient and reasonably accurate means to quantify seagrass cover and species composition.

Seagrass as pasture for seacows: Landscape-level dugong habitat evaluation

A 24 km 2 seagrass meadow in Hervey Bay, Queensland, Australia, was confirmed as important dugong habitat by a satellite tracking study. Marine videography, near-infrared spectroscopy (NIRS) and geographic information systems (GIS) were used to survey, analyse and map seagrass species composition, nutrient profile and patch structure at high resolution (200 m). Five species of seagrass covered 91% of the total habitat area. The total above and below-ground seagrass biomass was estimated to be 222.7 ± 19.6 t dry-weight. Halodule uninervis dominated the pasture (81.8%, 162.2 t), followed by Halophila ovalis (35.3%, 16.5 t), Zostera capricorni (15.9%, 22.2 t), Halophila spinulosa (14.5%, 21.9 t), and traces of Halodule pinifolia. Because seagrass distributions overlapped, their combined percentage totalled >100% of the survey area. The seagrass formed a continuous meadow of varying density. Abiotic variables explained relatively little of the spatial patterns in the seagrass. For all seagrass species, the above-ground component (shoots and leaves) possessed greater total nitrogen than the below-ground component (roots and rhizomes), which possessed greater total starch. Because of the relatively low intraspecific variation in nutrient composition, nutrients were concentrated according to seagrass biomass density. Halodule uninervis was the most nutritious seagrass species because of its superior whole-plant nitrogen (1.28 ± 0.05% DW) and starch (6.42 ± 0.50 DW %) content. Halodule uninervis formed large, clustered patches of dense biomass across the pasture and thus nitrogen and starch were concentrated where H. uninervis was prevalent. This seagrass meadow appears to be utilised well below its potential dugong carrying capacity. The survey and analytical techniques used enabled rapid, economical and accurate quantification and characterisation of seagrass habitat at scales relevant to a large forager.

Changes in the distribution of seagrass species along Florida's Central Gulf Coast: Iverson and Bittaker revisited

A broad-scale survey of seagrass species composition and distribution along Florida's central Gulf Coast (known as the Big Bend region) was conducted in the summer of 2000 to address growing concerns over the potential effects of increased nutrient loading from adjacent coastal rivers. Iverson and Bittaker (1986) originally surveyed seagrass distribution in this region between 1974–1980. We revisited 188 stations from the original survey, recording the presence or absence of all seagrass species. Although factors such as accuracy of station relocation, differences in sampling effort among studies, and length of time between surveys preclude statistical comparisons, several interesting patterns emerged. While the total number of stations occupied by the three most common seagrass species,Thalassia testudinum, Syringodium filiforme, andHalodule wrightii, was similar between the two time periods, we observed a change in the number of records of each species as well as changes in distribution with depth.T. testudinum andHalophila engelmanni occurrence declined in the deepest areas of the region, while the number of stations occupied byS. filiforme andH. wrightii increased in nearby areas. We observed several localized areas of seagrass loss, frequently associated with the mouths of coastal rivers. These results suggest that increased nutrient loading to coastal rivers that discharge into the Big Bend area may be affecting seagrasses by increasing phytoplankton abundance in the water column, thus changing water clarity characteristics of the region.

Decadal Changes in Seagrass Distribution and Abundance in Florida Bay

The Florida Bay ecosystem has changed substantially in the past decade, and alterations in the seagrass communities have been particularly conspicuous. In 1987 large areas ofThalassia testudinum (turtlegrass) began dying rapidly in western Florida Bay. Although the rate has slowed considerably, die-off continues in many parts of the bay. Since 1991, seagrasses in Florida Bay have been subjected to decreased light availability due to widespread, persistent microalgal blooms and resuspended sediments. In light of these recent impacts, we determined the current status of Florida Bay seagrass communities. During the summer of 1994, seagrass species composition, shoot density, shoot morphometrics, and standing crop were measured at 107 stations. Seagrasses had been quantified at these same stations 10 yr earlier by Zieman et al. (1989).T. testudinum was the most widespread and abundant seagrass species in Florida Bay in both 1984 and 1994, and turtlegrass distribution changed little over the decade. On a baywide basis,T. testudinum density and biomass declined significantly between surveys; mean short-shoot density ofT. testudinum dropped by 22% and standing crop by 28% over the decade.T. testudinum decline was not homogeneous throughout Florida Bay; largest reductions in shoot density and biomass were located principally in the central and western bay. Percent loss ofT. testudinum standing crop in western Florida Bay in 1994 was considerably greater at the stations with the highest levels of standing crop in 1984 (126–215 g dry wt m−2) than at the stations with lower levels of biomass. While turtlegrass distribution remained consistent over time, both the distribution and abundance of two other seagrasses,Halodule wrightii andSyringodium filiforme, declined substantially between 1984 and 1994. Baywide,H. wrightii shoot density and standing crop declined by 92%, andS. filiforme density and standing crop declined by 93% and 88%, respectively, between surveys. Patterns of seagrass loss in Florida Bay between 1984 and 1994 suggest die-off and chronic light reductions were the most likely causes for decline. If die-off and persistent water-column turbidity continue in Florida Bay, the long-term future of seagrasses in the bay is uncertain.

Fish and sea urchin herbivory and competition in Kenyan coral reef lagoons: the role of reef management

The impact of “overfishing” on coral-reef herbivores was studied using Thalassia and Sargassum bioassays at two reefs protected from fishing for over 15 yr, one “transition reef” protected for ~ 2 yr, and three unprotected reefs. The primary goals of this research were to (1) assess the ability of an herbivory assay to distinguish between (a) herbivore types such as sea urchins and herbivorous fishes (i.e. parrotfish and surgeonfish), (b) rates of herbivory by sea urchins and herbivorous fishes, (2) potential impacts of coral-reef herbivores on seagrass species composition and abundance, and (3) the role that fishing plays in mediating competitive interactions between sea urchins and herbivorous fishes. Studied reefs differed in their management regulations and enforcement such that impacts of fishing regulations could be partly distinguished from species interactions. Parrotfishes appear to be the dominant fishes feeding on the assay. The sea urchin Echinothrix diadema (Linnaeus) exhibited a much higher preference for the assay than other sea urchin species. Consequently, the variable species composition of herbivorous fishes and sea urchins and their feeding preferences make between-site comparisons of herbivory problematic. However, if sites with high numbers of Echinothrix are excluded and the majority offish bites on the Thalassia assay are attributabed to a few species of parrotfish, then between-site and between-treatment comparisons of relative herbivory can be made. Experimental reduction of sea urchin abundance led to increased bite rates on herbivory assays by parrotfishes at protected site but not the unprotected sites. These results imply that sea urchins can reduce grazing rates of some species of parrotfishes. The species composition of seagrass communities in protected and unprotected reefs appears to be partially affected by prey choices of the dominant grazers such that parrotfish and Echinothrix sea urchins favor Thallasodendron dominance while other species of sea urchin such as Diadema setosum, D. savignyi (Audoin) and Echinometra mathaei de Blainville favor Thalassia dominance.