Seagrass Cover Research Articles

Growth, biomass and productivity of the seagrass Thalassia hemprichii at Ashmore Reef, Australia

Ashmore Reef, located approximately 630 km north of Broome, Australia, is notable for its contribution to overall seagrass cover in the Timor Province bioregion, providing valuable ecosystem services including critical habitat for feeding and breeding green turtles (Chelonia mydas) and a small geographically isolated population of dugong (Dugong dugon). Yet, seagrass surveys at Ashmore Reef have not been completed since 2005. Seagrass was surveyed in 2019, with only two (Thalassia hemprichii and Halophila ovalis) of the five previously recorded species observed. Monitoring survey sites were established to determine density, biomass and productivity of the dominant species, T. hemprichii. Overall mean total biomass was 116.9 DW m−2 with an above- and below-ground biomass ratio of 1:2.5. Mean shoot density was 407 shoots m−2 (range 88.9–600 shoots m−2) with a mean canopy height of 42.8 mm (range 10–80 mm), and average blade length of 37.6 mm ( ± 1.1 SE) and blade width of 4.5 mm ( ± 0.2). Mean leaf growth was calculated as 2.2–2.4 mm day−1 and shoot growth 3.2–3.5 mm day−1. Canopy height increased by 5.3–5.7 % day−1 with a turnover rate of 17.5–21.3 days. Mean biomass of emergent leaves was 8 g DW m−2 with a grazing rate of 0.4 g DW m−2 day −1 or 123 kg DW ha−1 month−1. This study suggests that T. hemprichii at Ashmore Reef is productive and may support high turtle grazing rates.

Marine ecosystem restoration and aquaculture-based stock enhancement for fishery resource conservation in Abu Dhabi Emirate, United Arab Emirates

There is a necessity to protect and manage the marine ecosystem towards the conservation of the ecological equilibrium and in turn the sustainability of the fishery resources in Abu Dhabi, UAE. Marine stock enhancement (MSE) proved that releasing hatchery fingerlings of marine animals can improve the abundance of harvest beyond the level supported by natural recruitment. For such programme to be beneficial and economical, considerable investment and scientific efforts are required. Among the concerned entities in the United Arab Emirates, the Aquaculture and Marine Studies (AMSC) in Abu Al Abyad Island successfully released 59 million of juveniles of different finfish species and 96 million juveniles of prawn species in Abu Dhabi waters. In the context of conserving the ecological equilibrium, the AMSC was able to plant an area of 5,026 ha in various areas of Abu Dhabi Emirate with grey mangrove, Avicennia marina; culturing over 21,290 corals of Acropora sp., Porites sp., Cyphastrea sp and Psammocora sp. in an area of 0.5 ha nurseries in Abu Al Abyad and Yasat Islands which will later be transplanted in the degraded reef sites in both islands; and a well-established seagrass coverage of 2500 m2 was achieved in affected areas in Abu Al Abyad Island.

Evaluation of Water Column Correction Methods in Mapping Seagrass Bed Using Remote Sensing Data in Khanh Hoa Province, Vietnam

The use of remote sensing images for the interpretation of underwater substrate objects depends on their reflectance spectrum of different water depths. Thus, the water column correction step is important in the interpretation. Two commonly used water column correction methods are Lyzenga’s depth invariant index (DII) method and Sagawa’s bottom reflectance index (BRI) method. To evaluate the role of each method in Khanh Hoa waters, Tuan Le water, with a high seagrass coverage and moderate turbidity, and Thuy Trieu Lagoon, with high biodiversity of seagrass species and turbidity water, were selected as representatives. 70% of the survey data was used for ground training data of seagrass (dense and patchy), muddy-sand and sand features and the interpretation of both methods, whereas the remaining 30% of the survey data was used for validation of the mapping results. The maximum likelihood classification approach was used to extract the seagrass map and evaluated by overall accuracy as well as the Kappa coefficient. The processing results show that, in Tuan Le water, using the DII method gives an accuracy of 82.1% and the BRI of 80.1%; and in Thuy Trieu Lagoon, the DII method has an accuracy of 80.67% and the BRI of 80.0%. These results demonstrate that both methods have high accuracy results in both areas, but the method of BRI gives better results.

Seasonal Dynamics of Faunal Diversity and Population Ecology in an Estuarine Seagrass Bed

Biodiversity is important for communities to be resilient to a changing world, but patterns of diversity fluctuate naturally over time. Understanding these shifts — and the species driving community dynamics — is crucial for informing future ecological research and conservation management plans. We investigated the impacts of seasonality, small-scale changes in seagrass cover, and small-scale spatial location on the epifaunal communities occupying a temperate seagrass bed in the South Island of New Zealand. By sampling epifaunal communities using a fine-mesh push net two to three times per season for 1 year, and using a combination of multivariate and hierarchical diversity analyses, we discovered that season, seagrass cover, and the location within the bay, and their interactions, explained 88.5% of the variation in community composition. Community composition and abundances, but not numbers, of species changed over seasons. The most common taxa were commercially important Caridean shrimp and juvenile flounder (Rhombosolea spp.), and both decreased in abundance in summer (shrimp: 1.40/m2 in winter to 0.80/m2 in summer; flounder: 0.15/m2 in winter to 0.01/m2 in summer). Other commercially important species were captured as juveniles, including blue cod (Parapercis colias), kahawai (Arripis trutta), and whitebait (Galaxias spp.). The only adult fish captured in the study were two pipefish species (Stigmatopora nigra and Leptonotus elevatus), which had distinctly seasonal breeding patterns, with reproductively active adults most likely to be found in the spring and fall. Our study highlights the importance of estimating biodiversity parameters based on sampling throughout the year, as some species will be overlooked. We demonstrate that the temperate estuarine seagrass-affiliated animal communities differ in response to season and fine-scale local environments, causing fluctuations in biodiversity throughout the year.

Penyerapan Karbon Pada Ekosistem Lamun Di Kawasan Perairan Gili Maringkik Lombok, Provinsi Nusa Tenggara Barat, Indonesia

Seagrass beds are an important vegetation for reducing the impact of global warming besides the function of forest and mangroves vegetation. The research aims to find the composition of seagrass species, density, seagrass coverage, seagrass standing biomass, seagrass standing carbon and substrate carbon.The research was conducted in Gili Maringkik, Lombok, Indonesia. The research included observation of species composition and the sample was collected from 0.5 ×0.5 m plot area. Total plot area was 25 on five lanes with the space between plots 25 m and between lanes100 m.The researcher found that there were eight seagrass species (two families, six genera): Cymodocea rotundata, Cymodocea serrulata, Enhalus acoroides, Halodule pinifolia, Halophila minor, Halophila spinulosa, Syringodium isoetifolium and Thalassia hemprichii. Cymodocea rotundata(506.40±187.809 stand.m-2) was the species of highest density and while the species of Thalassia hemprichii (36.52±30.004 %) wasthe species of highest coverage. Total of seagrass standing biomass in Gili Maringkik was1081.85 g.DW.m-2with the carbon stock of seagrass beds was 483.86g.C.m-2 and substrate carbon content was0.09%–0.49%. The total carbon stock of seagrass beds in Gili Maringkik has 153.96 ton.C(4.84 ton.C.Ha-1).

Keanekaragaman Fitoplankton di Padang Lamun Kawasan Pesisir Mandalika Kabupaten Lombok Tengah

Phytoplankton is a source of food for zooplankton, fish larvae, and other organisms that live in seagrass beds. The purpose of this study was to determine the diversity of phytoplankton in seagrass beds in the coastal area of Mandalika, Central Lombok Regency. The location of water sample data collection was determined by purposive sampling by considering the percentage of seagrass cover. The sampling locations included 3 water locations, namely seagrass beds on the coast of Kuta, seagrass beds on the coast of Tanjung Ann, and seagrass beds on the coast of Gerupuk Bay. The research was conducted for three months from July to September 2021. The results of the study in the Mandalika coastal seagrass beds found 6 classes of phytoplankton, 26 orders, 30 families, 42 genera, and 46 phytoplankton species. Twenty one species of phytoplankton are from the Class Bacillariophyceae, 9 species from the Class Dinophyceae, 7 species from the Class Chlorophyceae, 5 species from the Class Cyanophyceae, 3 species from the Class Chrysophyceae, and 1 species from the Class Rhodophyceae. The abundance of phytoplankton at the three sampling locations ranged from 285-445 ind/L. The Phytoplankton Diversity Index in the three locations ranged from 1.35 to 2.75, which means communities with moderate diversity and dominance index ranged from 0.20-2.33.

Consistency assessment of multi-date PlanetScope imagery for seagrass percent cover mapping in different seagrass meadows

Seagrass percent cover is a crucial and influential component of the biophysical characteristics of seagrass beds and is a key parameter for monitoring seagrass conditions. Therefore, the availability of seagrass percent cover maps greatly assists in sustainable coastal ecosystem management. This research aimed to assess the consistency of PlanetScope imagery for seagrass percent cover mapping using two study areas, namely Parang Island and Labuan Bajo, Indonesia. Assessing the consistency of the PlanetScope imagery performance in seagrass percent cover mapping helps understand the effects of variations in the image quality on its performance in monitoring changes in seagrass cover. Percent cover maps were derived using object-based image analysis (image segmentation and random forest) and pixel-based random forest algorithm. Accuracy assessment and consistency analysis were conducted on the basis of the following three approaches: overall accuracy consistency, agreement percentage and consistent pixel locations. Results show that PlanetScope images can fairly consistently map seagrass percent cover for a specific area across different dates. However, these images produced different levels of accuracy when used for mapping in seagrass meadows with various characteristics and benthic cover complexities. The mapping accuracy (OA–overall accuracy) and consistency (AP–agreement percentage) in patchy seagrass meadows (Parang Island, mean OA 18.4%–38.6%, AP 44.1%–70.3%) are different from those in continuous seagrass meadows (Labuan Bajo, OA 43.0%–56.2%, and AP 41.8%–55.8%). Moreover, PlanetScope images are consistent when used for mapping seagrasses with low and high percent covers but strive to obtain good consistency for medium percent cover due to the combination of seagrass and non-seagrass in a pixel. Furthermore, images with relatively similar image acquisition conditions (i.e., winds, aerosol optical depth, signal-to-noise ratio, and sunglint intensity) produce better consistency. The OA is related to the image acquisition conditions, whilst the AP is related to variation in these conditions. Nevertheless, PlanetScope is still the best high spatial resolution image that provides daily acquisition and is highly beneficial for various applications in tropical areas with persistent cloud coverage.

Kerapatan Dan Penutupan Lamun Di Perairan Desa Tayando Yamtel Kecamatan Tayando Tam Kota Tual.

Research on the density and cover of seagrass in the waters of Tayando Yamtel Village, Tam District, Tual City, was carried out in March-May 2018, using the quadrant linear transect method. The results of the study found three species of seagrass, namely Cymodocea rotundata, Cymodocea serrulata and Thalassia hemprichii. The highest density values at stations I and III were Cymodocea rotundata, namely 90.598 ind/m2 and 80.02 ind/m2. The highest density value at station II was Cymodocea serrulata, which was 88.24 ind/m2. The total density of all species at each station is included in the dense category with the status of seagrass conditions in the waters of Tayando Yamtel Village at station II being in good, healthy and rich condition, while at stations I and III in less healthy and less rich conditions.

PROFIL BEBERAPA KOMUNITAS PERAIRAN TELUK BANYUWEDANG

Banyuwedang Bay is located in the West Bali National Park (TNBB), which is part of the conservation area. There were a lot of tourism activities developed in this area, which potentially caused disruption of the surrounding water conditions. This study aims to determine the profile of various communities in the waters of Banyuwedang Bay and as initial data to see the development of these communities due to the environmental impacts of various activities in the waters and coastal area of Banyuwedang Bay. Data was collected from five observation stations including: Community structure of Plankton, Macrozoobenthos, and seagrass, and coral reef cover. Result shows that the phytoplankton community consist of 9 genera where Nitzschia sp. (Bacillariophyceae) as the dominant species found. Phytoplankton have low diversity index ranging between 0.348 - 1.411. Zooplankton diversity index also has low value 0.693 with three genera found namely Copepoda sp., Nauplius sp. and Daphnia sp. Macrozoobenthos community is composed of 17 macrozoobenthos genera, with low diversity index ranging ??between 0.562-1.809. There are only two species of seagrass, Halophila ovalis and Cymodocea rotundata, which were found in Banyuwedang Bay. Coral reef cover seems to be ranged from poor to good conditions, with 25% - 73% coverage. Based on the result, we concluded that the Banyuwedang Bay has a moderate condition to the growth of plankton and seagrass, but good for coral reefs. Periodic monitoring is needed to find out changes in conditions that can occur in Banyuwedang Bay.
 Keywords: Banyuwedang Bay; community structure; macrozoobenthos

Habitat characteristics of bicolor Pen Shell (Pinna bicolor) in seagrass ecosystems in Poncan Gadang Island, Sibolga City, Sumatera Utara

Poncan Gadang Island has a complete coastal ecosystem, one of them is the seagrass ecosystem. One of the biotas that inhabit the seagrass ecosystem on this island is the Bicolor pen shell. This shellfish has an ecological function because of its role in the food chain in seagrass beds and has economic value. This study aims to determine the abundance of bicolor pen shells and their relationship with seagrass cover, water physical-chemical parameters. The sampling was conducted at the research location in July – September 2021, which was located on Poncan Gadang Island. This activity was carried out at 3 stations. The primary data taken were the abundance of Bicolor pen shell, seagrass, physic-chemical parameters of water and substrate. Sampling was carried out three times. The results showed that the abundance of bicolor pen shells ranged from 1.8 to 2.8 ind m-2. Station 1 has the lowest abundance and the highest is at station 3. The PCA results show that the abundance of shellfish is positively related to temperature, DO, salinity and C-organic, while percent cover of seagrass, pH, water transparency, depth, and current were negatively related to the abundance of these shellfish.

Improving Approaches to Mapping Seagrass within the Great Barrier Reef: From Field to Spaceborne Earth Observation

Seagrass meadows are a key ecosystem of the Great Barrier Reef World Heritage Area, providing one of the natural heritage attributes underpinning the reef’s outstanding universal value. We reviewed approaches employed to date to create maps of seagrass meadows in the optically complex waters of the Great Barrier Reef and explored enhanced mapping approaches with a focus on emerging technologies, and key considerations for future mapping. Our review showed that field-based mapping of seagrass has traditionally been the most common approach in the GBRWHA, with few attempts to adopt remote sensing approaches and emerging technologies. Using a series of case studies to harness the power of machine- and deep-learning, we mapped seagrass cover with PlanetScope and UAV-captured imagery in a variety of settings. Using a machine-learning pixel-based classification coupled with a bootstrapping process, we were able to significantly improve maps of seagrass, particularly in low cover, fragmented and complex habitats. We also used deep-learning models to derive enhanced maps from UAV imagery. Combined, these lessons and emerging technologies show that more accurate and efficient seagrass mapping approaches are possible, producing maps of higher confidence for users and enabling the upscaling of seagrass mapping into the future.

A half-century of demographic changes in a green turtle (Chelonia mydas) foraging aggregation during an era of seagrass decline

To understand the demographic responses of green turtles to seagrass decline, we examined a data set from study of a mixed-stock foraging aggregation of immature green turtles, Chelonia mydas, collected in Bermuda (32o18’N, − 64o46’W) over five decades. Average turtle size (SCLmin) and mass declined by 22.3% and 58.2%, respectively. Aggregation size structure shifted to smaller sizes and now consists of more small turtles and fewer large turtles. Density (turtles ha−1) increased significantly but biomass (kg ha−1) remained unchanged and low compared to C. mydas biomass observed elsewhere. Green turtles exhibited reduced site fidelity during two portions of the study period, suggesting increased foraging effort. Reduction in turtle body condition index and seagrass coverage occurred from offshore to inshore. Changes in aggregation composition and behavior were consistent with expectations given a documented decline in seagrass availability, combined with increased output from source rookeries. Apparent response to resource decline is traced back to 1976, well before seagrass loss was first documented. Green turtles and their primary food source (Thalassia testudinum) are at the northern limit of their range in Bermuda, where seagrasses would be expected to have a reduced tolerance for natural grazing pressure and increased susceptibility to synergistic stressors, especially temperature, bioturbation and phosphorus limitation. Our results suggest that synergistic stressors, and not green turtles alone, have produced the observed reduction in seagrasses on the Bermuda Platform. Given that seagrass declines have been reported worldwide, our findings may suggest how green turtles will respond elsewhere.

Seagrass Diversity and Distribution in Maribojoc Bay, Bohol, Philippines

Seagrasses are major parts of coastal and marine biodiversity. Unfortunately, these aquatic plants and their ecological values are virtually unknown to many Filipinos. This study assessed the seagrasses in Maribojoc Bay, particularly in the coastal areas of the three municipalities, namely Maribojoc, Dauis, Panglao, and the City of Tagbilaran. Ecological assessments were conducted to determine the composition, abundance, distribution, percent cover, diversity, dominance, and evenness of seagrass species. Eight sampling sites were surveyed from October to December 2020. Seven seagrass species were identified, with Thalassia hemprichii as the most abundant (52.79%). There was a significant difference (p>0.05) in seagrass species relative abundance. The Shannon diversity index implies low diversity (H’=1.40) of seagrass species. High dominance (2.98) and low evenness (0.72) were attributed to the high abundance of T. hemprichii in the seagrass beds. The seagrass coverage was characterized by patchy and continuous meadows, with percentage cover ranging from 17.45% (poor) – 60% (good). Maribojoc Bay had a seagrass percentage cover of 38.65%, which can be classified under “fair” conditions. Seagrass community structure implies sparse coverage and low diversity, probably due to the deterioration of once-continuous meadows. However, further studies concerning seagrass communities are recommended in order to implement rehabilitation program or improve current management in Maribojoc Bay.

Bioaccumulation Potential of Seagrass (Halophila sp.) for Mercury in the Selected Coastal Areas in Malita, Davao Occidental

This research aims to determine the potential of seagrass Halophila sp. in absorbing mercury in the selected coastal barangays in Malita, Davao Occidental. The study employs a quantitative type of research where strategies focus on quantifying the collection and analysis of data. The study was conducted from October 2020 to December 2021 in the three selected coastal Barangays in Malita, Davao Occidental namely Brgy.Culaman, Brgy. New Argao and Brgy. Tingolo. Transect line quadrat method was used to determine the percent cover and shoot density of seagrass. Using this method, the point of seagrass occurrence up to the outer limit where seagrasses disappears was determined. There were three (3) stations and three (3) transect lines were laid for each station with an interval of 50m. Five (5) quadrats were also laid in the seagrasses along the 50 meter transect line positioned perpendicular to the shoreline. The seagrasses were collected in each quadrat and the samples were transported to Davao Analytical Laboratories, Inc. for mercury analysis. The highest mean percent cover (10.27%), shoot density (128.91 shoots/m2) and Hg accumulation (572.9ppb) was recorded in Brgy. Tingolo, Malita, Davao Occidental while the lowest result was obtained by Brgy. Culaman, Malita, Davao Occidental. Halophila sp. has the capability to accumulate mercury content and exceeds the standard level of 20 ppb. There were no significant differences in the percent cover and Hg accumulation of seagrass Halophila sp. while significant difference is observed in the shoot density. However, there is a positive relationship (.664) between the shoot density and mercury concentration of Halophila sp.

Predicted Changes in Seagrass Cover and Distribution in the Face of Sea Level Rise: Implications for Bivalve Aquaculture in a US West Coast Estuary

Predicted Changes in Seagrass Cover and Distribution in the Face of Sea Level Rise: Implications for Bivalve Aquaculture in a US West Coast Estuary

Novel Lethal Clostridial Infection in Florida Manatees (Trichechus manatus latirostris): Cause of the 2013 Unusual Mortality Event in the Indian River Lagoon

The Indian River Lagoon (IRL) on Florida’s east coast is a biologically diverse estuary and an important habitat to the threatened Florida manatee (Trichechus manatus latirostris). An unusual mortality event (UME) was declared by the Working Group on Marine Mammal Unusual Mortality Events in 2013 after a marked increase in manatee deaths in the IRL of an unknown cause. This UME followed a dramatic reduction of seagrass coverage in the IRL due to chronic non-toxic phytoplankton blooms, with a resultant ecosystem shift to mixed macroalgal dominance. At least 199 manatee deaths fitting the UME case definition were documented in and adjacent to the IRL during 2012–2019; mortality was highest in 2013, when 111 of these deaths were documented. The case definition included carcasses in good nutritional condition, with multiorgan congestion or wet lungs consistent with drowning without trauma. The gastrointestinal compartments of manatee carcasses were filled with diverse macroalga species, and the contents were notably more fluid than usual. Gross intestinal findings included blebbing to segmental thickening of the wall. Microscopic lesions were primarily intestinal, including necrosis, edema, hemorrhage, mucosa-associated lymphoid changes, and inflammation, sometimes associated with Gram-positive bacterial rods. A multidisciplinary approach of environmental and carcass sampling found no causative evidence through tests for micro- and macroalgal biotoxins, trace metals, general toxin screening, or vitreum biochemistry. Microbiological, cytological, immunohistochemical, and molecular analyses of Clostridiales from intestinal samples identified Clostridioides difficile toxin A, toxins A/B and toxin A gene; Paeniclostridium sordellii lethal gene (and other potential virulence factors from a sequenced strain); and Clostridium perfringens alpha and epsilon toxin genes. The results from this 8 year-long investigation are indicative that the cause of death in this manatee UME was associated with clostridial infection, initiated by a shift to a predominantly macroalgal diet.

Tides, Topography, and Seagrass Cover Controls on the Spatial Distribution ofPinna nobilison a Coastal Lagoon Tidal Flat

AbstractIn the last two decadesPinna nobilis, the largest bivalve mollusk endemic to the Mediterranean Sea, has recolonized the tidal flats of some coastal lagoons along the Italian Adriatic coast. In this study, we investigate the influence of tides on the spatial distribution and density of aP. nobilispopulation developing on a tidal flat of the Venice lagoon (NE Italy) by exploiting remote sensing technologies. Our results show that there is a threshold topographic elevation (about 0.5 m below mean sea level for the studied tidal flat) above which the number and duration of emersions become limiting factors ofP. nobilisabundance. Above this elevation, the population density decreases sharply. Densely populated areas tend to occur in tidal flat depressions, where the duration and frequency of emersions are low. We find, however, that the population density has large spatial variability in response to other factors, such as the seagrass percentage cover. The density of the shells increases with increasing seagrass percentage cover, and the denseP. nobilispopulation (0.8–2.68 N/m2) grows within theCymodocea nodosameadow that has a high seagrass percentage cover (>85%). However, within the seagrass meadow with the highest percentage cover, the bivalve preferentially colonizes the portion closest to the main channel, a possible source of nutrients. The shift toC. nodosa—dominated meadows in the Venice lagoon occurred during the last two decades has probably facilitated the observed colonization byP. nobilis. Our findings provide management information for protection and/or restoration ofP. nobilisin coastal lagoons.

Fish Assemblages in Subtidal Seagrass Meadows Surrounding the West Sand, South China Sea

Both beach seine (BS) and line transect-based underwater visual census (UVC) methods were applied to investigate fish assemblages of the West Sand (Xisha Islands, South China Sea) seagrass meadows and their relationship with seagrass coverage. The West Sand seagrass meadows were divided into four survey areas, two (areas A and B) to the north and two (areas C and D) to the south. The average seagrass coverages (%) in the north were significantly higher than those in the south (p < 0.05). The dominant seagrass species is Thalassia hemprichii, contributing to over 70% substrate coverage in all four areas. A total of 843 individuals from 25 fish species belonging to 22 genera, 11 families, and ten orders were recorded, with five species obtained by both BS and UVC methods. The labrids Halichoeres trimaculatus and Stethojulis strigiventer were the only two species that dominated (index of relative importance IRI > 500) in both BS and UVC methods. Fish assemblages among the four areas were significantly different (BS, p = 0.002; UVC, p = 0.002); area A was significantly different from areas B−D (p < 0.05), which was mainly driven by the variation in H. trimaculatus density. Furthermore, fish assemblages of the four areas (abundance data) revealed by both BS (r2 = 0.8455) and UVC (r2 = 0.9354) methods showed high correlation with the seagrass coverage. The majority of the individuals (>93%) recorded by BS and UVC methods were shorter than the total length at maturity (TLm), with 80.36 and 46.13% of individuals less than half the sizes of their TLm, respectively. Our results, for the first time, proved the importance of seagrass meadows at the West Sand as fish nursery grounds. The ecological roles of seagrass habitats in South China Sea merit further investigation.

Community Structure of Seagrass in Harapan Island, Seribu Islands, Indonesia

Seagrass is an aquatic biota that is ecologically and economically beneficial. This study aims to determine the community structure of seagrass in Harapan Island, Seribu Islands, Indonesia. The research was conducted using purposive sampling method, that divided into two sites which is considered to represent the ecology’s condition of Harapan Island. The site 1 was located adjacent to the mangroves area, while the site 2 was placed near a reclamation development and ship anchoring. Every site had three subsites as replications, each was 50 meters apart. Water parameters temperature, salinity, turbidity, pH, dissolved oxygen (DO), depth, and organic compound of each site were measured. The results showed that coverage of seagrass in site 2 is higher than site 1 that categorized as frequent-common coverage. The site 2 had the density of 220 individuals seagrass per square meter that dominated by Cymodocea rotundata. The highest diversity index was found in the site 1 at 1.15, with the lowest dominance index being 0.39. The distribution pattern in site 1 is clustered and uniform, while site 2 is uniform. The Importance Value Index in site 1 was 114,8 by Halodule uninervis, while site 2 was 125,5 by C. rotundata. It is thought that the low diversity of seagrass at the site 2 due to it was near a reclamation development and ship anchoring that disturb the abundance and community structure of seagrass in Harapan Island. Habitat destruction and anthropogenic impact of seagrass need to be minimized to support environmental coastal and fisheries sustainability.

Drone-Based Characterization of Seagrass Habitats in the Tropical Waters of Zanzibar

Unmanned automatic systems (UAS) are increasingly being applied as an alternative to more costly time-consuming traditional methods for mapping and monitoring marine shallow-water ecosystems. Here, we demonstrate the utility of combining aerial drones with in situ imagery to characterize the habitat conditions of nine shallow-water seagrass-dominated areas on Unguja Island, Zanzibar. We applied object-based image analysis and a maximum likelihood algorithm on the drone images to derive habitat cover maps and important seagrass habitat parameters: the habitat composition; the seagrass species; the horizontal- and depth-percent covers, and the seascape fragmentation. We mapped nine sites covering 724 ha, categorized into seagrasses (55%), bare sediment (31%), corals (9%), and macroalgae (5%). An average of six seagrass species were found, and 20% of the nine sites were categorized as “dense cover” (40–70%). We achieved high map accuracy for the habitat types (87%), seagrass (80%), and seagrass species (76%). In all nine sites, we observed clear decreases in the seagrass covers with depths ranging from 30% at 1–2 m, to 1.6% at a 4–5 m depth. The depth dependency varied significantly among the seagrass species. Areas associated with low seagrass cover also had a more fragmented distribution pattern, with scattered seagrass populations. The seagrass cover was correlated negatively (r2 = 0.9, p < 0.01) with sea urchins. A multivariate analysis of the similarity (ANOSIM) of the biotic features, derived from the drone and in situ data, suggested that the nine sites could be organized into three significantly different coastal habitat types. This study demonstrates the high robustness of drones for characterizing complex seagrass habitat conditions in tropical waters. We recommend adopting drones, combined with in situ photos, for establishing a suite of important data relevant for marine ecosystem monitoring in the Western Indian Ocean (WIO).