Seagrass Density Research Articles

Seasonal variation in species composition and abundance of demersal fish and invertebrates in a Seagrass Natural Reserve on the eastern coast of the Shandong Peninsula, China

Seagrass habitats are structurally complex ecosystems, which support high productivity and biodiversity. In temperate systems the density of seagrass may change seasonally, and this may influence the associated fish and invertebrate community. Little is known about the role of seagrass beds as possible nursery areas for fish and invertebrates in China. To study the functioning of a seagrass habitat in northern China, demersal fish and invertebrates were collected monthly using traps, from February 2009 to January 2010. The density, leaf length and biomass of the dominant seagrass Zostera marina and water temperature were also measured. The study was conducted in a Seagrass Natural Reserve (SNR) on the eastern coast of the Shandong Peninsula, China. A total of 22 fish species and five invertebrate species were recorded over the year. The dominant fish species were Synechogobius ommaturus, Sebastes schlegelii, Pholis fangi, Pagrus major and Hexagrammos otakii and these species accounted for 87% of the total number of fish. The dominant invertebrate species were Charybdis japonica and Octopus variabilis and these accounted for 98% of the total abundance of invertebrates. There was high temporal variation in species composition and abundance. The peak number of fish species occurred in August–October 2009, while the number of individual fish and biomass was highest during November 2009. Invertebrate numbers and biomass was highest in March, April, July and September 2009. Temporal changes in species abundance of fishes and invertebrates corresponded with changes in the shoot density and leaf length of the seagrass, Zostera marina.

Precise indices based on n-alkane distribution for quantifying sources of sedimentary organic matter in coastal systems

Precise indices based on n-alkane signatures were developed in order to determine the sources and composition of sedimentary organic matter (SOM) in coastal systems. The Arcachon Bay (France), a well-studied temperate lagoon, was used as an example of a complex coastal system sheltering a wide diversity of OM sources. Three main groups of sources were well discriminated from their n-alkane signatures: seagrass (Zostera sp.) produced mainly n-C17, n-C19, n-C21, n-C23 and n-C25 alkanes, algae (Rhodophyta, Chlorophyta) produced n-C15 and n-C17 and the terrigenous input [Quercus sp., Spartina sp. and river suspended particulate OM (SPOM)] was characterized by n-C25, n-C27, n-C29, n-C31 and n-C33. From the above and literature n-alkane fingerprints, we developed a set of indices (n-alkane ratios) to quantify the contribution of these three major sources of the SOM. At the Arcachon Bay scale, they indicated that SOM was composed mainly of seagrass (ca. 53±19%) and terrestrial (ca. 41±17%) material, followed by algae (ca. 6±9%). Moreover, the new n-alkane indices exhibited more relevant spatial patterns than classical ones – the TAR (C27+C29+C31/C15+C17+C19; terrestrial to aquatic ratio) and the Paq (C23+C25/C23+C25+C29+C31; aquatic plant %) – with a greater contribution from marine sources in the central part of the lagoon where a high density of Zostera seagrass was observed. Therefore, the development of precise indices adapted to the local diversity of OM sources is needed when using n-alkanes for quantifying the source composition of SOM in complex coastal systems.

Are large macroalgal blooms necessarily bad? Nutrient impacts on seagrass in upwelling-influenced estuaries.

Knowledge of nutrient pathways and their resulting ecological interactions can alleviate numerous environmental problems associated with nutrient increases in both natural and managed systems. Although not unique, coastal systems are particularly prone to complex ecological interactions resulting from nutrient inputs from both the land and sea. Nutrient inputs to coastal systems often spur ulvoid macroalgal blooms, with negative consequences for seagrasses, primarily through shading, as well as through changes in local biogeochemistry. We conducted complementary field and mesocosm experiments in an upwelling-influenced estuary, where marine-derived nutrients dominate, to understand the direct and indirect effects of nutrients on the macroalgal-eelgrass (Zostera marina L.) interaction. In the field experiment, we found weak evidence that nutrients and/or macroalgal treatments had a negative effect on eelgrass. However, in the mesocosm experiment, we found that a combination of nutrient and macroalgal treatments led to strongly negative eelgrass responses, primarily via indirect effects associated with macroalgal additions. Together, increased total light attenuation and decreased sediment oxygen levels were associated with larger effects on eelgrass than shading alone, which was evaluated using mimic algae treatments that did not alter sediment redox potential. Nutrient addition in the mesocosms directly affected seagrass density; biomass, and morphology, but not as strongly as macroalgae. We hypothesize that the contrary results from these parallel experiments are a consequence of differences in the hydrodynamics between field and mesocosm settings. We suggest that the high rates of water movement and tidal submersion of our intertidal field experiments alleviated the light reduction and negative biogeochemical changes in the sediment associated with macroalgal canopies, as well as the nutrient effects observed in the mesocosm experiments. Furthermore, adaptation of ulvoids and eelgrass to high, but variable, background nutrient concentrations in upwelling-influenced estuaries may partly explain the venue-specific results reported here. In order to manage critical seagrass habitats, nutrient criteria and macroalgal indicators must consider variability in marine-based nutrient delivery and local physical conditions among estuaries.

Decreasing seagrass density negatively influences associated fauna.

Seagrass meadows globally are disappearing at a rapid rate with physical disturbances being one of the major drivers of this habitat loss. Disturbance of seagrass can lead to fragmentation, a reduction in shoot density, canopy height and coverage, and potentially permanent loss of habitat. Despite being such a widespread issue, knowledge of how such small scale change affects the spatial distribution and abundances of motile fauna remains limited. The present study investigated fish and macro faunal community response patterns to a range of habitat variables (shoot length, cover and density), including individual species habitat preferences within a disturbed and patchy intertidal seagrass meadow. Multivariate analysis showed a measurable effect of variable seagrass cover on the abundance and distribution of the fauna, with species specific preferences to both high and low seagrass cover seagrass. The faunal community composition varied significantly with increasing/decreasing cover. The faunal species composition of low cover seagrass was more similar to sandy control plots than to higher cover seagrass. Shannon Wiener Diversity (H′) and species richness was significantly higher in high cover seagrass than in low cover seagrass, indicating increasing habitat value as density increases. The results of this study underline how the impacts of small scale disturbances from factors such as anchor damage, boat moorings and intertidal vehicle use on seagrass meadows that reduce shoot density and cover can impact upon associated fauna. These impacts have negative consequences for the delivery of ecosystem services such as the provision of nursery habitat.

Simulation of seagrass bed mapping by satellite images based on the radiative transfer model

Seagrass and seaweed beds play important roles in coastal marine ecosystems. They are food sources and habitats for many marine organisms, and influence the physical, chemical, and biological environment. They are sensitive to human impacts such as reclamation and pollution. Therefore, their management and preservation are necessary for a healthy coastal environment. Satellite remote sensing is a useful tool for mapping and monitoring seagrass beds. The efficiency of seagrass mapping, seagrass bed classification in particular, has been evaluated by mapping accuracy using an error matrix. However, mapping accuracies are influenced by coastal environments such as seawater transparency, bathymetry, and substrate type. Coastal management requires sufficient accuracy and an understanding of mapping limitations for monitoring coastal habitats including seagrass beds. Previous studies are mainly based on case studies in specific regions and seasons. Extensive data are required to generalise assessments of classification accuracy from case studies, which has proven difficult. This study aims to build a simulator based on a radiative transfer model to produce modelled satellite images and assess the visual detectability of seagrass beds under different transparencies and seagrass coverages, as well as to examine mapping limitations and classification accuracy. Our simulations led to the development of a model of water transparency and the mapping of depth limits and indicated the possibility for seagrass density mapping under certain ideal conditions. The results show that modelling satellite images is useful in evaluating the accuracy of classification and that establishing seagrass bed monitoring by remote sensing is a reliable tool.

Terrestrial Laser Scanning Reveals Seagrass Microhabitat Structure on a Tideflat

Local-scale environmental heterogeneity can provide microhabitats that influence the spatial distribution of competing species. Microhabitats may influence the distribution of seagrasses along elevation gradients, but difficulty measuring intertidal microtopography has hindered quantification. Using a terrestrial laser scanner (TLS), we mapped and monitored a 1.84 ha study site for three years to understand spatial and temporal patterns of sediment microtopography. We performed high-accuracy GPS surveys and vegetation surveys of a native and an invasive seagrass. TLS provided sub-decimeter scale precision in digital elevation models (DEMs) of the tideflat. The location and shape of microtopographic features were stable from year to year, but the magnitude of local relief varied. A simple index of topographic context predicted the shoot density of the native seagrass, Zostera marina and the invasive seagrass, Zostera japonica, but the shoot density of the invasive seagrass was better predicted by the shoot density of Z. marina than by topographic context. Microtopographic relief at this site appears to exert a strong influence on the meter-scale distribution of seagrass. We demonstrate the potential for TLS mapping of habitat-relevant microtopography in a soft sediment intertidal environment where TLS faces substantial challenges but promises unique insights.

Let it flow: how does an underlying current affect wave propagation over a natural seagrass meadow?

Temperate seagrass beds can be found within intertidal and tidal areas of variable hydrodynamic forcing. To investigate the interaction between hydrodynamics and seagrass plants, Zostera noltii meadows were exposed to a range of combinations of waves and flow in a flume. Velocity profiles were obtained to observe the evolution of hydrodynamic forcing due to the presence of a seagrass canopy. Observations under flow without waves agreed with those of previous studies and showed that turbulent kinetic energy (TKE) at the top of the canopy increased with increasing flow velocity as well as with distance from the leading edge. The highest TKE values were achieved for the highest flow velocity without waves, and the presence of waves generally led to reduced TKE values. Profiles of time-averaged downstream velocity were not affected by the presence of waves. However, flow reduction near the bed increased with higher seagrass density, resulting in an S-shaped profile for the highest density and flow velocity. Underlying currents affected wave orbital velocities, causing a reduction of orbital diameters in the free-stream region. In the canopy region, this process was superimposed by the damping effect of the seagrass meadow, but only positive flow components were reduced by seagrass presence. Streamlining and oscillating motion of seagrass leaves were observed under flow and waves, respectively. These processes did not affect the wave/flow attenuation capacity of the meadow, which indicates that the ecosystem-engineering traits of seagrasses are not compromised by increased flow velocity or in the presence of waves.

ANALISIS KONDISI LAMUN (SEAGRASS) DI PERAIRAN PULAU PRAMUKA, KEPULAUAN SERIBU

ABSTRACT
 This research aims to analyze the condition of seagrass ecosystem to see water quality data of the seagrass habitat and percentage cover of seagrass in the waters of the Pramuka Island, Seribu Islands. The research was conducted over two months from October to November 2014.This research used a descriptive method with field survey approach. The population in this study is the seagrass in Waters Pramuka Island. Determining the location with purposive sampling of the sampling is divided into three stations is North, East and South. Data collection techniques include primary data and secondary data. Primary data is data of seagrass (type, percentage cover and density of seagrass) and data of seagrass habitat environmental parameters (water temperature, current speed, brightness, depth, salinity, substrate, TSS, DO, pH) were obtained by direct measurement in the field, while secondary data include the general state of the research sites. Data analysis techniques used in this study using analysis of community structure of seagrass and water quality analysis. The results showed that seagrass species found in the Pramuka Island there are 6 types of seagrass Cymodocea rotundata, Cymodocea serrulata, Enhalus acoroides, Halophila ovalis, Halodule uninervis, Thalassia hemprichii. Conditions of seagrass in the waters of the Pramuka Island included into the category of less healthy-poor seagrass. At station 1 percentage by 31% classified seagrass less healthy conditions, while the other two stations are stations 2 and 3 belong to the category of the poor condition of seagrass, with each percentage cover of seagrass 19.4% and 20.3%. Of all water quality parameters measured, all the parameters are still in normal circumstances, but there are some parameters whose value is high at some stations TSS and pH value is high at station 2 with a value of TSS 18 mg/l and a pH value of 8.2. Water quality and seagrass communities in station 1 is still in good condition for the growth of seagrass, because at this station is an unspoiled area away from human activity, while the stations 2 and 3 have undergone changes in community structure of seagrass because at this station has several anthropogenic activities that disrupt the lives of seagrass, mostly from human activities such as domestic sewage and hoarding/reclamation, which affects the condition of seagrass at station 2 and 3 are poor seagrass.
 Keyword: Seagrass, Water Quality, Pramuka Island

Age and size compositions, habitats, growth and reproductive characteristics of a terapontid (Pelates octolineatus) in coastal waters

This study of Pelates octolineatus is the first to use individually aged fish to describe the life cycle of a terapontid, a speciose and abundant Indo-West Pacific family. On the lower west Australian coast, this species uses dense seagrass as a nursery area and, after ~1 year of life when approaching 100mm in total length (TL), moves into deeper waters over sparser seagrass where it matures at the end of its second year at ~140–170mm. The maximum TL and age were 256mm and 10 years. A modified von Bertalanffy curve, allowing for a linear increase in the growth coefficient with age, improved the fit to the lengths at age of older P. octolineatus. Growth was even better described by extending this model to allow for seasonality through incorporating a sine-based curve. This model described well the seasonality exhibited by modal progressions in monthly length–frequency distributions. Instantaneous growth rates, particularly of the youngest age classes, peaked in the warm, summer months and the amplitude of seasonal change in these rates declined with increasing age. Gonadal recrudescence occurred in early spring as temperature and day length increased and spawning peaked in late-spring to mid-summer when temperatures were approaching their maxima.

The application of oyster and seagrass models to evaluate alternative inflow scenarios related to Everglades restoration

The South Florida landscape is highly engineered featuring ∼3380km of canals, ∼1225 water control structures, >70 pumping stations, managed wetlands, densely populated coastal watersheds, and impacted estuaries. Landscape scale agricultural and urban modifications require flood control which sometimes results in the release of excess freshwater that potentially damages estuarine ecology. The Central Everglades Planning Project (CEPP) is a partnership between the U.S. Army Corps of Engineers and the South Florida Water Management District focused on restoring natural patterns of freshwater flow to the Everglades and associated estuaries. One of the primary project goals is to reduce deleterious inflows eastward to the St. Lucie Estuary (SLE) and westward to the Caloosahatchee River Estuary (CRE) by diverting freshwater south from Lake Okeechobee. Performance evaluation of the CEPP relies upon integrated modeling which links the watershed engineering projects to estuarine salinities and biotic responses. The objective of this study was to utilize seagrass and oyster models to evaluate the effectiveness of alternative inflow scenarios. There are three different scenarios: the existing condition base (ECB), the future without proposed CEPP projects (FWO) but with other landscape features that could alter freshwater inflows, and alternative 4R (ALT4R) that incorporates a suite of restoration projects including those associated with the CEPP. Each of these inflow scenarios was used to generate daily salinity time series from 1965 to 2005 at multiple locations in the SLE and CRE which were used to run site-specific seagrass and oyster models. The hydrodynamic and ecological effects of freshwater inflow were greater in the SLE compared to the much larger CRE. Predicted densities of oysters (individualsm−2) and seagrass (shootsm−2) were greater in the wet season (May–Oct) vs. the dry season (Nov–Apr) in both estuaries. Oyster and seagrass densities increased under the ALT4R inflow scenario that mitigated high inflows to the estuaries in the wet season. This result improved environmental conditions while increasing freshwater availability for other parts of the Greater Everglades and the southern estuaries such as Florida Bay and Biscayne Bay.

Sunspots drive seagrasses

Seagrasses are underwater flowering plants that form an important marine habitat worldwide. They respond to watershed and climate influences and are considered a good indicator of environmental health. Investigations into seagrass photosynthesis, response to high irradiance and UV light are providing new insights into controls on plant production. We show that the size and density of the intertidal seagrass directly relates to sunspot activity. The influence of sunspot activity has been largely overlooked in plants and never examined in seagrasses. The correlation between seagrass canopy height and the number of sunspots was highly significant, with height decreasing steadily as sunspots increase and recover only after a substantial decline in sunspot activity. The density of seagrass shoots correlated positively with sunspots until, at high sunspot numbers (>110 per month), plant density dropped and then rebounded. High sunspot activity inhibits the production and alters the meadow structure of tropical intertidal seagrass habitats.

Interactions between seagrass complexity, hydrodynamic flow and biomixing alter food availability for associated filter-feeding organisms.

Seagrass shoots interact with hydrodynamic forces and thereby a positively or negatively influence the survival of associated species. The modification of these forces indirectly alters the physical transport and flux of edible particles within seagrass meadows, which will influence the growth and survivorship of associated filter-feeding organisms. The present work contributes to gaining insight into the mechanisms controlling the availability of resources for filter feeders inhabiting seagrass canopies, both from physical (influenced by seagrass density and patchiness) and biological (regulated by filter feeder density) perspectives. A factorial experiment was conducted in a large racetrack flume, which combined changes in hydrodynamic conditions, chlorophyll a concentration in the water and food intake rate (FIR) in a model active filter-feeding organism (the cockle). Results showed that seagrass density and patchiness modified both hydrodynamic forces and availability of resources for filter feeders. Chlorophyll a water content decreased to 50% of the initial value when densities of both seagrass shoots and cockles were high. Also, filter feeder density controlled resource availability within seagrass patches, depending on its spatial position within the racetrack flume. Under high density of filter-feeding organisms, chlorophyll a levels were lower between patches. This suggests that the pumping activity of cockles (i.e. biomixing) is an emergent key factor affecting both resource availability and FIR for filter feeders in dense canopies. Applying our results to natural conditions, we suggest the existence of a direct correlation between habitat complexity (i.e. shoot density and degree of patchiness) and filter feeders density. Fragmented and low-density patches seem to offer both greater protection from hydrodynamic forces and higher resource availability. In denser patches, however, resources are allocated mostly within the canopy, which would benefit filter feeders if they occurred at low densities, but would be limiting when filter feeder were at high densities.

Toxic effects of increased sediment nutrient and organic matter loading on the seagrass Zostera noltii

As a result of anthropogenic disturbances and natural stressors, seagrass beds are often patchy and heterogeneous. The effects of high loads of nutrients and organic matter in patch development and expansion in heterogeneous seagrass beds have, however, poorly been studied. We experimentally assessed the in situ effects of sediment quality on seagrass (Zostera noltii) patch dynamics by studying patch (0.35m diameter) development and expansion for 4 sediment treatments: control, nutrient addition (NPK), organic matter addition (OM) and a combination (NPK+OM). OM addition strongly increased porewater sulfide concentrations whereas NPK increased porewater ammonium, nitrate and phosphate concentrations. As high nitrate concentrations suppressed sulfide production in NPK+OM, this treatment was biogeochemically comparable to NPK. Sulfide and ammonium concentrations differed within treatments, but over a 77 days period, seagrass patch survival and expansion were impaired by all additions compared to the control treatment. Expansion decreased at porewater ammonium concentrations >2000μmolL−1. Mother patch biomass was not affected by high porewater ammonium concentrations as a result of its detoxification by higher seagrass densities. Sulfide concentrations >1000μmolL−1 were toxic to both patch expansion and mother patch. We conclude that patch survival and expansion are constrained at high loads of nutrients or organic matter as a result of porewater ammonium or sulfide toxicity.

Ecosystem metabolism along a colonization gradient of eelgrass (Zostera marina) measured by eddy correlation

The Virginia coastal bays experienced local extinction of eelgrass (Zostera marina) during the early 1930s, and restoration beginning in 2001 has generated an ecosystem state change from bare to vegetated sediments. Oxygen fluxes were measured seasonally using the eddy correlation technique at three sites representing different stages of seagrass colonization: unvegetated (bare), 5 yr, and 11 yr since seeding. Derived seasonal ecosystem respiration (R) and gross primary production (GPP) increased up to 10‐fold and 25‐fold, respectively, with meadow age. Although hourly oxygen (O2) fluxes were highly correlated with light at the vegetated sites, no identifiable trends with light were observed at the bare site. The light compensation point where O2 production and respiration are in balance increased from 46 µmol photons m−2 s−1 to 257 µmol photons m−2 s−1 and 63 µmol photons m−2 s−1 to 472 µmol photons m−2 s−1 at the 5 yr and 11 yr seagrass sites, respectively, with increasing seasonal temperatures from 12.3°C to 27.9°C and 9.3°C to 30.5°C, respectively. This suggests that more light, and thus more O2 production, is required to offset increasing respiration with both temperature and meadow age. Photosynthesis—irradiance curves generated from hourly O2 fluxes throughout the seasons were used to estimate annual net ecosystem metabolism (NEM). Annual NEM rates at the bare, 5 yr, and 11 yr sites were −7.6, 8.6, and −7.0 mol O2 m−2 yr−1, respectively. Although the system went through a period of net autotrophy during early stages of colonization, the ecosystem state change from unvegetated sediments to dense seagrass meadows changed the magnitude of both GPP and R, but not the overall metabolic balance of the system.

Bimini Islands: a characterization of the two major nursery areas; status and perspectives.

Bimini Islands (Bahamas, 25°44′ N 79°16′ W) are characterized by a unique tropical marine environment which provides critical nursery habitats and food resources for many important species of ecological and economical value. Two areas are particularly important in the function and dynamics of the local marine environment: North Sound and South Bimini. Since 1998 the northern part of the island has been subject to an intense urbanization process that involves the construction of an extensive touristic complex. Over the years this activity has radically modified a substantial portion of the land, and part of the underwater environment as well, threatening the fragile balance of the North Sound nursery ground. Effects on marine habitats and on local species have been reported, and although some measures to limit the damage have already been taken, the local ecosystem could ultimately suffer from continuation of the construction work on the area. In 2010, we performed surveys of both main nursery grounds to assess the current ecological status and the main differences between the two areas, investigating macrobenthic epifauna abundance, seagrass density and abiotic parameters. The results of this study indicate that the ecosystem still appears in reasonably healthy condition, although showing some concerning trends. These data provide baseline conditions to assess further changes, and possibly to support the development of plans for the conservation of the North Sound and South Bimini coastal ecosystems.

Comparison of remotely-sensed surveys vs. in situ plot-based assessments of sea grass condition in Barnegat Bay-Little Egg Harbor, New Jersey USA

With the increasing appreciation that sea grass habitats are in global decline, there is a great need to be able to efficiently and effectively assess and characterize the status and trends of sea grass in our coastal ecosystems. This paper examines the utility of remotely sensed vs. in situ plot-based monitoring using the Barnegat Bay-Little Egg Harbor (BB-LEH), New Jersey, USA estuarine system as a case study. Eelgrass (Zostera marina) is the dominant species, while widgeon grass (Ruppia maritima) is also common in lower salinity regions of the BB-LEH. Aerial imagery collected during the months of July and August 2009 was interpreted and mapped using object based image analysis techniques, similar to techniques used in the 2003 mapping survey of this system. Boat-based in situ monitoring data were collected concurrently with the aerial photography to assist the image interpretation and for an independent accuracy assessment. We compared the remotely-sensed mapping of sea grass cover change (in 2003 vs. 2009) vs. in situ plot-based monitoring conducted from 2004 through 2009. Comparison of the remotely-sensed vs. the in situ plot-change analysis suggests that the two methodologies had broadly similarly results, with the percent area showing declines in sea grass cover greater than those that exhibited increases. In conclusion, the two studies provide corroborating evidence that sea grass has declined in percent cover in the BB-LEH system during the decade of the 2000’s. While remotely-sensed surveys provide synoptic information for a “big picture” view on sea grass distribution, site specific in situ sampling is required to determine other aspects of sea grass status, e.g. above vs. below-ground biomass, blade length, shoot density, epiphytic loading, etc. Either method alone gives an incomplete picture. As demonstrated in this study, to fully characterize the spatial extent, health, and density of sea grass meadows across the entire estuary, combining remote sensing surveys concomitantly with comprehensive in situ assessment provides the most robust approach.

Foraging behavior, prey distribution, and microhabitat use by bottlenose dolphins Tursiops truncatus in a tropical atoll

The study of habitat use by top predators is important for understanding community interactions and is necessary for sound ecosystem management. In marine systems, top predators such as sharks and cetaceans have a strong impact on the structure and function of communities. While the observation of habitat use and foraging behavior of most marine predators is logistically difficult, bottlenose dolphins Tursiops truncatus offer less of a challenge due to visible surface behavior and well-documented populations. We examined bottlenose dolphin behavior in relation to microhabitat classes at Turneffe Atoll, Belize. The dolphins were found to feed proportionally more in boundary microhabitats, areas where dense seagrass beds adjoined open sand flats, than in other microhabitats. Fish density, particularly schools of grunts (family Haemulidae), were higher in the boundary microhabitat than in seagrass or sand microhabitats. Extensive acoustic recordings yielded few fish calls, suggesting that passive listening for soniferous fish was not the dominant means of diurnal prey detection. The dolphins’ disproportionate use of boundary microhabitats for feeding was likely due to the abundance and accessibility of prey.

Evaluating Light Availability, Seagrass Biomass, and Productivity Using Hyperspectral Airborne Remote Sensing in Saint Joseph’s Bay, Florida

Seagrasses provide a number of critical ecosystem services, including habitat for numerous species, sediment stabilization, and shoreline protection. Ariel photography is a useful tool to estimate the areal extent of seagrasses, but recent innovations in radiometrically calibrated sensors and algorithm development have allowed identification of benthic types and retrieval of absolute density. This study demonstrates the quantitative ability of a high spatial resolution (1 m) airborne hyperspectral sensor (3.2 nm bandwidth) in the complex coastal waters of Saint Joseph’s Bay (SJB). Several benthic types were distinguished, including submerged and floating aquatic vegetation, benthic red algae, bare sand, and optically deep water. A total of 23.6 km2 of benthic vegetation was detected, indicating no dramatic change in vegetation area over the past 30 years. SJB supported high seagrass density at depths shallower than 2 m with an average leaf area index of 2.0 ± 0.6 m2 m−2. Annual seagrass production in the bay was 13,570 t C year−1 and represented 41 % of total marine primary production. The effects of coarser spatial resolution were investigated and found to reduce biomass retrievals, underestimate productivity, and alter patch size statistics. Although data requirements for this approach are considerable, water column optical modeling may reduce the in situ requirements and facilitate the transition of this technique to routine monitoring efforts. The ability to quantify not just areal extent but also productivity of a seagrass meadow in optically complex coastal waters can provide information on the capacity of these environments to support marine food webs.

A test of the use of computer generated visualizations in support of ecosystem-based management

Systematic scenario analysis is increasingly being used as an approach to evaluate ecosystem-based management options, often using “storylines” communicated through computer-generated (CG) images or visualizations. To explore potential issues associated with using CG imagery for conveying scenarios of habitat restoration we performed experiments in the Puget Sound, Washington region in which we asked whether respondents could differentiate among images of varying seagrass density and spatial extent, and if the presence of humans in the images affected these assessments and their perceptions of ecosystem health. Respondents were able to grossly determine relative seagrass density in the images, but only about 50% of them were able to determine this perfectly. Most errors occurred when the difference in density was small: approximately 20shootm−2. The ability to correctly distinguish among images was inversely correlated with educational level. The presence or absence of people in the imagery did not influence the ability of respondents to correctly sort images, nor did it affect perceptions of ecosystem “health”. Taken together, the results suggest that such imagery can be useful as basis for communicating large differences in ecological conditions, but may be less informative as means to convey marginal changes in ecological structure. This work begins to highlight some of the pitfalls, but also the promise, of the use of CG visualization in marine resource management.

Effects of nutrient enrichment on seagrass population dynamics: evidence and synthesis from the biomass–density relationships

Summary The available data from experimental and descriptive studies on seagrass biomass and density responses to nutrient enrichment were analysed to assess the intraspecific mechanisms operating within seagrass populations and whether biomass–density relationships can provide relevant metrics for monitoring seagrasses. The response of shoot biomass and density to nutrient enrichment was dependent on the type of study; the short‐term positive response of biomass and density in experimental studies reveals context‐specific nutrient limitation of seagrasses. The long‐term negative response of descriptive studies probably results from ecosystem‐scale events related to nutrient enrichment such as increased turbidity, algal blooms, epiphyte loads and anoxia. Most seagrass species analysed lie in the nonthinning part of the theoretical biomass–density curves. A simultaneous increase in biomass and decrease in density, evidence of self‐thinning, were only observed in 4 of 28 studies. The analysis of both the static and the dynamic biomass–density relationships revealed that the slopes increase under nutrient enrichment. Surprisingly, the species‐specific slopes (log B‐log D) were higher than one, revealing that the B/D ratio, that is, the average shoot biomass, increases with density in all seagrass species analysed. Nutrient enrichment further enhanced this effect as biomass–density slopes increased to even higher values. The main drivers behind the increasing biomass–density slopes under nutrient enrichment were the increase in shoot biomass at densities above a species‐specific threshold and/or its decrease below that threshold. Synthesis. Contrasting short‐ and long‐term responses of both biomass and density of seagrasses to nutrient enrichment suggest that the former, positive ones result from nutrient limitation, whereas the later, negative ones are mediated by whole ecosystem responses. In general, shoot biomass of seagrasses increases with density, and nutrient enrichment enhances this effect. Experimental testing of facilitation processes related to clonal integration in seagrasses needs to be done to reveal whether they determine the low incidence of self‐thinning and the intriguing biomass–density relationships of seagrass species. The increasing slopes and decreasing intercepts of the species‐specific dynamic biomass–density relationships of seagrasses and the decreasing coefficients of variation of both biomass and density constitute relevant, easy‐to‐collect metrics that may be used in environmental monitoring.