Presence Of Seagrass Research Articles

Sediment modification by seagrass beds: Muddification and sandification induced by plant cover and environmental conditions

Seagrasses are well-known ecosystem engineers. They reduce water dynamics and sediment resuspension, and trap fine sediments. However, exceptions of this paradigm have been reported. To test whether these exceptions could be related to plant cover and environmental conditions, we investigated sediment modification under influence of seagrass presence in various annual eelgrass ( Zostera marina) beds with varying plant cover and sediment composition. At the relatively wave-exposed, sandy sites, dense vegetation caused muddification (increase in fine sediments and organic content) of the sediments. Sparse vegetation (<35% cover) had no effect, as such confirming the classical sediment trapping paradigm. In contrast, at the sheltered sites with muddy sediments, dense vegetation had no effect on the sediment composition, and in sparse vegetation sandification (decrease in fine sediments and organic content) was recorded. Sandification was never recorded before and was probably related to turbulence enhancement. Both, muddification and sandification are likely to provide a feedback on seagrass performance. Muddification may increase the nutrient input and, depending on the nutrient status of the system, either stimulate or reduce seagrass development. Similarly, sandification may postpone and even prevent extinction of seagrass beds when it occurs in areas that may have become too muddy for seagrass growth.

Frequency and success of ambush and chase predation in fish assemblages associated with seagrass and bare sediment in an Adriatic lagoon

Predation of fish assemblages in seagrass meadows was examined in the field and in tank experiments. Lure trolling indicated that (1) total abundance of fish was higher on bare sediment where small fish ( 15 cm) predominated; (3) large ambush predators, primarily the grass goby and European eel, were almost completely restricted to seagrass; (4) the predation mode in seagrass was almost entirely ambushing or stalk-attacking, while the predation mode on bare sediment was almost entirely chase-attacking; (5) ambush predation was far more successful than chase-attack predation; and (6) overall predation risk was approximately three times higher in seagrass. Tank experiments showed that piscivory success of the grass goby was higher than that of the most common chase-attacker, the black goby, and the presence or absence of artificial seagrass, regardless of density, had no significant effect on predation success of either species. Guts of the grass goby contained food items of a wider size range that averaged twice the size of those of the black goby. Our results confirm our prediction that the risk of predation, especially of small/juvenile fish, is higher in seagrass meadows than at adjacent bare substrate, and this risk differential is explained by the presence of larger, more efficient ambush predators restricted to seagrass, and the scarcity of large chase-attack predators in the Novigrad Sea.

An ecosystem‐scale predictive model of coastal seagrass distribution

Abstract Maintaining ecological processes that underpin the functioning of marine ecosystems requires planning and management of marine resources at an appropriate spatial scale. The Great Barrier Reef World Heritage Area (GBR) is the world's largest World Heritage Area (approximately 348 000 km2) and second largest marine protected area. It is difficult to inform the planning and management of marine ecosystems at that scale because of the high cost associated with collecting data. To address this and to inform the management of coastal (approximately 15 m below mean sea level) habitats at the scale of the GBR, this study determined the presence and distribution of seagrass by generating a Geographic Information System (GIS)‐based habitat suitability model. A Bayesian belief network was used to quantify the relationship (dependencies) between seagrass and eight environmental drivers: relative wave exposure, bathymetry, spatial extent of flood plumes, season, substrate, region, tidal range and sea surface temperature. The analysis showed at the scale of the entire coastal GBR that the main drivers of seagrass presence were tidal range and relative wave exposure. Outputs of the model include probabilistic GIS‐surfaces of seagrass habitat suitability in two seasons and at a planning unit of cell size 2 km×2 km. The habitat suitability maps developed in this study extend along the entire GBR coast, and can inform the management of coastal seagrasses at an ecosystem scale. The predictive modelling approach addresses the problems associated with delineating habitats at the scale appropriate for the management of ecosystems and the cost of collecting field data. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Regional and latitudinal variation in the diversity, dominance and abundance of microphagous microgastropods and other benthos in intertidal beds of dwarf eelgrass, Nanozostera spp.

The smaller macroscopic members of the epifauna and shallowly-burrowing infauna of comparable intertidal beds of dwarf eelgrass and associated areas of non-vegetated sediment were investigated with uniform methodology in the cool-temperate English southern North Sea (Nanozostera noltii), warm-temperate southern coast of the Western Cape, South Africa (N. capensis) and in subtropical southern Queensland, Australia (N. muelleri capricorni), together with equivalent seagrass sites in tropical Sulawesi, Indonesia, and Seychelles, Western Indian Ocean. Epifaunal microphagous microgastropods dominated both the eelgrass and non-vegetated cool- and warm-temperate sites with >80% of macrofaunal individuals, but decreased markedly in density and dominance with decreasing latitude, down to near zero in the tropics; microgastropod species diversity in the Nanozostera increased with decreasing latitude, whilst their species richness per core sample was highest in the warm temperate zone. Other co-existing—largely infaunal—taxa (mainly annelid worms, bivalve molluscs and crabs), however, showed less marked latitudinal variation in density and no relationship of taxon diversity with latitude. With few exceptional cases, microgastropod density, dominance, species richness and diversity were greater in the eelgrass beds than in adjacent non-vegetated sediments, as were the densities and taxon diversities of the associated faunal groups, although within the beds themselves there were no significant correlations between seagrass density and the density or diversity of either the microgastropods or their associated fauna. The extent to which the presence or absence of seagrass influenced the underlying community composition of the benthic fauna varied between localities. These results are consonant with an increasing effect of predation in low latitudes on small epifauna.

Nursery use patterns of commercially important marine fish species in estuarine systems along the Portuguese coast

Analysing the estuarine use patterns of juveniles of marine migrant fish species is vital for identifying important sites for juveniles as well as the basic environmental features that characterize these sites for different species. This is a key aspect towards understanding nursery function. Various estuarine systems along the Portuguese coast (Minho, Douro, Ria de Aveiro, Mondego, Tejo, Sado, Mira, Ria Formosa and Guadiana) were sampled during Spring and Summer 2005 and 2006. Juveniles of commercially important marine fish species Solea solea, Solea senegalensis, Platichthys flesus, Diplodus vulgaris and Dicentrarchus labrax, predominantly 0-group individuals, were amongst the most abundant species and had distinct patterns of estuarine use as well as conspicuous associations with several environmental features. Juvenile occurrence and density varied amongst estuaries and sites within them, and differed with species. Sites with consistently high juvenile densities were identified as important juvenile sites (i.e. putative nursery grounds). Through generalized linear models (GLM), intra-estuarine variation in occurrence and density of each of the individual species was largely explained by environmental variables (temperature; salinity; depth; percentage of mud in the sediment; presence of seagrass; importance of intertidal areas; relative distance to estuary mouth; macrozoobenthos densities; and latitude). Decisive environmental factors defining important sites for juveniles varied depending on the system as a result of different environmental gradients, though there were common dominant features for each species regardless of the estuary considered. Analysed environmental variables in the GLM also accounted for inter-estuarine variation in species' occurrence and density. In several estuaries, the identified important juvenile sites were used by many of these species simultaneously and may be of increased value to both management and conservation. Overall, the variability in site features amongst estuaries highlighted the tolerance of these species to different available environmental conditions and provided fundamental information for future spatially explicit modelling of their distribution. This should ultimately enable the prediction of species response to habitat alterations.

Spatial distribution of deepwater seagrass in the inter-reef lagoon of the Great Barrier Reef World Heritage Area

Seagrasses in waters deeper than 15 m in the Great Barrier Reef World Heritage Area (adjacent to the Queensland coast) were surveyed using a camera and dredge (towed for a period of 4 to 6 min); 1426 sites were surveyed, spanning from 10 to 25°S, and from inshore to the edge of the reef (out to 120 nautical miles from the coast). At each site seagrass presence, species, and biomass were recorded; together with depth, sediment, secchi, algae presence, epibenthos, and proximity to reefs. Seagrasses in the study area extend down to water depths of 61 m, and are difficult to map other than by generating distributions from point source data. Statistical modeling of the seagrass dis- tribution suggests 40 000 km 2 of the sea bottom has a probability of some seagrass being present. There is strong spatial variation driven in part by the constraint of the Great Barrier Reef's long, thin shape, and by physical processes associated with the land and ocean. All seagrass species found were from the genus Halophila. Probability distributions were mapped for the 4 most common species: Halophila ovalis, H. spinulosa, H. decipiens, and H. tricostata. Distributions of H. ovalis and H. spin- ulosa show strong depth and sediment effects, whereas H. decipiens and H. tricostata are only weakly correlated with environmental variables, but show strong spatial patterns. Distributions of all species are correlated most closely with water depth, the proportion of medium-sized sediment, and visibility measured by Secchi depth. These 3 simple characteristics of the environment correctly pre- dict the presence of seagrass 74% of the time. The results are discussed in terms of environmental dynamics, management of the Great Barrier Reef province, and the potential for using surrogates to predict the presence of seagrass habitats.

Predicting habitat suitability in temperate seagrass ecosystems

The worldwide observed dramatic decline of seagrasses has typically been attributed to multiple stressors such as eutrophication, disease, sedimentation, and toxicity events. Using principal component analysis and (multivariate) logistic regression, we investigated the importance of 30 commonly measured variables in explaining the presence and absence of the temperate seagrass species Zostera marina and Zostera noltii at 84 Western European locations. Although many interrelated variables influence seagrass presence in our dataset, presence or absence of both species could be reliably predicted by using only two easy‐to‐measure variables. A logistic regression model of Z. marina correctly predicted 77% of all observations by including water column light attenuation and sediment pore‐water reduction oxidation potential (RedOx). The Z. noltii model had an 86% accuracy based on only tidal location (intertidal or subtidal zone) and pore‐water RedOx. Applying the models to five evaluation sites demonstrated that both models can be usefully applied as tools for seagrass ecosystem restoration and conservation.

Habitat selection by emperor fish larvae

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 6:61-65 (2009) - DOI: https://doi.org/10.3354/ab00169 NOTE Habitat selection by emperor fish larvae Yohei Nakamura1,*, Takuro Shibuno2, David Lecchini3, Yoshiro Watanabe4 1Graduate School of Kuroshio Science, Kochi University, 200 Monobe, Nankoku, Kochi 783-8502, Japan 2National Research Institute of Aquaculture, 422–1, Nakatsuhamaura, Minami-ise, Mie 516-0193, Japan 3Centre IRD Nouméa, Institut de Recherche pour le Développement, UR 128 CoReUs, New Caledonia, France 4Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan *Email: ynakamura@kochi-u.ac.jp ABSTRACT: One of the great puzzles of coral reef fish ecology is how pelagic larvae locate the habitat in which they settle. The present study explored whether offshore emperor fish (Lethrinidae) larvae selected a specific reef and/or habitat at settlement. Although older juveniles are known to use back-reef seagrass beds as a potential nursery habitat, information is scarce regarding pre-settlement larvae. Using light traps anchored on the reef slopes at seagrass-replete and seagrass-free reefs (Ishigaki Island, Japan), lethrinid larvae were sampled weekly from April to June 2008. Larval abundance in light traps was similar on the 2 reef types. This result implies that lethrinid larvae did not or could not differentiate between the presence or absence of seagrass before settlement. However, in the laboratory, larvae collected from both reef types selected seagrass substrate rather than bare substrate or coral. In underwater visual censuses, newly settled juveniles were not observed on the seagrass-free reef, whereas they were abundant on the seagrass-replete reef. Together, these findings suggest that lethrinid larvae did not settle near the seagrass-free reef but rather preferentially settled near the seagrass-replete reef. KEY WORDS: Settlement · Coral reef fish · Lethrinidae · Seagrass bed · Habitat selection Full text in pdf format PreviousNextCite this article as: Nakamura Y, Shibuno T, Lecchini D, Watanabe Y (2009) Habitat selection by emperor fish larvae. Aquat Biol 6:61-65. https://doi.org/10.3354/ab00169 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 6. Online publication date: June 30, 2009 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2009 Inter-Research.

Seagrass habitat complexity does not always decrease foraging efficiencies of piscivorous fishes

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 377:43-49 (2009) - DOI: https://doi.org/10.3354/meps07869 Seagrass habitat complexity does not always decrease foraging efficiencies of piscivorous fishes Masahiro Horinouchi1,*, Naoki Mizuno2, Yuka Jo2, Masashi Fujita2, Mitsuhiko Sano3, Yuzuru Suzuki2 1Research Center for Coastal Lagoon Environments, Shimane University, 1060 Nishikawatsu-cho, Matsue, Shimane 6908504, Japan 2Fisheries Laboratory, Graduate School of Agricultural and Life Sciences, University of Tokyo, 2971-4 Bentenjima, Maisaka, Hamamatsu, Shizuoka 4310214, Japan 3Laboratory of Global Fisheries Science, Graduate School of Agricultural and Life Sciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 1138657, Japan *Email: hori@soc.shimane-u.ac.jp ABSTRACT: The foraging efficiencies of predators in relation to seagrass density and microhabitat use patterns of prey during predator presence/absence were examined by laboratory experiments. The foraging efficiency of the permanent resident piscivorous sculpin Pseudoblennius cottoides, which employed ambush and stalk-and-attack tactics, was higher in the presence of seagrass, while that of the transient piscivorous gnomefish Scombrops boops, which employed chase-and-attack tactics, decreased in denser seagrass. When coexisting with gnomefish, juveniles of the prey goby Gymnogobius heptacanthus sometimes entered seagrass areas to avoid attacks by such predators. In the absence of gnomefish, however, goby juveniles almost always stayed outside the seagrass areas (instead inhabiting open areas adjacent to the outer seagrass edge), even when sculpin were absent. These phenomena suggested that dense seagrass may be characterized by a constant potential risk (for prey) resulting from permanent resident predators whose foraging efficiency was improved by the habitat structure. Accordingly, prey species lacking tactics to efficiently decrease risks from such predators may remain outside the seagrass habitat, although close enough to successfully retreat to it in response to acute temporal risks offered by transient chase-and-attack predators. Such differential habitat use patterns corresponding to different predator types may be at least partly responsible for the concentrations of small fishes sometimes observed in open areas adjoining the outer margins of natural seagrass habitats. KEY WORDS: Permanent resident piscivorous fish · Foraging efficiency · Seagrass · Habitat complexity · Prey habitat choice Full text in pdf format PreviousNextCite this article as: Horinouchi M, Mizuno N, Jo Y, Fujita M, Sano M, Suzuki Y (2009) Seagrass habitat complexity does not always decrease foraging efficiencies of piscivorous fishes. Mar Ecol Prog Ser 377:43-49. https://doi.org/10.3354/meps07869 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 377. Online publication date: February 26, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Use of a high-resolution profiling sonar and a towed video camera to map a Zostera marina bed, Solent, UK

Seagrasses are flowering plants that develop into extensive underwater meadows and play a key role in the coastal ecosystem. In the last few years, several techniques have been developed to map and monitor seagrass beds in order to protect them. Here, we present the results of a survey using a profiling sonar, the Sediment Imager Sonar (SIS) and a towed video sledge to study a Zostera marina bed in the Solent, southern UK. The survey aimed to test the instruments for seagrass detection and to describe the area for the first time. On the acoustic data, the bed produced the strongest backscatter along a beam. A high backscatter above the bottom indicated the presence of seagrass. The results of an algorithm developed to detect seagrass from the sonar data were tested against video data. Four parameters were calculated from the SIS data: water depth, a Seagrass Index (average backscatter 10–15 cm above the bed), canopy height (height above the bed where the backscatter crosses a threshold limit) and patchiness (percentage of beams in a sweep where the backscatter 10–15 cm above the bed is greater than a threshold limit). From the video, Zostera density was estimated together with macroalgae abundance and bottom type. Patchiness calculated from the SIS data was strongly correlated to seagrass density evaluated from the video, indicating that this parameter could be used for seagrass detection. The survey area has been classified based upon seagrass density, macroalgae abundance and bottom type. Only a small area was occupied by a dense canopy whereas most of the survey area was characterised by patchy seagrass. Results indicated that Zostera marina developed only on sandy bottoms and was not found in regions of gravel. Furthermore, it was limited to a depth shallower than 1.5 m below the level of Lowest Astronomical Tide and present in small patches across the intertidal zone. The average canopy height was 15 cm and the highest density was 150 shoots m −2.

Small is beautiful: An inverted view of habitat fragmentation in seagrass beds

An intertidal Zostera marina landscape in Torbay, Devon, UK, was sampled to investigate the relationship between patch size, diversity and infaunal assemblage composition with the intention of defining a minimum Zostera patch size where the infaunal seagrass assemblage becomes distinct from the bare sand assemblage. All Zostera patches were found to support a higher level of biodiversity than the surrounding bare sand. However, the size of the Zostera patch had no impact on the level of diversity; it was just the presence or absence of seagrass that made a difference. The sediment and seagrass variables were not significantly different across the range of Zostera patch sizes, indicating that the environment characteristics were homogeneous within the Zostera patches at the patch scale. Multivariate analysis revealed that assemblage composition did vary between the patch types, although the opportunistic polychaete Capitella capitata was present in all patch types and was the most abundant species overall. The presence of opportunistic species and the homogeneity of the Zostera patch variables may be due to the location of this intertidal seagrass bed, which is relatively exposed compared to the locations of other seagrass beds along the south coast of Devon, resulting in a more dynamic and disturbed environment. Nevertheless, our results demonstrate that even small patches of seagrass comprising a few plants support a higher abundance and diversity of infaunal invertebrates than bare sand, indicating that Zostera patches have conservation value whatever their size.

Asociación a sustratos de los erizos regulares (Echinodermata: Echinoidea) en la laguna arrecifal de Isla Verde, Veracruz, México

Regular sea urchins substrate association (Echinodermata: Echinoidea) on Isla Verde lagoon reef, Veracruz, Mexico. The diversity, abundance, distribution and substrate association of the regular sea urchins found at the South part of Isla Verde lagoon reef, Veracruz, Mexico is presented. Four field sampling trips where made between October, 2000 and October, 2002. One sampling quadrant (23 716 m2) the more representative, where selected in the southwest zone of the lagoon reef, but other sampling sites where choose in order to cover the south part of the reef lagoon. The species found were: Eucidaris tribuloides tribuloides, Diadema antillarum, Centrostephanus longispinus rubicingulus, Echinometra lucunter lucunter, Echinometra viridis, Lytechinus variegatus and Tripneustes ventricosus. The relation analysis between the density of the echinoids species found in the study area and the type of substrate was made using the Canonical Correspondence Analysis (CCA). The substrates types considerate in the analysis where: coral-rocks, rocks, rocks-sand, and sand and Thalassia testudinum. The sand substrate with pieces of shell fragments was omitted from the analysis, since D. antillarum was the only species present in this habitat. During the two sampling climatic seasons the distribution of E. tribuloides tribuloides and C. longispinus rubicingulus was investigated to determine their substrate association on coral-rock and rock. The distribution of D. antillarum, E. lucunter lucunter and E. viridis was mainly associated to coral-rock, rock and sand substrates, while which the distribution of L. variegatus and T. ventricosus was more associated with presence of sea grass. The distribution of sea urchins is associated with the substrate type and their feeding habits which are also intimately linked. The seven species reached their biggest abundance during the raining season. E. lucunter lucunter and D. antillarum are considered in this study as bioindicators of the ecological quality of the coral reef. Rev. Biol. Trop. 56 (Suppl. 3): 281-295. Epub 2009 January 05.

Influence of environmental and prey variables on low tide shorebird habitat use within the Robbins Passage wetlands, Northwest Tasmania

Shorebirds feed primarily on tidal flats, and their distribution over these flats is influenced by their prey and abiotic factors. These factors act by influencing the distribution and abundance of the prey, or the shorebirds ability to exploit it. The aims of this study were to investigate the low tide foraging distribution of shorebirds at four sites within the Robbins Passage wetlands, and the environmental and invertebrate factors that may influence their distribution. The greatest densities and number of shorebirds were found at Shipwreck Point and East Inlet. The shorebirds within-site distribution was also non-random, with the shorebirds present in greatest densities at the water's edge and low intertidal stratum, although this varied among species. Generally, on a small spatial scale, invertebrate diversity was positively correlated, and seagrass leaf mass was negatively correlated, with shorebird feeding density. On a large spatial scale, invertebrate biomass and seagrass root mass were positively correlated with shorebird feeding density. Invertebrate biomass and seagrass root mass explained 71% of the variance in total shorebird feeding density on the tidal flats. The variation in shorebird feeding density and diversity was therefore partly explained by invertebrate diversity and biomass, as well as the environmental factors seagrass roots and leaf mass and tidal flat area, although the strength of these relationships was influenced by the two different spatial scales of the study. The strength of the relationships between shorebird feeding density and the invertebrate and environmental variables was stronger on a large spatial scale. The presence of seagrass may have influenced shorebird-feeding density by affecting the invertebrate abundance and composition or the shorebirds ability to detect and capture their prey. The area of the tidal flat had opposing effects on the shorebird species. These results can be used to assist in the development of management plans for the Robbins Passage wetlands and the conservation of important shorebird areas.

Ecosystem engineering by annual intertidal seagrass beds: Sediment accretion and modification

Seagrasses are generally known as ecosystem engineers, as they reduce flow velocities in their canopies. In perennial subtidal meadows, this usually leads to increased net sedimentation rates and reduction of the grain size. The present study aims to describe the contribution of annual seagrass populations to these processes and elucidate the temporal dynamics. Sediment accretion and grain size modification were experimentally tested by transplanting seedlings of an annual intertidal eelgrass population to an unvegetated tidal flat. Within the planting units (79 shoots m −2) 4.7 mm of sediment accreted, whereas in the most dense parts of these units (199 shoots m −2) accretion amounted to 7.1 mm. The silt fraction (<63 μm) increased and the sand fraction (63–500 μm) decreased in the eelgrass beds, which provides evidence that higher silt content in seagrass beds is the result and not the cause of seagrass presence. Annual intertidal eelgrass beds significantly contribute to the immobilisation of sediment during the growing season with its magnitude depending on canopy density. During winter, the accumulated sediments were released again and could even induce additional erosion. Possible consequences of these sediment dynamics for the larger scale functioning of estuarine ecosystems are discussed.

Distribuição e abundância dos caranguejos Uca Leach (Crustacea, Decapoda, Ocypodidae) na Baía de Guaratuba, Paraná, Brasil

Um estudo sobre distribuição espacial e abundância dos chama-marés Uca Leach, 1814 foi realizado na Baía de Guaratuba, Estado do Paraná. Foram coletados chama-marés de dez biótopos ao longo de um gradiente de salinidade de zero a 32 dentro da Baía de Guaratuba. Foram obtidas sete espécies, entre as quais, Uca mordax (Smith, 1870) que foi registrada somente em biótopos inundados por águas de baixas salinidades (de zero a 16). As demais espécies mostraram tolerância a uma ampla variação de salinidade, mas Uca maracoani (Latreille, 1802-1803) e Uca leptodactyla Rathbun, 1898 predominaram em águas mais salinas, de 14 a 32, enquanto U. burgersi Holthuis, 1967, Uca rapax (Smith, 1870), Uca thayeri Rathbun, 1900 e Uca uruguayensis Nobili, 1901 foram coletadas em mais de três biótopos e mostraram uma tendência ao eurihalismo, suportando salinidades de 4 a 32. Entretanto, outras características do substrato tais como porcentagem relativa de cascalho/areia/silte/argila, teor de matéria orgânica e presença de marismas, também, influenciaram a distribuição espacial destes caranguejos. U. leptodactyla foi registrada com densidade máxima de 240 ind.m-2, o mais alto valor conhecido.

Investigation of the Influence of Seagrass on Fisheries Catch Level Using Isotonic Regression

The relationship between seagrass area and the commercial catch of some economically important South Australian fish species is not known in any quantitative sense. However, there is evidence in the literature to suggest that seagrass decline will detrimentally impact on species abundance and composition, and consequently that the presence or absence of seagrass will compromise the commercial and recreational fishery. In this paper we describe the construction of models based on order restricted (isotonic) regression and discuss the role of seagrass area, in terms of its influence on the level of commercial catch through modifying the effect of fishing 'effort'.

Responses to Predation Cues and Food in Two Species of Sympatric, Tropical Sea Urchins

Abstract.We examined the responses of two tropical sea urchins,Lytechinus variegatusandTripneustes ventricosus, to cues from predators, simulated predation events and food. Cues released from damaged conspecifics, heterospecifics and heteroclassics (holothurian) were used to simulate predation events in field experiments. Responses to the presence of seagrass, spiny lobster or both were tested in tanks. Findings were supplemented by natural history observations of dispersion patterns off Bermuda. In field experiments, neither species formed groupings in the presence of conspecific, heterospecific or heteroclassic cues. Flight responses were greatest in conspecific treatments; responses to heterospecific cues were intermediate to control and conspecific cues. Urchins in pre‐assembled associations remained in groups in control trials but dispersed when exposed to predation cues.Lytechinusexhibited greater sensitivity to predation cues thanTripneustes. Cues from a damaged sea cucumber invoked a response fromLytechinusbut notTripneustes. Both species employed a two phased response to cues from damaged conspecifics: initially a rapid, but ephemeral (2 min), alarm response followed by a slower (≈ 35 % lower) sustained flight phase for 6+ min, which in nature would disperse urchins downstream and away from a predator. In tank experiments,Lytechinusformed groupings only around food or food + predators. The presence of a predator reduced the aggregation response to food, suggesting thatLytechinusemployed a risk aversion strategy.Tripneustesexhibited escape or refractory behavior in both control and experimental treatments in laboratory tanks.

Patterns in fish assemblages 25 years after major seagrass loss

Inferences about the effects of habitat loss may be derived from comparisons of differ- ent places when pre-impact data are unavailable. We compared the fish faunas of sandy substrata off beaches with and without seagrass in Cockburn Sound (Western Australia), a bay where >3000 ha of seagrass was lost from the 1950s to the 1970s, and for which there are no pre-impact data. Numbers of species and individuals varied considerably in space and time, and this variation obscured patterns that might have been associated with the presence or absence of seagrass. Multivariate tests indi- cated some significant differences in species composition between beaches with and without sea- grass; however, patterns among species were complex. Catches were strongly influenced by depth and the quantity of drifting vegetation. In general, fewer species were captured at shallower beaches. Hauls with a lot of drift captured more species, more individuals and higher biomasses of fishes that eat invertebrates and are associated with plants. When widespread seagrass loss occurs, resultant decreases in the supply of drift are likely to negatively affect densities of some fauna. We can make no firm conclusions about the effects of seagrass loss in Cockburn Sound, but suggest that declines in the production of drift have caused changes in fish assemblages.

Elements of habitat complexity that influence harpacticoid copepods associated with seagrass beds in a temperate bay.

The influence of habitat structure on abundance and taxonomic richness of epibenthic harpacticoid copepods in seagrass beds of Port Phillip Bay, Australia was investigated using artificial seagrass plants. The density and length of artificial seagrass plants was manipulated at three sites over two sampling times. Results for artificial plants were also compared with controls without plants. The presence of habitat structure in the form of artificial seagrass resulted in a significant increase in harpacticoid abundance at all sites and taxonomic richness at one site. In terms of artificial seagrass treatments, higher blade density resulted in higher harpacticoid abundance, but blade length and surface area had no significant effect. Taxonomic richness did not vary amongst artificial seagrass treatments. At the site where taxonomic richness was increased in the presence of artificial seagrass, rarefaction showed that the result was consistent with a passive increase related to increased sample size. In contrast, although abundances in artificial seagrass were significantly higher than in controls at the other two sites, the taxonomic richness was similar to controls, suggesting that the full range of taxa available was represented in control samples. This study shows that structural aspects of complexity can have importance beyond the simple provision of complexity in the form of increased surface area of habitat, and may depend on the scale examined. Further, the study emphasises the importance of spatial and temporal replication of experiments to give generality to results.

Competition between the tropical alga, Halimeda incrassata, and the seagrass, Thalassia testudinum

In this study, we experimentally investigate the interspecific interactions operating between the dominant seagrass and an abundant, native rhizophytic macroalga within a mature seagrass community. Treatments consisted of density manipulations of the dominant seagrass species, Thalassia testudinum, and of the most common representative of the rhizophytic algae, Halimeda incrassata. Evidence of interaction was measured by changes in relative short-term productivity and biomass of both algae and seagrass over a 4-month period. There was an asymmetrical effect of density manipulation: the presence of seagrass decreased the size of algal thalli by 20.4% and the macroalgal growth rate by 33.3%, but the presence of macroalgae had no significant impact on seagrass growth rate and decreased the mean size of short-shoots of T. testudinum by only 10.3%. These results support a competitive interaction theory for T. testudinum and rhizophytic macroalgae. Nutrient and light limitation were investigated as possible underlying mechanisms of the interaction. Seagrass leaf tissue C:N was significantly lower ( P=0.06) in algal removal treatments whereas no significant treatment effect was demonstrated for substrate-level irradiance, suggesting that competition for N was the mechanism of the interaction.