Responses of two Mediterranean seagrasses to experimental changes in salinity

The aim of this study is to examine the effects of variations in salinity levels on growth and survival of two fast-growing Mediterranean seagrasses, Cymodocea nodosa and Zostera noltii. We also tested the capacity of C. nodosa to acclimate to a gradual increase in salinity and to discover how it responds to a sharp rise in salinity in combination with other factors, such as increases in temperature, seasonality and different plant-population origins. Several short-term (10 days) experiments were conducted under controlled conditions. For each experiment, ten marked shoots were placed in 5-l aquaria, where they were exposed to different salinity treatments (ranging from 2 to 72 psu). Growth and survival of both species were significantly affected by salinity. A significant effect between salinity and temperature on the shoot growth rate of C. nodosa was also detected, but not on shoot mortality. When C. nodosa plants were acclimated by gradually increasing the salinity level, it was observed that acclimatisation improved tolerance to salinity changes. A different response to salinity variations, depending on the origin of the plants or the season of the year, was also detected. These results indicated that Z. noltii plants tolerate conditions of hyposalinity better than C. nodosa, and that the tolerance range of C. nodosa may change depending on the temperature, the season or the population.This research was financed by an ACUAMED contract and by an FPI grant (FPI 01 A 002) from the Generalitat Valenciana

Diel behaviour and trophic ecology of Scolopsis bilineatus (Nemipteridae)

Nemipterids are ubiquitous mid-sized fishes on Indo-Pacific reefs. We investigated the trophic ecology of the nemipterid species Scolopsis bilineatus at two locations on the Great Barrier Reef: One Tree Island and Orpheus Island. Fish ate a variety of benthic invertebrate taxa represented by rank: polychaetes, ophuiroids, sipunculids, nemerteans and small crustaceans. Polychaetes dominated the diet of fish of all sizes. Feeding behaviour and habitat utilization varied with the size of fish. Juveniles fed diurnally and adults nocturnally. Most juveniles fed rapidly in sand and rubble habitat during the day. In contrast, adults occupied shelter sites during the day, but dispersed onto sand to feed at night. A manipulative experiment demonstrated that small adult S. bilineatus exhibit opportunistic behaviour by responding to disturbance of the substratum for the purposes of feeding. Diurnal opportunistic feeding probably has a minimal influence on overall dietary intake. Identification of nocturnal feeding for adult S. bilineatus is of significant ecological importance, as nocturnal fishes often play unique and important roles in energy and nutrient cycling on reefs

The effects of manipulation of sedimentary iron and organic matter on sediment biogeochemistry and seagrasses in a subtropical carbonate environment

The microbial metabolism of organic matter (OM) in seagrass beds can create sulfidic conditions detrimental to seagrass growth; iron (Fe) potentially has ameliorating effects through titration of the sulfides and the precipitation of iron-sulfide minerals into the sediment. In this study, the biogeochemical effects of Fe availability and its interplay with sulfur and OM on sulfide toxicity, phosphorous (P) availability, seagrass growth and community structure were tested. The availability of Fe and OM was manipulated in a 2 × 2 factorial experiment arranged in a Latin square, with four replicates per treatment. The treatments included the addition of Fe, the addition of OM, the addition of both Fe and OM as well as no addition. The experiment was conducted in an oligotrophic, iron-deficient seagrass bed. Fe had an 84.5% retention efficiency in the sediments with the concentration of Fe increasing in the seagrass leaves over the course of the experiment. Porewater chemistry was significantly altered with a dramatic decrease in sulfide levels in Fe addition plots while sulfide levels increased in the OM addition treatments. Phosphorus increased in seagrass leaves collected in the Fe addition plots. Decreased sulfide stress was evidenced by heavier δ34S in leaves and rhizomes from plots to which Fe was added. The OM addition negatively affected seagrass growth but increased P availability; the reduced sulfide stress in Fe added plots resulted in elevated productivity. Fe availability may be an important determinant of the impact that OM has on seagrass vitality in carbonate sediments vegetated with seagrasses