Leaf consumption by <Emphasis Type="Italic">Sesarma plicata</Emphasis> in a mangrove forest at Jiulongjiang Estuary,China

The feeding ecology of Sesarma plicata (Grapsidae: Sesarminae), the most abundant crab species in a mangrove forest dominated by Kandelia candel at Jiulongjiang Estuary, China, was investigated through field and laboratory experiments. Feeding preference and consumption rates were determined on mature, senescent and decomposed leaves of Kandelia candel, Bruguiera gymnorrhiza and Aegiceras corniculatum. In the laboratory, S. plicata preferred leaves of K. candel over those of B. gymnorrhiza and A. corniculatum, and consumed significantly more decomposed leaves than mature and senescent ones, irrespective of crab size. Field experiments with limited power failed to reveal detectable species preferences despite more consumption of K. candel, but decomposed leaves of each species were again preferred. Leaf characteristics associated with preference changed with plant species and leaf state. Low tannins and high water content characterized the preference for decomposed state of leaves. Species preference was significantly and negatively related to crude fibers and C:N ratios for mature leaves, and crude fiber for senescent leaves, but significantly and positively related to water content for decomposed leaves. Leaf consumption rates averaged for all leaf categories from laboratory no-choice feeding experiments were 0.101, 0.055 and 0.017 gDW ind⁻¹ d⁻¹ for large, medium and small crabs, respectively. In this forest, mean density of S. plicata was 20.5 ind m⁻² as assessed by a manual catching method. Leaf litter removal rate during neap tides by sesarmid crabs was about 1.33 gDW m⁻² d⁻¹ in April 2006. The leaves removed by crabs were grazed on the sediment surface or taken into crab burrows, shredded and stored before being eaten.     

Microbial activity and carbon, nitrogen, and phosphorus content in a subtropical seagrass estuary (Florida Bay): evidence for limited bacterial use of seagrass production

Bacterial abundance, production, and extracellular enzyme activity were determined in the shallow water column, in the epiphytic community of Thalassia testudinum, and at the sediment surface along with total carbon, nitrogen, and phosphorus in Florida Bay, a subtropical seagrass estuary. Data were statistically reduced by principle components analysis (PCA) and multidimensional scaling and related to T. testudinum leaf total phosphorus content and phytoplankton biomass. Each zone (i.e., pelagic, epiphytic, and surface sediment community) was significantly dissimilar to each other (Global R = 0.65). Pelagic aminopeptidase and sum of carbon hydrolytic enzyme (esterase, peptidase, and α- and β-glucosidase) activities ranged from 8 to 284 mg N m⁻² day⁻¹ and 113–1,671 mg C m⁻² day⁻¹, respectively, and were 1–3 orders of magnitude higher than epiphytic and sediment surface activities. Due to the phosphorus-limited nature of Florida Bay, alkaline phosphatase activity was similar between pelagic (51–710 mg P m⁻² day⁻¹) and sediment (77–224 mg P m⁻² day⁻¹) zones but lower in the epiphytes (1.1–5.2 mg P m⁻² day⁻¹). Total (and/or organic) C (111–311 g C m⁻²), N (9.4–27.2 g N m⁻²), and P (212–1,623 mg P m⁻²) content were the highest in the sediment surface and typically the lowest in the seagrass epiphytes, ranging from 0.6 to 8.7 g C m⁻², 0.02–0.99 g N m⁻², and 0.5–43.5 mg P m⁻². Unlike nutrient content and enzyme activities, bacterial production was highest in the epiphytes (8.0–235.1 mg C m⁻² day⁻¹) and sediment surface (11.5–233.2 mg C m⁻² day⁻¹) and low in the water column (1.6–85.6 mg C m⁻² day⁻¹). At an assumed 50% bacterial growth efficiency, for example, extracellular enzyme hydrolysis could supply 1.8 and 69% of epiphytic and sediment bacteria carbon demand, respectively, while pelagic bacteria could fulfill their carbon demand completely by enzyme-hydrolyzable organic matter. Similarly, previously measured T. testudinum extracellular photosynthetic carbon exudation rates could not satisfy epiphytic and sediment surface bacterial carbon demand, suggesting that epiphytic algae and microphytobenthos might provide usable substrates to support high benthic bacterial production rates. PCA revealed that T. testudinum nutrient content was related positively to epiphytic nutrient content and carbon hydrolase activity in the sediment, but unrelated to pelagic variables. Phytoplankton biomass correlated positively with all pelagic components and sediment aminopeptidase activity but negatively with epiphytic alkaline phosphatase activity. In conclusion, seagrass production and nutrient content was unrelated to pelagic bacteria activity, but did influence extracellular enzyme hydrolysis at the sediment surface and in the epiphytes. This study suggests that seagrass-derived organic matter is of secondary importance in Florida Bay and that bacteria rely primarily on algal/cyanobacteria production. Pelagic bacteria seem coupled to phytoplankton, while the benthic community appears supported by epiphytic and/or microphytobenthos production.     

Spatial and temporal variation in the elemental and stable isotopic content of the seagrasses <Emphasis Type="Italic">Posidonia oceanica</Emphasis> and <Emphasis Type="Italic">Cymodocea nodosa</Emphasis> from the Illes Balears,Spain

Morphology, elemental content and isotopic composition of leaves of the seagrasses Posidonia oceanica and Cymodocea nodosa were highly variable across the Illes Balears, a Spanish archipelago in the western Mediterranean, and varied seasonally at one site in the study area. The data presented in this paper generally expand the reported ranges of nitrogen, phosphorus, iron and arsenic content and δ¹³C and δ¹⁵N for these species. Nitrogen and phosphorus content of P. oceanica leaves also showed significant seasonal variability; on an annual basis, P. oceanica leaves averaged 1.55% N and 0.14% P at this monitoring site. Both N and P were more concentrated in the leaves in winter than in summer, with winter maxima of 1.76% N and 0.17% P and summer minima of 1.34% N and 0.11% P. There was no significant annual pattern observed in the δ¹³C of P. oceanica leaves, but there was a repeated 0.6‰ seasonal fluctuation in δ¹⁵N. Mean annual δ¹⁵N was 4.0‰; δ¹⁵N was lowest in May and it increased through the summer and autumn to a maximum in November. Over the geographic range of our study area, there were interspecific differences in the carbon, nitrogen and phosphorus content of the two species. Posidonia oceanica N:P ratios were distributed around the critical value of 30:1 while the ratios for C. nodosa were lower than this value, suggesting P. oceanica we collected was not consistently limited by N or P while C. nodosa tended toward nitrogen limitation. Nutrient content was significantly correlated to morphological indicators of plant vigor. Fe content of P. oceanica leaves varied by a factor of 5×, with a minimum of 31.1 μg g⁻¹ and a maximum of 167.7 μg g⁻¹. Arsenic was present in much lower tissue concentrations than Fe, but the As concentrations were more variable; the maximum concentration of 1.60 μg g⁻¹ was eight times as high as the minimum of 0.20 μg g⁻¹. There were interspecific differences in δ¹³C of the two species; C. nodosa was consistently more enriched (δ¹³C = −7.8 ± 1.7‰) than P. oceanica (−13.2 ± 1.2‰). The δ¹³C of both species decreased significantly with increasing water depth. Depth related and regional variability in the δ¹³C and δ¹⁵N of both species were marked, suggesting that caution needs to be exercised when applying stable isotopes in food web analyses.     

Comparative analysis of macrofaunal species richness and composition in Posidonia oceanica, Cymodocea nodosa and leaf litter beds

We investigated macrofaunal species richness and composition in Posidonia oceanica, Cymodocea nodosa and Leaf litter beds within a coastal area of the Gulf of Oristano in proximity of the Cabras lagoon (western Sardinia, Italy). A total of 124 taxa were found, of which 116 were identified at the species level. They were analyzed based on both taxonomic and substrate affinity classification. Presence/absence analysis revealed that P. oceanica, C. nodosa and Leaf litter were all characterized by a conspicuous number of soft-bottom polychaetes (e.g., Prionospio multibranchiata and Ampharete acutifrons) and crustaceans (e.g., Corophium sextonae and Dynamene bidentatus), also known as detritivores. There were also major differences between the three habitats investigated. Consistent with its structural complexity, P. oceanica showed the highest species richness [E(S ₅₀)] and the most diversified macrofaunal assemblages, both in terms of taxonomic groups and taxa associated with different substrates. The two seagrasses, however, showed a similar species composition and differed from Leaf litter for the exclusive presence of hard-bottom species (e.g., the tunicate Phallusia fumigata) and seagrass-associated species (e.g., the polychaete Syllis garciai and the decapod Paguristes syrtensis). In contrast, Leaf litter showed the most differences between the habitats, and was characterized by the bivalves Abra alba and Cerastoderma glaucum, not found in seagrass beds, and by Loripes lacteus and Ruditapes decussatus. Leaf litter also had the highest content of organic matter (26.7% ± 1.4) and total organic carbon (10.3% ± 0.4). Our results confirmed the facilitative role of living seagrasses, in particular P. oceanica, as related to their structural complexity, for numerous species from different substrates (e.g., hard bottom species). This study also showed that leaf litter beds act as a particular environment where sediment instability, leaf breakdown, and organic matter enrichment and decomposition strongly influence animal distribution. Finally, our results highlighted the ecological and trophic importance of seagrass-derived detritus and the associated macroinvertebrate detritivores within seagrass-dominated systems.     

Seasonal growth and biomass of the subtropical seagrassHalodule wrightii in relation to continuous measurements of underwater irradiance

Continuous year-round measurements of photosynthetically active radiation (PAR) were collected in relation to leaf elongation and plant biomass in the shoal-grass,Halodule wrightii Aschers., within three different estuarine systems on the south Texas coast (Laguna Madre: May 1989 to September 1993; Corpus Christi Bay: February 1990 to September 1993; San Antonio Bay; May 1990 to April 1991). Large differences in water transparency at all three sites masked seasonal variations in surface insolation as reflected in average diffuse attenuation coefficient (k) values ranging from 0.7 to 2.9 and differences in the maximum depth penetration ofH. wrightii, which varied from 0.6 to about 1.3 m. The continuous presence of a chrysophyte (brown tide) algal bloom in Laguna Madre since 1990 led to significant decreases in spring leaf elongation rates and a nearly 50% decline in below-ground biomass, which was reflected in root:shoot ratio (RSR) values that declined from 5.4 in 1989 to 2.3 in 1992. Increased turbidity and lower light levels in San Antonio Bay also corresponded with diminished plant biomass and the subsequent loss of plants; at both locations, the annual quantum flux ranged from 2200 to 2400 mol m^-2yr^-1, or about 18% of surface irradiance (SI). In contrast,H. wrightii populations growing at ca.1.2 m depths and characterized by high RSR values (4.0) were exposed to 5100 to 5700 mol m^-2yr^-1, or about 41 to 46% SI. Under these conditions, plants were exposed to daily saturating levels of PAR (H _sat) of 3 to 8 h during the spring/summer period of maximum growth, compared to an average of 2 h in Laguna Madre (after 1990) and San Antonio Bay based on field-derived measurements of photosynthetic parameters. Leaf elongation inH. wrightii exhibited a clear circannual rhythm at all sites, regardless of underwater light levels and therefore was not a sensitive indicator of light stress. Instead, chronic long-term reductions in underwater PAR were most strongly reflected in total plant biomass. The higher light demand (18% SI) forH. wrightii in relation to many other seagrasses (11% SI; Duarte 1991) may be related to its higher photosynthetic light requirement, but may also reflect the different methods used to evaluate the minimum light requirements of seagrasses. In estuarine and coastal waters, which are characterized by large and unpredictable variations in water transparency, continuous measurements of in situ PAR are invaluable in assessing the growth and photosynthetic response of seagrasses to variations in underwater irradiance.The University of Texas at Austin, Marine Science Institute Contribution No. 913     

Depletion of heavy isotopes of oxygen and hydrogen in tissue water of intertidal plants: implications for water economy

Stable oxygen and hydrogen isotope ratios of leaf and thallus water of the intertidal seagrasses Phyllospadix scouleri and P. torreyi and the marine algae Egregia menziesii, Gelidium coulteri, and Corallina vancouverensis from three locations in California, USA, were determined in 1987. Compared with subtidal seawater, most plant-water samples were depleted in the heavy isotopes ¹⁸O and deuterium. Depletion of heavy isotopes was greatest in plants growing at the highest intertidal elevations. This was an unexpected result, because enrichment of heavy isotopes occurs during evapotranspiration in terrestrial plants. Two possible mechanisms for this isotopic depletion are proposed: direct uptake of heavy isotope-depleted water vapor and preferential diffusion of ¹⁶O and ¹H into littoral plants from water remaining in the intertidal zone at low tide.     

Nutrient resorption from seagrass leaves

The resorption of nutrients (C, N and P) from senescent leaves of six seagrass species from nine different locations in tropical (Indonesia and Kenya), Mediterranean (Spain) and temperate (The Netherlands) regions has been investigated. Resorption was quantitatively assessed by calculating the difference in nutrient content between the leaves with the highest content, and the oldest leaves. In order to do so, the leaves were classified according to their age. The nutrient contents of leaves of a given age category were calculated by multiplying the measured nutrient concentration in this age category with its corresponding modelled leaf biomass. N- and P-concentrations declined during ageing and senescence of the leaves in all of the investigated situations but two. The decline in concentration varied up to 58% for N and up to 66% for P. The C-concentration declined on three of the investigated occasions and varied up to 24%. Despite a decline in concentration, the leaf C-content did not change, indicating no resorption of carbon. The efficiency of N-resorption from intact seagrass leaves varied between 3.8 and 29% (average: 15%), while the efficiency of phosphorus resorption varied between 0 and 51% (average: 21%). The resorption efficiency was not significantly different in seagrasses with a relatively high and a relatively low nutrient concentration, although within-species comparisons showed that in some cases resorption efficiency was positively related to the nutrient concentration of the leaves. Premature loss of leaves and leaf fragments (by e.g. herbivory) may substantially interfere with the resorption process. In Indonesian seagrasses we estimated that as a result of fragmentation and premature detachment only between 56 and 77% of the physiological resorption potential actually was realised. It is concluded that internal resorption may play a role in the nutrient dynamics of seagrass plants, but that its quantitative importance probably is limited. Nutrient resorption from senescent seagrass leaves may reduce the nutrient requirements for seagrass leaf production by approximately 10% for nitrogen and 15% for phosphorus. Received: 28 September 1996 / Accepted: 7 November 1996     

Clearance rates of the great scallop (<Emphasis Type="Italic">Pecten maximus</Emphasis>) and blue mussel (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) at low natural seston concentrations

Great scallop, Pecten maximus, and blue mussel, Mytilus edulis, clearance rate (CR) responses to low natural seston concentrations were investigated in the laboratory to study (1) short-term CR variations in individual bivalves exposed to a single low seston diet, and (2) seasonal variations in average CR responses of bivalve cohorts to natural environmental variations. On a short temporal scale, mean CR response of both species to 0.06 μg L⁻¹ chlorophyll a (Chl a) and 0.23 mg L⁻¹ suspended particulate matter (SPM) remained constant despite large intra-individual fluctuations in CR. In the seasonal study, cohorts of each species were exposed to four seston treatments consisting of ambient and diluted natural seston that ranged in mean concentration from 0.15 to 0.43 mg L⁻¹ SPM, 0.01 to 0.88 μg L⁻¹ Chl a, 36 to 131 μg L⁻¹ particulate organic carbon and 0.019 to 0.330 mm³ L⁻¹ particle volume. Although food abundance in all treatments was low, the nutritional quality of the seston was relatively high (e.g., mean particulate organic content ranged from 68 to 75%). Under these low seston conditions, a high percentage of P. maximus (81–98%) and M. edulis (67–97%) actively cleared particles at mean rates between 9 and 12 and between 4 and 6 L g⁻¹ h⁻¹, respectively. For both species, minimum mean CR values were obtained for animals exposed to the lowest seston concentrations. Within treatments, P. maximus showed a greater degree of seasonality in CR than M. edulis, which fed at a relatively constant rate despite seasonal changes in food and temperature. P. maximus showed a non-linear CR response to increasing Chl a levels, with rates increasing to a maximum at approximately 0.4 μg L⁻¹ Chl a and then decreasing as food quantity continued to increase. Mean CR of M. edulis also peaked at a similar concentration, but remained high and stable as the food supply continued to increase and as temperatures varied between 4.6 and 19.6°C. The results show that P. maximus and M. edulis from a low seston environment, do not stop suspension-feeding at very low seston quantities; a result that contradicts previous conclusions on the suspension-feeding behavior of bivalve mollusks and which is pertinent to interpreting the biogeographic distribution of bivalve mollusks and site suitability for aquaculture.     

Distinctive types of leaf tissue damage influence nutrient supply to growing tissues within seagrass shoots

Herbivory is now recognized as an important structuring agent in seagrass meadows but the attack pattern and tissue damage of consumers are highly variable. Nutritional preferences of herbivores and/or easy access to resources may cause differences in biomass loss among tissues that damage the plant in functionally distinctive ways. The two main Mediterranean herbivores, the fish Sarpa salpa (L.) and the sea urchin Paracentrotus lividus (Lmk.), remove higher amounts of intermediate and external shoot leaves, respectively. To test whether this selective feeding can have different consequences on the allocation patterns of nutrient within plants, we simulated the effect of both herbivores by clipping external and intermediate leaves (plus unclipped controls) of Posidonia oceanica (L.) and we measured plant tolerance in terms of shoot growth and leaf nutrient supply to new tissue using isotopic markers. As expected, control treatments displayed high carbon and nutrient supply from external leaves (83% of the total ¹⁵N and 84% of the total ¹³C incorporated by the shoot). When subjected to clipping, the remaining leaves enhanced carbon and nitrogen supply compared with the control by 16% of N and 36% of C—in the intermediate clipping—and by over 100% of N and 200% of C—in the external clipping—to compensate for the nutrient lost. However, only in the case of fish herbivory (intermediate clipping), enhanced supply alone was able to fully compensate for the nutrient losses. In contrast, this mechanism is not completely effective when external leaves are clipped (urchin herbivory). Yet, the consequences of this nutrient loss under sea urchin herbivory are not apparent from the nutrient content of the new tissue, suggesting that there are other sources of nitrogen income (uptake or reallocation from rhizomes). Our study does not only confirm the tolerance of P. oceanica to herbivory, but also constitutes the first evidence of leaf-specific, compensatory nutrient supply in seagrasses.     

Thalassia testudinum leaf dynamics in a Mexican Caribbean coral reef lagoon

Shoot density, leaf growth, initiation, biomass and primary production in Thalassia testudinum (Banks ex König) were monitored at monthly intervals from August 1990 until January 1992 at three stations in the tropical coral reef of Puerto Morelos lagoon, Mexico. Leaf growth decreased with increasing leaf length, declining rapidly once the tips of leaves had started to decay; however, the leaves continued to grow until complete senescence. Maximum potential leaf age was>90 d. Leaf growth, biomass and primary production were highest at the station in the vicinity of mangrove discharges, intermediate at the nearshore fringe of the seagrass meadow, and lowest at the back-reef station. Leaf growth, leaf initiation, biomass and primary production were minimum in the winter months and maximum in the summer. Leaf growth and primary production were significantly correlated with water temperature or/and the hours of daylight. This is the first report of temperature-or/and hours of daylight-related seasonal variability in T. testudinum production from the tropical Caribbean.     

Nutrient enrichment of seagrass beds in a rhode island coastal lagoon

Seagrass and algal beds showed a variety of reponses when the water column was treated with low level additions of ammonium, nitrate and phosphate. The nutrients were added separately to 3 uniform seagrass beds of a temperature coastal lagoon during 1979 and 1980. (1) Ammonium caused the production of dense mats of free-floating green algae Enteromorpha plumosa and Ulva lactuca. It also stimulated growth in both the leaf and root-rhizome fractions of Zostera marina. This growth response in Z. marina was greater in the area where current reached 12 cm · s^-1 than in the area with little or no current. The concentration of nitrogen in the tissue did not change. In contrast, where current was lacking, Z. marina growth increase with ammonium was small, but the concentration of nitrogen in the tissue doubled over that in control plots. The growth of Ruppia maritima was inversely related to the growth of green algae in the same plots. The red alga Gracilaria tikvahiae did not grow better in ammonium, but its tissue reddened. (2) Nitrate additions enhanced the growth of the green seaweeds Enteromorpha spp. and U. lactuca, but not Z. marina or R. maritima. G. tikvahiae, when fertilized in isolation from other plants, showed a marginal response to this nutrient, and the tissue always reddened. (3) Phosphate enhanced growth in Z. marina and R. maritima exposed to moderate current. G. tikvahiae growing alone showed a small growth response to phosphate. The phosphate made no difference in the growth of the green seaweeds. (4) None of the nutrient supplements noticeably altered the species composition of either epiphytic or planktonic algae associated with the beds, although we did detect small increases in their numbers. The rapid and dense growth of green algae in nitrogen-enriched water probably limited growth of adjacent seagrasses and red algae. Because these seaweeds did not use the phosphate, it became available to other plant components. The overall floral response to nutrient addition in seagrass communities depends, therefore, upon the particular nutrient supplied, the ability of alternate species in the area to compete for that nutrient and the velocity of current in the specific area.     

Life history and production of the mysid <Emphasis Type="Italic">Orientomysis robusta</Emphasis>: high <Emphasis Type="Italic">P/B</Emphasis> ratio in a shallow warm-temperate habitat of the Sea of Japan

Although mysids play important roles in marine food chains, studies on their production are scarce, especially for warm-water species. We investigated life history and production of Orientomysis robusta in a shallow warm-temperate habitat of the Sea of Japan. Its spawning and recruitment occurred throughout the year; 19 overlapping cohorts were recognizable over an annual cycle. The summer cohorts recruited in July–September exhibited rapid growth, early maturity, small brood size, and small body size. A converse set of life history traits characterized the autumn–winter cohorts recruited in October–March. The spring cohorts recruited in April–June had intermediate characteristics of both cohorts. Life spans were 19–33, 21–48, and 69–138 days for summer, spring, and autumn–winter cohorts, respectively, and mortality rates were high for spring and summer cohorts, especially during June–August but were low for autumn–winter cohorts. Production calculated from the summation of growth increments was 488.8 mg DW m⁻² year⁻¹ with an annual P/B ratio of 21.26. The short life span seems to be responsible for such an extremely high P/B ratio. A method not requiring recognition and tracking cohorts gave similar values (534.0 mg DW m⁻² year⁻¹ and 20.49). The close agreement in production values between the two methods indicates our estimates are valid.     

Primary production of Posidonia oceanica in the Mediterranean Basin

Primary production of the marine phanerogam Posidonia oceanica (Linnaeus) Delile was measured by lepidochronological analyses at 22 sites in the Mediterranean Sea (Corsica, France, Italy, Sardinia and Turkey), between 1983 and 1992, to determine spatial and temporal variations. Leaf production (blade and sheath) ranged from 310 to 1 540 mg dry wt shoot^–1 yr^–1, depending on site and depth. Rhizome production ranged from 24 to 120 mg dry wt shoot^–1 yr^–1 (6% of average leaf production). At some sites the results obtained by lepidochronological analysis were consistent with earlier results obtained by classic methods (e.g. leaf-marking). While primary production per shoot (mg dry wt shoot^–1 yr^–1) displayed no significant differences between sites, primary production of the P. oceanica meadow (g dry wt m^–2 yr^–1) decreased with increasing depth at all sites studied. This decrease correlated with reduced density of the meadow (number of shoots per m²) with increasing depth. Past primary production was also extrapolated at three sites at the island of Ischia (Italy) for a period of 5 yr in order to determine interannual variations over a period of several years. While major variations were recorded for the surface stations (5 and 10 m depth), production remained stable at the deepest station (20 m depth). Given the large geographical scale of the study (location, depth range), it would appear that while P. oceanica production remains considerable, the values recorded in the literature on the basis of classical analyses (surface stations) represent maxima, and cannot be generalised for meadows as a whole.     

Light and carbon balance in the seagrass Thalassia testudinum : evaluation of current production models

The production dynamics and carbon balance of Thalassia testudinum in the lower Laguna Madre, Texas, USA, were examined during the 1995 summer period based on in situ photosynthesis vs irradiance (PI) measurements and continuous measurements of underwater photon-flux density (PFD). The validity of applying the H _sat model, used to calculate production for Zostera marina as the product of the maximum rate of photosynthesis (P _max) and daily hours of saturating irradiance (H _sat) was assessed for T. testudinum by comparison with integrated production estimates derived through numerical integration. Gross integrated production values were combined with dark-respiration measurements of photosynthetic (PS) and non-photosynthetic (NPS) tissues and areal biomass to generate daily whole-plant carbon balance. Production and whole-plant carbon balance are discussed in relation to surface and underwater PFD measurements, biomass and other physical and chemical parameters collected during a 1 yr period from January to December 1995. The H _sat model significantly underestimated production during all summer months, averaging 70% of integrated production over the entire study period. Gross integrated production ranged between 11.5 mg C g⁻¹ leaf dry wt d⁻¹ in June (during a period of unseasonably low PFDs caused by a drift-alga mat covering the seagrass bed) to 26.7 mg C g⁻¹ leaf dry wt d⁻¹ in July. Modeled net carbon gain was highest in July at 454 mg C m⁻² d⁻¹ (1.4 g dry wt m⁻² d⁻¹), sufficient to account for measured rates of leaf production in the study area and representative of T. testudinum populations of low productivity. During part of the summer period, however, the population was in negative carbon balance. The relatively low productivity of this population and the periods of negative carbon balance are attributed to low net photosynthesis:dark respiration (P _net:R _d) ratios, sporadic low-light periods, the small fraction of PS tissue relative to whole-plant biomass (5 to 13%) and nutrient limitation. Production models are sensitive to both light availability and the proportion of PS tissue supporting NPS biomass as reflected in whole-plant P _net:R _d ratios. Received: 13 August 1997 / Accepted: 6 March 1998     

Net negative growth detected in a population of <Emphasis Type="Italic">Leptogorgia sarmentosa</Emphasis>: quantifying the biomass loss in a benthic soft bottom-gravel gorgonian

Gorgonian species may contribute to the three-dimensional seascape in soft bottom-gravel areas, but the information on their biology and ecology is very scarce. The biometry and secondary production of the Mediterranean soft bottom-gravel passive suspension feeder Leptogorgia sarmentosa (Cnidaria: Octocorallia) was studied using photographic monitoring of the primary branches from May 1998 to September 2000. The primary branches observed had a high density of polyps (2.2 ± 0.2 SD polyps mm⁻¹) and a high organic matter content (63.2 ± 9.1 SD %). During the two-year sampling period, there was a net negative growth in 90% of the gorgonian population. The mean loss during the 27-month period was −2.9 ± 0.9 SD cm per branch (5.7 mg C branch⁻¹). However, considering only the initial and final diameters and maximum height in the 27 months elapsed time, the gorgonians showed positive growth, which meant that photographic sampling of single branches was a more appropriate method for gorgonian secondary production monitoring. A water mass anomaly detected in 1999 in the north-western Mediterranean Sea may have been the cause of the net negative growth in L. sarmentosa in the studied area. Partial mortality due to different factors, such as strong currents, predation, disease, etc., could be a common strategy in sessile colonial benthic populations that would facilitate their maintenance even during very stressful circumstances.     

Population dynamics,reproduction and growth of the Indo-Pacific horned sea star, <Emphasis Type="Italic">Protoreaster nodosus</Emphasis> (Echinodermata; Asteroidea)

The horned sea star (Protoreaster nodosus) is relatively common in the Indo-Pacific region, but there is little information about its biology. This study of the population biology of P. nodosus was carried out in Davao Gulf, The Philippines (7°5′N, 125°45′E) between September 2006 and May 2008. Protoreaster nodosus was found in sand and seagrass dominated habitats at a mean density of 29 specimens per 100 m² and a mean biomass of 7.4 kg per 100 m², whereas a significantly lower density and biomass was found in coral and rock dominated habitats. Adult specimens (mean radius R = 10.0 cm) were found at depths of 0–37 m, whereas juveniles (R < 8 cm) were only found in shallow sandy habitats with abundant seagrass (water depth ≤2 m). Increased gonad weights were found from March to May (spawning period), which coincided with an increasing water temperature and a decreasing salinity. Density and biomass did not change significantly during reproduction, but sea stars avoided intertidal habitats. All specimens with R > 8 cm had well developed gonads and their sex ratio was 1:1. Protoreaster nodosus grew relatively slowly in an enclosure as described by the exponential function G = 7.433 e^{−0.257 × R} . Maturing specimens (R = 6–8 cm) were estimated to have an age of 2–3 years. Specimens with a radius of 10 cm (population mean) were calculated to have an age of 5–6 years, while the maximum age (R = 14 cm) was estimated as 17 years. Potential effects of ornamental collection on the sea star populations are discussed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Utilisation of mangroves and seagrasses by fishes in the Negombo Estuary,Sri Lanka

In two years of sampling, 10 287 fishes belonging to 62 species (39 families) were collected from mangroves and seagrasses of the Negombo Estuary. Of the species collected, 70% were marine, 27% estuarine and 3% freshwater. Aplocheilus melastigma, Ambassis dayi, Etroplus suratensis, E. maculatus, Bathygobius fuscus and Arothron immaculatus constituted 90% of the catch. Sixteen species were caught only from mangroves, 6 only from seagrasses, and 40 from both. The number of species, individuals, biomass and diversity were higher in seagrasses than in mangroves. Gobies and some siganids moved to the mangrove habitat as they grew, but cichlids, triacanthids, some eleotrids and tetraodontids preferred seagrasses. The number of species and individuals caught increased at the beginning of the north-east monsoon when the rainfall was low. The number of species and individual numbers were positively correlated with salinity and dissolved oxygen and negatively correlated with water temperature. Of the 33 species examined, seagrasses formed >25% of the gut content in juvenile Lates calcarifer, Apogon thermalis, E. suratensis, E. maculatus, Siganus javus, Butis butis and Bathygobius fuscus. Mangrove litter was the main gut content of the family Syngnathidae. E. suratensis, E. maculatus and S. javus from seagrasses contained more filamentous algae than seagrasses in their stomachs. S. javus collected from mangroves also contained more seagrasses in their stomachs than mangrove litter, indicating that the association of juvenile fishes with mangroves or seagrasses may not be obligate in estuaries subject to low tidal fluctuations.     

Tolerance of Mediterranean seagrasses (<Emphasis Type="Italic">Posidonia oceanica</Emphasis> and <Emphasis Type="Italic">Cymodocea nodosa</Emphasis>) to hypersaline stress: water relations and osmolyte concentrations

The present study examines for the first time the effects of increased salinity on water relations and osmolyte (carbohydrates and amino acids) concentrations in two Mediterranean seagrass species, Posidonia oceanica and Cymodocea nodosa, which are adapted to growth in environments with contrasting salinity and have a known differential sensitivity to alterations in ambient salinity. The specific aim was to obtain insights into their respective capacities to cope with natural or anthropogenically induced (e.g. desalination plants) hypersaline stress and its ecological implications. To this end, large plant fragments of both seagrass species were maintained for 47 days in a laboratory mesocosm system under ambient salinity (37 psu; control) and three chronic hypersaline conditions (39, 41 and 43 psu). Analyses of leaf-tissue osmolality indicated that both species followed a dehydration avoidance strategy, decreasing their leaf water potential (Ψ_w) as the external salinity increased, but using different physiological mechanisms: whereas P. oceanica leaves exhibited a reduction in osmotic potential (Ψ_π), C. nodosa leaves maintained osmotic stability through a decrease in turgor pressure (Ψ_p) probably mediated through cell-hardening processes. Accordingly, the concentrations of soluble sugars and some amino acids (mainly Pro and Gly) suggested the activation of osmoregulatory processes in P. oceanica leaves, but not in C. nodosa leaves. Osmotic adjustments probably interfered with leaf growth and shoot survival of P. oceanica under hypersaline stress, whereas C. nodosa showed a more efficient physiological capacity to maintain plant performance under the same experimental conditions. These results are consistent with the more euryhaline ecological behaviour of C. nodosa and contribute to understanding the high vulnerability shown by P. oceanica to even mild increments in seawater salinity.     

Heterotrophic nitrogen fixation (acetylene reduction) during leaf-litter decomposition of two mangrove species from South Florida, USA

Heterotrophic nitrogen-fixation (acetylene reduction) was measured during decomposition (under dark conditions) of Rhizophora mangle L. and Avicennia germinans (L.) Stearn leaf litter. Nitrogen-fixation rates in leaf litter increased following 24 d incubation, then decreased after ≃44 d for both species. Maximum rates of 66.2 and 64.6 nmol C₂H₄ g⁻¹ dry wt h⁻¹ were reached by R. mangle and A. germinans leaf litter, respectively. Higher fixation rates of leaf litter were associated with an increase in water content and sediment particles on leaf surfaces of both species. Rates of nitrogen fixation by diazotrophs attached to sediment particles were not significantly different from zero. With additions of d-glucose, ethylene production rates increased by factors of 625-, 34- and 7-fold for sediment, R. mangle and A.␣germinans leaf litter, respectively, compared to rates prior to enrichment. These organically enhanced rates of nitrogen fixation on leaves could be accounted for by increased activity associated with attached sediment particles and not the leaf material. Total phenolics [reported as tannic acid equivalent (TAE) units] decreased nitrogen-fixation rates when added to d-glucose-enriched sediment at >20 mg TAE l⁻¹. Phenolic compounds could explain the initial lag in rates of nitrogen fixation during leaf-litter decomposition of R. mangle (initial content of 110.8 mg TAE g⁻¹ dry wt), but not of A. germinans (initial content of 23.4 mg TAE g⁻¹ dry wt). The higher phenolic content and reportedly lower carbon substrate of R. mangle did not result in species-specific differences in either the magnitude or temporal pattern of nitrogen fixation compared to A. germinans leaf litter. We conclude that the availability of organic substrates leached from the leaf litter along with colonization by the heterotrophic diazotrophs (as indicated by sediment accumulation) controls nitrogen-fixation rates in a similar manner in the leaf litter of both species. Received: 8 August 1997 / Accepted: 4 December 1997     

Light absorption,photosynthesis and growth of Nannochloris atomus in nutrient-saturated cultures

Nannochloris atomus was maintained in exponential growth at photon flux densities (PFD) from 400 to 700 nm, ranging from 10 to 200 mol m^-2 s^-1. Growth was lightsaturated at PFDs in excess of 100 mol m^-2 s^-1, with a mean light-saturated growth rate at 23 °C of 1.5×10^-5s^-1 (1.2 d^-1). The light-limited growth rates extrapolated to a compensation PFD for growth that was not significantly different from zero, although no changes in cell numbers were observed in a single culture incubated at a PFD of 1.0 mol m^-2s^-1. Dark-respiration rates were independent of PFD, averaging 1.7×10^-6 mol O₂ mol^-1 C s^-1 (0.14 mol O₂ mol^-1 C d^-1). The maximum photon (quantum) efficiency of photosynthesis was also independent of PFD, with a mean value of 0.12 mol O₂ mol^-1 photon. The chlorophyll a-specific light absorption cross-section ranged from 3 to 6×10^-3 m² mg^-1 chl a and was lowest at low PFDs due to intracellular self-shading of pigments associated with high cell-chlorophyll a contents. The C:chl a ratio increased from 10 to 40 mg C mg^-1 chl a between PFDs of 14 and 200 mol m^-2 s^-1. These new observations for N. atomus are compared with our previous observations for the diatom Phaeodactylum tricornutum in terms of an energy budget for microalgal growth.