Influences of season and water depth on the clonal biology of the seagrassThalassia testudinum

Ramet density, leaf relative growth rates, leaf chlorophyll levels, and proximate constituent levels were determined on three dates in 1988 at three water depths for aThalassia testudinum Banks ex Konig meadow in lower Tampa Bay, Florida, USA. Density varied seasonally in patterns unique to each depth. Leaf relative growth rates indicated a unimodal, rather than bimodal, growth pattern at this site. Leaf chlorophyll levels reflected both depth-related and seasonal influences, with levels at all depths increasing in times of high light attenuation. Based on results from leaf relative growth rates and proximate constituent levels, the degree of ramet interdependence appears to vary with sampling date and water depth. The degree of ramet interdependence appears to be maximal in times of genet expansion (June and August) and minimal in times of energy storage (November). Interdependence of ramets varies spatially as well; ramets are maximally integrated in shallow regions (on those dates when genet proliferation occurs), and minimally integrated at deep edges. Depth-related differences in ramet integration are probably due to the spatial heterogeneity of the primary resource limitation varying with water depth, and to depth-related differences in ramet population dynamics. Patterns of ramet physiological integration inT. testudinum are similar to the patterns reported for terrestrial clonal plants.     

Thalassia testudinum productivity and grazing by green turtles in a highly disturbed seagrass bed

There has been an historical decline in the seagrass beds in Maho and Francis Bays, St. John, U.S. Virgin Islands: presently (1986) there are only five small seagrass beds in shallows water. These seagrass beds are highly disturbed by heavy boat usage and are intensively grazed by the green turtle Chelonia mydas L. Fifteen to 50 boats anchor each night in the bays: anchor scars cause a loss of up to 6.5 m² d^-1 or 1.8% yr^-1 of the seagrass beds. Seagrasses regrew into such scars only minimally within a period of 7 mo. The size of the green turtle population was estimated at 50 subadults and their feeding behavior was determined by direct observation and radiotelemetry. The behavior of the green turtles differed from other observations published on the species. Here, the turtles grazed all available Thalassia testudinum, their preferred seagrass food, rather than creating discrete grazing scars, and spent all their waking hours (9 h per day) feeding. Areal productivity of T. testudinum leaves (33 to 97 mg dry wt m^-2d^-1) in the bays was at least an order of magnitude lower than published values or than the productivity of another, lessdisturbed seagrass bed on St. John, despite having comparable leaf-shoot density. Leaf shoots were stunted, fragile, achlorotic, and had only two leaves as opposed to the five leaves per shoot more typically seen. The green turtle population was near the estimated carrying capacity of T. testudinum, based on the standing crop and productivity of T. testudinum and the grazing rate of the turtles. The effect of disturbance of T. testudinum from boats and turtles was assessed by excluding these with emergent fences. Within 3 mo of protection, the areal and shoot-specific productivity of T. testudinum leaves as well as leaf size increased significantly compared to unprotected areas. Conservation efforts are recommended in Maho Bays and Francis because seagrass productivity is low, disturbance rates are higher than recovery rates, the turtles cannot increase further their feeding rate in order to compensate for such factors, and there are few alternate sources of T. testudinum on the north shore of St. John.Contribution No. 175 from West Indies Laboratory, Teague Bay, Christiansted, St. Croix, U.S. Virgin Islands 00820, USA     

Leaf reddening in the seagrass Thalassia testudinum in relation to anthocyanins, seagrass physiology and morphology, and plant protection

Numerous seagrass species growing in high-light environments develop red coloration in otherwise green leaves, yet the ecophysiology of leaf reddening in seagrasses is poorly understood. To increase our understanding of the process of leaf reddening in Thalassia testudinum found in the lower Florida Keys (USA), we identified the molecules responsible for red coloration in leaves and compared physiological, morphological, and growth attributes of entirely red-leafed shoots to entirely green-leafed shoots. We determined that four anthocyanin molecules are responsible for red coloration in leaves. In addition, we found that red leaves had higher concentrations of photoprotective pigments (anthocyanins and UV-absorbing compounds), higher effective quantum yields (ΔF/F _m′) at midday, and were shorter, narrower, and weighed less than green leaves. No significant difference in growth rates was observed between red- and green-leafed shoots, but patches of red-leafed shoots had shorter canopy heights and smaller LAI compared to patches of green-leafed shoots. Our results demonstrate that leaf reddening in T. testudinum is caused by high concentrations of anthocyanins, is associated with physiological and morphological attributes, and acts as a sunscreen since red leaves were able to maintain high effective quantum yields at high light intensities.     

Nitrogen fixation (acetylene reduction) in the phyllosphere of Thalassia testudinum

N₂ fixation (C₂H₂ reduction) associated with the leaves of the sea grass Thalassia testudinum was investigated at 5 sites in South Florida (Biscayne Bay) and one site in the Bahamas (Bimini Harbor). Significant activities were correlated with the occurrence of a heterocystous blue-green alga (Calothrix sp.) on the leaves. C₂H₂ reduction was not stimulated by organic compounds, either aerobically or anaerobically in the light or dark. Therefore, other physiological types of microbes were not important in N₂ fixation. Diurnal and seasonal variations in N₂ fixation occurred, with maximal rates during the daytime and in the late spring and early summer. N₂ fixation was negligible at four stations in Biscayne Bay. At the fifth station, near Fowey Rock, about 5 kg N ha^-1 year^-1 was fixed. In the summer, the N₂ fixed per day (4–5 mg N m^-2) could provide 4 to 23% of the foliar productivity demands of T. testudinum at this site and the station in Bimini Harbor. N₂ fixation at the periphery of a sea-grass patch, near Fowey Rock, could provide 8 to 38% of the daily nitrogen requirement for leaf production, and thereby might compensate for a less effective trapping and recycling of nitrogen from dead leaves in such regions.     

Leaf traits are good predictors of plant performance across 53 rain forest species

We compared the leaf traits and plant performance of 53 co-occurring tree species in a semi-evergreen tropical moist forest community. The species differed in all leaf traits analyzed: leaf life span varied 11-fold among species, specific leaf area 5-fold, mass-based nitrogen 3-fold, mass-based assimilation rate 13-fold, mass-based respiration rate 15-fold, stomatal conductance 8-fold, and photosynthetic water use efficiency 4-fold. Photosynthetic traits were strongly coordinated, and specific leaf area predicted mass-based rates of assimilation and respiration; leaf life span predicted many other leaf characteristics. Leaf traits were closely associated with growth, survival, and light requirement of the species. Leaf investment strategies varied on a continuum trading off short-term carbon gain against long-term leaf persistence that, in turn, is linked to variation in whole-plant growth and survival. Leaf traits were good predictors of plant performance, both in gaps and in the forest understory. High growth in gaps is promoted by cheap, short-lived, and physiologically active leaves. High survival in the forest understory is enhanced by the formation of long-lived well protected leaves that reduce biomass loss by herbivory, mechanical disturbance, or leaf turnover. Leaf traits underlay this growth-survival trade-off; species with short-lived, physiologically active leaves have high growth but low survival. This continuum in leaf traits, through its effect on plant performance, in turn gives rise to a continuum in species' light requirements.     

Peculiar growth dynamics of the surfgrass <Emphasis Type="Italic">Phyllospadix japonicus</Emphasis> on the southeastern coast of Korea

The surfgrass Phyllospadix japonicus is endemic to exposed shores of the northeastern Pacific Ocean. Unlike the majority of seagrasses, P. japonicus grows on rocky substrata. The specific physical features of the habitat are likely related to the peculiar ecological characteristics of P. japonicus. However, few studies have been conducted thus far on the growth dynamics of Phyllospadix spp., largely due to the turbulent water conditions in its habitat. P. japonicus is a dominant seagrass species, and it plays critical ecological roles on the eastern coast of Korea. Here, we examined its growth dynamics for the first time on the Korean coast. We measured shoot density, biomass, leaf production, phenology, morphology, tissue nutrient content, as well as environmental factors including underwater photon flux density (PFD), water temperature and water column nutrient concentrations from March 2003 to December 2005. Shoot density, biomass, leaf productivity and morphological characteristics exhibited significant seasonal variations; maximum values of these variables occurred in winter and early spring, and the minima were recorded in late summer and early fall. PFD and water temperature were, respectively, positively and negatively correlated with leaf production. Nutrient availability fluctuated substantially, but there was no evidence of distinct seasonal variation, nor was it correlated with leaf production. Leaf chlorophyll concentrations were correlated strongly with leaf production, whereas tissue nutrient contents were unrelated to leaf production. Maximum potential seed production ranged from 1,200 seeds m⁻² in 2004 to 3,445 seeds m⁻² in 2003; however, seedlings were rarely detected through the observation period. Thus, P. japonicus meadows at the study site appeared to persist through vegetative propagation. Leaf C content varied bimodally, with peaks in spring and fall. Leaf N content was minimal during months in which leaf productivity was lowest. These patterns in tissue nutrient content are clearly different from those of the majority of soft-substratum seagrasses and appear to relate to the reduced physiological tolerance of high temperature in P. japonicus compared to other temperate seagrasses.     

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     

Influence of sediment nitrogen-availability on carbon and nitrogen dynamics in the seagrass Thalassia testudinum

Variations in tissue carbon (C), nitrogen (N) content, and non-structural carbohydrate (NSC) reserves in the turtle grass Thalassia testudinum Banks ex König were examined in relation to changes in sediment-N availability in Corpus Christi Bay (CCB) and lower Laguna Madre (LLM), Texas, USA, from May to October 1997. Under natural conditions, sediment pore-water NH⁺ ₄-concentrations were higher at CCB (100?μM) than at LLM (30?μM); this difference was reflected in a significantly higher leaf and rhizome N-content at CCB than at LLM. However, sediment NH⁺ ₄-enrichment using a commercial fertilizer resulted in significantly higher tissue N-content relative to controls at both sites. N enrichment also influenced plant carbon metabolism, as reflected by distinct increases in leaf C-content at both sites. Significant decreases in rhizome NSC-content was recorded during the first two months of the experiment, suggesting that C was reallocated from rhizomes to leaves to support stimulated leaf growth at both sites. At LLM, leaf growth-rates increased and leaf turnover-time decreased as a result of sediment NH⁺ ₄-enrichment. With respect to chlorophyll, concentrations did not change significantly at CCB, but increased steadily at LLM after the first month following fertilization. In general however, chlorophyll concentrations in control plots were significantly higher at CCB than that at LLM. These observations suggest that leaf function related to C-fixation is enhanced under higher sediment N conditions, as reflected in higher leaf growth-rates and increased blade chlorophyll-content. In contrast, under low-N conditions, below-ground tissue production is enhanced at the expense of the above-ground shoots and leaves, resulting in the high below: above-ground biomass ratios often observed in seagrass beds of oligotrophic environments.     

Changes in nutrient content and stable isotope ratios of C and N during decomposition of seagrasses and mangrove leaves along a nutrient availability gradient in Florida Bay,USA

The decomposition of the mangrove Rhizophora mangle and the seagrass Thalassia testudinum was examined using litterbags along a natural gradient in nutrient availability. Seagrass leaves had a higher fraction of their biomass in the labile pool (57%), compared to mangrove leaves (36%) and seagrass rhizomes (29%); the overall decomposition rates of the starting material reflected the fractionation into labile and refractory components. There was no relationship between the N or P content of the starting material and the decomposition rate.

Nutrient availability had no influence on decomposition rate, and mass was lost at the same rate from litterbags that were buried in the sediment and litterbags that were left on the sediment surface. The dynamics of N and P content during decomposition varied as a function of starting material and burial state. N content of decomposing mangrove leaves increased, but seagrass rhizomes decreased in N content during decomposition while there was no change in seagrass leaf N content. These same general patterns held for P content, but buried seagrass leaves increased in P content while surficial leaves decreased. δ¹³C and δ¹⁵N changed by as much as 2‰ during decomposition.     

Phytoplankton-zooplankton relationships in the Western Pacific Ocean and adjacent seas

Regressions of biomass and daily food requirements of herbivorous zooplankton on daily primary production were calculated, using assumptions based on data collected in various sea areas of the western Pacific Ocean and adjacent seas. A regression coefficient (1.470) of calculated herbivorous biomass on observed daily primary production is significantly higher than unity (P<0.01). This indicates that the herbivorous biomass sustained by unit amount of primary production is large in the more productive high latitudes, and small in the less productive tropical sea areas. This is attributed to relatively larger food requirements per unit biomass of the tropical herbivores as compared with those found in cold waters. Despite distinct areal differences in the herbivorous biomass-primary production ratios, the calculated daily food requirement of herbivores was in direct proportion to the daily primary production, when equilibrium had been established between phytoplankton and zooplankton. Under conditions of limited food supplies, the small body size of the tropical herbivores may be advantageous both in reducing the total energy consumption per individual, and in inducing rapid growth and reproduction. Therefore, the low ratio of biomass to primary production in the tropics could beregarded as a result of possible regulation of tropical herbivores to scarce food conditions rather than as evidence of failure of adaptation to such conditions.     

Utilization of growth parameters of eelgrass,Zostera marina,for productivity estimation under laboratory and in situ conditions

Allometry was used for monitoring aboveground growth of the marine angiosperm Zostera marina L. Dry weight was regressed with leaf length and width, allowing estimation of aboveground net productivity and biomass of individual plants. At the termination of the experiment, rhizome productivity of the same plants was determined by harvesting. Plants in shaded and unshaded seawater tanks were monitored from June until September, 1976; in situ plants were also monitored at Point Judith Pond, Rhode Island, USA. Unshaded plants had shorter leaves, a lower net productivity, lower biomass, and a lower aboveground-torhizome productivity ratio than shaded plants. Unshaded plants had a higher rate of rhizome branching and the resulting new shoot formation than in situ plants.     

Growth dynamics of the seagrass <Emphasis Type="Italic">Halophila nipponica</Emphasis>, recently discovered in temperate coastal waters of the Korean peninsula

Seagrass species in the genus Halophila are usually distributed in tropical or subtropical areas, but a Halophila species identified as H. nipponica was first observed in temperate coastal regions of Korea in 2007. Since this species mainly occurs in warm temperate regions influenced by warm currents, we hypothesized that H. nipponica may exhibit different growth patterns from those of other temperate seagrass species in Korea, instead showing similar growth dynamics to tropical/subtropical species. The growth and morphology of H. nipponica in relation to coincident measurements of environmental factors were investigated from July 2008 to September 2009 to examine the growth dynamics of this species. Water temperature at the study site ranged from 9.7°C in January to 25.1°C in August. Shoot density, biomass, and productivity exhibited significant seasonal variation, increasing during summer and decreasing during winter. Productivity was severely restricted to nearly ceasing at water temperatures less than 15°C, and winter minimum growth lasted until May. The optimal temperature for H. nipponica growth was approximately 25°C, which was the maximum water temperature at the study site, and no growth reduction in high summer water temperature was observed. Thus, H. nipponica on the temperate coast of Korea exhibited a distinctly different growth pattern from those of temperate seagrass species in Korea, which have shown great reductions in growth at water temperatures higher than 20°C. Higher below- to above-ground ratio and leaf burial into sediments with shorter leaf petioles during winter might be overwintering strategies in this species. The growth patterns of H. nipponica at the study site imply that this species still possess the tropical characteristics of the genus Halophila.     

Phytoplankton photosynthetic activity and growth rates in the NW Adriatic Sea

Taxonomic composition, biomass, primary production and growth rates of the phytoplankton community were studied in two stations in the NW Adriatic Sea on a seasonal basis, in areas characterized by differing hydrological and trophic conditions. The main differences between the two stations were quantitative rather than qualitative, most phytoplankton species being common to both stations. The effects of differing nutrient concentrations and plume spreading were evident. Biomass and primary production rates were significantly higher in the coastal station (S1), and the phytoplankton distribution in the water column was markedly stratified in S1 and more even in the offshore station (S3). However, chlorophyll a specific production, potential growth rate and production efficiencies were very similar in both stations, even when phosphorus concentrations were limiting. A discrepancy between potential and actual growth rate was observed: as a feature common to both stations, comparisons between potential and actual growth rates revealed that little carbon produced by phytoplankton accumulated as algal biomass; therefore, very high loss rates were estimated.     

Spatial and temporal pattern in seagrass community composition and productivity in south Florida

We document the distribution and abundance of seagrasses, as well as the intra-annual temporal patterns in the abundance of seagrasses and the productivity of the nearshore dominant seagrass (Thalassia testudinum) in the south Florida region. At least one species of seagrass was present at 80.8% of 874 randomly chosen mapping sites, delimiting 12,800 km² of seagrass beds in the 17,000-km² survey area. Halophila decipiens had the greatest range in the study area; it was found to occur over 7,500 km². The range of T. testudinum was almost as extensive (6,400 km²), followed by Syringodium filiforme (4,400 km²), Halodule wrightii (3,000 km²) and Halophila engelmanni (50 km²). The seasonal maxima of standing crop was about 32% higher than the yearly mean. The productivity of T. testudinum was both temporally and spatially variable. Yearly mean areal productivity averaged 0.70 g m⁻²day⁻¹, with a range of 0.05–3.29 g m⁻² day⁻¹. Specific productivity ranged between 3.2 and 34.2 mg g⁻¹ day⁻¹, with a mean of 18.3 mg g⁻¹ day⁻¹. Annual peaks in specific productivity occurred in August, and minima in February. Integrating the standing crop for the study area gives an estimate of 1.4 × 1011 g T. testudinum and 3.6 × 10¹⁰ g S. filiforme, which translate to a yearly production of 9.4 × 10¹¹ g T. testudinum leaves and 2.4 × 10¹¹ g S. filiforme leaves. We assessed the efficacy of rapid visual surveys for estimating abundance of seagrasses in south Florida by comparing these results to measures of leaf biomass for T. testudinum and S. filiforme. Our rapid visual surveys proved useful for quantifying seagrass abundance, and the data presented in this paper serve as a benchmark against which future change in the system can be quantified. Received: 30 January 2000 / Accepted: 24 July 2000     

Nutrition and grazing behavior of the green turtle Chelonia mydas

The apparent digestibility coefficients for 4 size classes of the green turtle Chelonia mydas feeding on the seagrass Thalassia testudinum were measured in Union Creek, Great Inagua, Bahamas, from September 1975 to August 1976. The values ranged from 32.6 to 73.9% for organic matter; from 21.5 to 70.7% for energy; from 71.5 to 93.7% for cellulose; from 40.3 to 90.8% for hemicellulose; and from 14.4 to 56.6% for protein. Digestive efficiency increased with increases in water temperature and body size. There was no seasonal variation in the nutrient composition of T. testudinum blades. Grazing on T. testudinum may be limited by its low quality as a forage, a result of its high fiber content and possible low protein availability. Turtles did not graze at random over the extensive beds of T. testudinum, but maintained grazing plots of young leaves by consistent recropping. They thus consumed a more digestible forage-higher in protein and lower in lignin-than the ungrazed, older leaves of T. testudinum. The selectivity of green turtles for either a seagrass or algal diet may reflect the specificity of their intestinal microflora.     

Responses of decomposition rate and nutrient release of floating-leaved and submerged aquatic macrophytes to vertical locations in an urban lake (Nanhu Lake,China)

A leaf-bag field experiment was conducted to investigate the decomposition and release of nutrients from leaves of two aquatic macrophytes (floating-leaved Trapa bispinosa and submerged Vallisneria natans) deposited in the four vertical locations (i.e. air-water interface, AW; sediment-water interface, SW; buried at a depth of 10?cm, B10; buried at a depth of 20?cm, B20) of littoral zone in Nanhu Lake, China, for 60 days from July to August 2015. Leaf initial quality significantly influenced mass loss and nutrient release except TN (total nitrogen) remaining. Compared to V. natans, T. bispinosa leaves decomposed faster under the same treatments. The decomposition was greatly affected by both leaf chemical quality and the location of deposition. With the increasing depth of vertical locations, leaf biomass loss and nutrient release of both T. bispinosa and V. natans decreased. In addition, initial N:P ratio and cellulose were the major determinants for decomposition in AW and SW treatments while total phenol in B10 and B20. Our results suggest that the combined effect of leaf chemical quality and burial could mediate macrophyte mass loss and release of nutrients and carbon, which in turn can influence organic matter accumulation and nutrient cycling in shallow freshwater lakes.     

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.     

Grazing rates of organic matter and living fungal biomass of decaying Spartina alterniflora by three species of salt-marsh invertebrates

 The pathway for the flow of salt-marsh grass production into marsh food-webs is still not well defined. We compared the abilities of three marsh macroinvertebrates [salt marsh periwinkles, Littoraria irrorata (Say) (=Littorina irrorata), salt-marsh coffee-bean snails, Melampus bidentatus (Say); and a talitrid amphipod, Uhlorchestia spartinophila Bounsfield and Heard] to access standing-dead leaves of smooth cordgrass (Spartina alterniflora Loisel). The invertebrates were incubated with naturally-decaying leaves, and the rates of removal of organic matter and living fungal biomass (ergosterol) were measured. The impact of invertebrate activity upon fungal growth rates was measured as rates of fungal-membrane synthesis (incorporation of radioacetate into ergosterol). The removal rates of organic leaf biomass per mg individual biomass were highest for amphipods (700 μg mg⁻¹ d⁻¹) and lowest for periwinkles (90 μg mg⁻¹ d⁻¹), but the relatively large biomass of the snails made their removal rates per individual greater than those of amphipods. Net removal of ergosterol by all three invertebrates was >50% for yellow-brown (early-decay) leaf blades. For fully-brown (advanced-decay) blades, >50% removal of ergosterol was found only for periwinkles; exposure to coffee-bean snails and amphipods resulted in a net ergosterol reduction of ≤20%. The lower net reduction of living fungal biomass by coffee-bean snails and amphipods may have been due to fungal-growth stimulation (2.3-fold stimulation in coffee-bean snails and 1.5-fold stimulation in amphipods). Grazing by periwinkles did not stimulate fungal growth, possibly because of its high intensity. Grazing by these three salt-marsh shredders may affect marsh-grass shoot-decay in different ways. Periwinkles may abbreviate the period of fungal production, and incorporate the decaying material relatively quickly into snail biomass and fecal-pellet rain to the sediments. Coffee-bean snails and amphipods may enhance and prolong fungal production, along with the formation of fecal-pellet rain. All three invertebrates fed preferentially on leaf blades rather than leaf sheaths, and feeding rates of gastropods were higher during the night than during the day. Received: 25 November 1998 / Accepted: 4 November 1999     

Primary productivity of the tropical western Atlantic Ocean near Barbados

Forty-four measurements of net primary productivity by ¹⁴C fixation were made at a station near Barbados, West Indies, over a period of 22 months; in addition 12 measurements were made at 7 deep-water stations in the tropical Western Atlantic Ocean. Average daily production was 0.288 g C/m² and estimated annual net production 105 g C/m². There was no evidence of seasonal variations in the rate of primary production nor of significant differences between production at the principal station and the additional deep-water stations. The data are, therefore, considered to be representative of the rate of primary production in this part of the tropical Western Atlantic Ocean and not significantly affected by proximity to land.     

Biomass and production in polar planktonic and sea ice microbial communities: a comparative study

Algal and bacterial biomass and production were measured in the plankton, platelet ice and congelation ice communities at one station in McMurdo Sound, Antarctica during September and October 1986. Bacterial abundances and particulate organic carbon and nitrogen were 10 to 100 times greater in the plankton than in the sea ice, whereas the chlorophyll a concentrations in the plankton and sea ice microbial communities (SIMCO) were similar Rates of both light-limited and light-saturated photosynthesis and daily primary production were 2 to 6 times greater in the plankton than in the SIMCO. Bacterial growth rates ranges from 0.7 to 1.5 d^-1 in all three communities; however, because of the greater bacterial biomass in the plankton, bacterial production was 15 to 20 times higher there than in the SIMCO. These results suggest that during the early austral spring, planktonic production contributes significantly to total production in ice-covered environments.