Effect of macrophyte secondary metabolites on feeding preferences of the herbivorous parrotfish Sparisoma radians

Experiments were conducted to test the role of secondary metabolites in determining the natural feeding preference hierarchy of the bucktooth parrotfish Sparisoma radians. The two least preferred food genera of S. radians, Halimeda and Penicillus, both contain 1,4-diacetoxy-1,3-butadiene terpenes, while the most preferred species, Thalassia testudinum, does not. Experiments with agar cylinders containing macrophyte homogenates showed that macrophyte biteability was not a factor. Instead preference could be altered by the application of the diacetoxybutadiene containing terpenes 4,9-diacetoxyudoteal and caulerpenyne or fractions or extracts containing them at naturally occurring concentrations. Concentration of the active terpenes affected the intensity of the fish's preference for the control in pairwise comparisons. Extracts and fractions which did not contain 4,9-diacetoxyudoteal or caulerpenyne did not affect fish feeding preferences at naturally occurring concentrations. Experiments in which S. radians were given no plant choice showed that coating T. testudinum with H. incrassata organic crude extract reduced the number of bites consumed and the biomass consumed to a level equivalent to that obtained for H. incrassata plants.     

An acoustic investigation of seagrass photosynthesis

The use of high-frequency acoustics has recently emerged as a viable method for mapping the areal coverage of seagrasses. Since the bubbles produced by seagrass plants are partly responsible for the observed acoustic signature, it is likely that sound transmission throughout a seagrass canopy varies on circadian cycles coinciding with photosynthetic bubble production. This study examined the propagation of high-frequency (100 kHz) sound energy through the seagrass canopies of Syringodium filiforme, Halodule wrightii and Thalassia testudinum in a shallow outdoor mesocosm. Relative changes in the received acoustic energy were recorded every hour during a 24-h period and compared to independently measured rates of oxygen production. The mean acoustic intensity of energy transmitted throughout the seagrass canopy varied by 3.5 dB for S. filiforme, 4.4 dB for T. testudinum and 4.7 dB for H. wrightii over a 24-h period. These transmission characteristics are encouraging for the future development of in situ acoustic assessments of seagrass photosynthesis.     

Nitrogen fixation in the rhizosphere of marine angiosperms

High rates of acetylene reduction were observed in systems containing excised rhizomes of the Caribbean marine angiosperms Thalassia testudinum, Syringodium filiforme and Diplanthera wrightii, and the temperate marine angiosperm Zostera marina. For 4 plant and plant-sediment systems the ratio of acetylene reduced/N₂ fixed varied from 2.6 to 4.6. For T. testudinum the estimated rates of nitrogen fixation are in agreement with estimated requirements of the plant for nitrogen. For a typical T. testudinum stand, N₂ fixation is estimated to be 100 to 500 kg N/hectare per year. Numbers of N₂-fixing bacteria in the rhizosphere sediments were roughly 50 to 300 times more abundant than those in the nonrhizosphere sediments, and in both types of sediments were of the same orders as the estimated numbers of heterotrophic aerobes.Canadian IBP Contribution No. 137.     

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     

Herbivory and detritivory among gammaridean amphipods from a Florida seagrass community

The gammaridean amphipods Cymadusa compta (Smith), Gammarus mucronatus Say, Melita nitida Smith and Grandidierella bonnieroides Stephensen from a seagrass community in the Indian River estuary of Florida (USA) fed variously upon large drift algae, small algae epiphytic on seagrasses and seagrass leaf debris and detritus. Consumption was measured in the laboratory using an index (CI) equivalent to mg ingested mg^-1 amphipod day^-1. Observations revealed that the amphipods fed by macrophagy, an attack upon large algae and seagrass debris, and by microphagy, small particle detritus feeding and scraping of plant surfaces for diatoms and other epiphytic algae. C. compta was a macrophagous feeder with a generalized diet of algae and seagrass debris, preferring epiphytic algae and drift algae at mean rates of 1.10 and 0.87 CI, respectively. Gammarus mucronatus fed upon epiphytes and seagrass debris equally at mean rates of 0.90 and 0.97 CI, respectively. The diet of M. nitida condisted primarily of epiphytes, consumed at an average rate of 1.05 CI. Grandidierella bonnieroides fed in a specialized microphagous manner, grooming plant surfaces for small particle detritus and diatoms at an approximate CI rate of 1.45. Assimilation of plants ingested, as reflected by carbon-14 uptake, varied similarly among the 4 amphipods. Epiphytic algae appeared to be most useful as food, providing means of 41 to 75% carbon-14 uptake as ingesta. Drift algae and seagrass debris were of less value, with means varying between 11 and 24 % of carbon-14 uptake by the amphipods. The data show a pattern of feeding which resembles resource partitioning of food both by size and kind. Other evidence, however, including population limitation by predators and an apparent overabundance of food, indicate that resource partitioning as seen may be an artifact, and one which has no co-evolutionary basis among the present species.Contribution No. 102 of Harbor Branch Foundation, Inc.     

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.     

Abundance,spatial distribution,and size structure of populations of Oreaster reticulatus (Echinodermata: Asteroidea) on sand bottoms

Three populations of Oreaster reticulatus (Linnaeus, 1758) inhabiting shallow-water (<4 m) seagrass habitats in the Grenadines (West Indies, Caribbean Sea) were associated predominantly with beds of Halodule wrightii. Occupation of fringing inshore areas of bare sand was inversely related to wave action; even where sandy patches occurred offshore, the preferred substratum was H. wrightii. The association of O. reticulatus with H. wrightii is related to the asteroid's microphagous feeding habit and the availability of food resources associated with the seagrass. O. reticulatus rarely occurred on dense beds of Thalassia testudinum, but was moderately abundant in areas of sparse cover. Differences in the occurrence of O. reticulatus among seagrass types may be related to factors afdecting foraging effort, such as the tractability of the substratum and mobility upon it. Populations of O. reticulatus exhibited an aggregated dispersion within beds of H. wrightii, possibly attributable to local substratum heterogeneity and/or reproductive behavior. Increased turbulence induced migration to deeper water and markedly increased aggregation along offshore boundaries. The populations were primarily adults, with some late juvenile stages. The paucity of juveniles and their cryptic behavior and coloration suggest that settlement and early postmetamorphic development occurs in alternate habitats, such as dense beds of T. testudinum. Interpopulation differences in size structure may be associated with differences in the quality and availability of food sources.     

Food resources of postlarval brown shrimp (Penaeus aztecus) in a Texas salt marsh

Field and laboratory experiments were conducted to identify the sources of food in the natural diet of postlarval brown shrimp (Penaeus aztecus Ives). A series of enclosures placed in East Lagoon (29°20N; 94°45W) on Galveston Island, Texas, USA, in May 1985, were used to evaluate the individual and combined contribution of Spartina alterniflora detritus, epiphytes of S. alterniflora, plankton, and demersal fauna in terms of differences in shrimp growth and carbon assimilation (stable carbon-isotope analysis). Demersal fauna (harpacticoid copepods, amphipods, tanaids and polychaete annelids), and plankton (>0.095 mm) accounted for approximately 53 and 47% of the growth of the postlarvae (11 to 22 mm rostrum-telson length), respectively, while the autochthonous plant substrates, S. alterniflora detritus and epiphytes, contributed little. Laboratory experiments confirm that a mixed diet consisting of both animal protein and phytoplankton promotes maximum growth. Our results indicate that plankton may be an important allochthonous source of carbon contributing to the growth and development of shrimp in the salt marsh.     

13C/12C ratios and the trophic importance of algae in Florida Syringodium filiforme seagrass meadows

Over 380 stable carbon isotope (¹³C) analyses made during 1981–82 showed that Syringodium filiforme Kutz seagrass meadows in the Indian River lagoon of eastern Florida have food webs based on algal rather than seagrass carbon. Seagrasses averaging approximately-8 were isotopically distinct from algae epiphytic on seagrass blades (X=-19.3) and particulate organic matter in the water column X=-21.6. ¹³C values of most fauna ranged between-16 and-22, as would be expected if food web carbon were derived solely from algal sources. These results counter the idea that seagrass detritus is the dominant carbon source in seagrass ecosystems. Two factors that may contribute to the low apparent importance of seagrass in the study area are high algal productivities that equal or exceed S. filiforme productivity and the high rates of seagrass leaf export from meadows.     

Nutrients increase epiphyte loads: broad-scale observations and an experimental assessment

There is a global trend towards elevated nutrients in coastal waters, especially on human-dominated coasts. We assessed local- to regional-scale relationships between the abundance of epiphytic algae on kelp (Ecklonia radiata) and nutrient concentrations across much of the temperate coast of Australia, thus assessing the spatial scales over which nutrients may affect benthic assemblages. We tested the hypotheses that (1) percentage cover of epiphytic algae would be greater in areas with higher water nutrient concentrations, and (2) that an experimental enhancement of nutrient concentrations on an oligotrophic coast, to match more eutrophic coasts, would cause an increase in percentage cover of epiphytic algae to match those in more nutrient rich waters. Percentage cover of epiphytes was most extensive around the coast of Sydney, the study location with the greatest concentration of coastal chlorophyll a (a proxy for water nutrient concentration). Elevation of nitrate concentrations at a South Australian location caused an increase in percentage cover of epiphytes that was comparable to percentage covers observed around Sydneys coastline. This result was achieved despite our inability to match nutrient concentrations observed around Sydney (<5% of=" sydney=" concentrations),=" suggesting=" that=" increases=" to=" nutrient=" concentrations=" may=" have=" disproportionately=" larger=" effects=" in=" oligotrophic=">     

Standing crop of foraminifera in sublittoral epiphytic communities of a Long Island salt marsh

During the summers of 1966 and 1967, 12 field trips were made to stations in North Sea Harbor, Southampton, Long Island (USA). From representative locations, 228 small samples of larger algae and their epiphytes (0.2 g dry weight) were taken aseptically. Enteromorpha intestinalis, the most widely distributed aquatic plant, was the most frequently collected. The large standing crop of Zostera marina and Zanichellia palustris was also sampled. Foraminifera were most abundant in epiphytic communities of Enteromorpha in early summer and later spread to Zostera, Zanichellia, Ulva, Polysiphonia, and Ceramium. Foraminifera were rarely found in epiphytic communities of Fucus or Codium. By summers' end Enteromorpha rarely had a standing crop of foraminifera. One of the most abundant foraminiferan species, Protelphidium tisburyensis, was found most frequently on Enteromorpha; Quinqueloculina spp occurring on Enteromorpha, less frequently. Ammonia beccarri and Elphidium spp were abundant in the environment, and showed little substrate preference. Patches of decaying Enteromorpha had the greatest standing crop of foraminifera and low species diversity index (0.581). Young green patches had a much higher species diversity index (0.94). Indices for Zostera, Zanichellia, Polysiphonia, Fucus, Ulva and Codium were, respectively, 0.82, 0.99, 0.86, 0.70, 0.77, and 0.196. No correlation was found between epiphytic community weight and total number of foraminifera recovered. The standing crop of epiphytes/g substrate dry weight was lower at some field stations; possibly explained by stronger current. Of the total samples, 50.4% were positive, with an average of 18 foraminifera/sample, or 40 foraminifera/g substrate plant and epiphytes. Twenty six samples were classified as bloom with 50 or more forams (57 to 425)/sample. Species diversity indices for these blooms ranged from 0.38 to 1.12. Ammonia beccarii was the dominant form in 18 blooms, Allogromia laticollaris and Protelphidium tisburyensis in 3; Elphidium incertum, Quinqueloculina seminulum and Trochammina inflata in 1 each.Supported by US AEC Contracts AT (30-1) 3396 and AT (30-1) 3995. Ref. numbers NYO 3396-17 and NYO 3995-2.Much of this study was carried out in the Living Foraminifera Laboratory, Department of Micropaleontology of the American Museum of Natural History.     

Respiration and nitrogen excretion by the squid Loligo forbesi

The amphipod Corophium volutator (Pallas) of the Bay of Fundy is a major food resource for migrating shorebirds. A relatively low total lipid content (ca 1.7%) was found in summer and fall samples of 1986. Analysis of triacylglycerides which would normally reflect an animal diet through fatty acid input, revealed high proportions of fatty acids characteristic of algae. C. volutator contains hydrocarbons (0.2% of total lipids) with a unimodal distribution in the range C₁₈ to C₃₅ with maximum at C₂₅, and a weak odd-carbon preference (carbon preference index, C.P.I. =1.22). This hydrocarbon pattern shown by C. volutator seems to be biogenic, but due to both a weak odd-carbon preference and a significant content of the isoprenoid phytane, the presence of petroleum hydrocarbons cannot be ruled out. Comparison of the hydrocarbon composition be ruled out. Comparison of the hydrocarbon composition found in C. volutator with hydrocarbons described in the literature for its most likely sources of food suggests that highly degraded material and aged detritus are components of the diet of this deposit feeder, and as important as the local diatom population. No organohalogen pesticides (DDT, dichlorodiphenyl-trichloroethane, or derivatives) or polychlorinated biphenyls (PCBs) were detected in the lipid extracts of C. volutator. Absence of these materials and low hydrocarbon levels explain the continued successful exploitation of this food resource by migrating shorebirds.     

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     

Effect of phosphate and ammonium levels on photosynthetic and respiratory responses of the red alga Gracilaria verrucosa

Gracilaria verrucosa (Hudson) Papenfuss exposed to nutrient enriched media (0.1 mM PO₄; 1.0 mM NH ₄ ⁺ ) by pulse feeding 2 h every third day for a period of 5 wk at 20°C and 25–30 salinity showed significantly higher rates of photosynthesis regardless of photon flux density correlated with increased pigment levels. Algae in nonenriched media showed significantly higher levels of soluble carbohydrates and decreased levels of phycoerythrin and chlorophyll a. Photosynthetic and respiratory responses to temperature 15°, 25°, 30°C and salinity (15, 25, 30 S) combinations indicate broad tolerances by both nutrient enriched and non-nutrient enriched algae. Photosynthetic and respiratory rates were highest at the high temperatures. Pulse-fed algae had significantly higher photosynthetic rates than non-nutrient enriched plants at all temperature and salinity combinations. Non-nutrient enriched algae had significantly higher respiratory rates than nutrient enriched algae at only 30°C and 15. The respiratory rates of both nutrient enriched and non-nutrient algae decreased under combinations of higher temperatures and salinities. G. verrucosa, grown without nutrients, has lower tolerances to environmental stresses.     

Alkanes and alkenes in marine benthic algae

Saturated and olefinic hydrocarbons were determined in additional species of benthic marine algae from the Cape Cod (Massachusetts, USA) area (see: Youngblood et al., 1971). The distribution of homologous and isomeric olefins was studied in plants of different age and in morphologically different parts of the same specimen. With two minor exceptions, only normal alkanes and alkenes are present. The methylene-interrupted C₁₉- and C₂₁-polyolefins are particularly abundant; ¹-heneicosahexaene and the corresponding pentaene are common to all brown algae, while the corresponding ³-isomers occur in green algae. The hydrocarbon concentration, the alkene-to-alkane ratio and the polyolefin content are highest in young plants or in rapidly growing tissues of older plants. This suggests a deeper involvement in cell biochemistry of straight-chain hydrocarbons than previously considered. The biosynthesis of the plant polyolefins remains to be explored; no immediately obvious precursors of the ¹-polyolefins were found among the algal fatty acids. The hydrocarbon composition of these benthic algae differs greatly from that of fossil fuels in its simplicity and predominately unsaturated nature. The separation of the isomers by gas chromatography and their structural elucidation by mass spectrometry, alone and in combination with hydrogenation and ozonolysis, are discussed.Contribution No. 3155 of the Woods Hole Oceanographic Institution.     

Effects of plant size and distribution on the numbers of invertebrate species and individuals inhabiting the brown alga Pelvetia fastigiata

The invertebrates living on specimens of the brown alga Pelvetia fastigiata, growing in the intertidal zone at La Jolla, California (USA) in November 1975 were enumerated. Within 7 collecting sites, larger plants generally shelter more animal species and individuals than smaller plants. The number of species on a given plant can be described as an equilibrium number; numbers of species and individuals can become similar on defaunated and transplanted algae of comparable sizes placed at the same experimental site. Such equilibria are site-specific because the colonization, immigration and loss rates of animals differ among the sites studied. Relationships between plant size and the number of animal species and individuals on P. fastigiata also differ among the collecting sites. The between-site differences are related to tidal level, to habitat diversity and to habitat patch-size. Small isolated plants without epiphytes shelter few species. The faunas of larger isolated plants at mid-tide levels generally include many thallus-dwelling, tubiculous, vagrant and epiphyte-dwelling species, but few such species commonly inhabit plants within aggregations of P. fastigiata. Within aggregations, the plants host relatively few epiphytes and thus lack habitat diversity, and the net emigration rates of many animals including epiphyte-dwellers are relatively high. Plants within aggregations, however, usually shelter more animal indivuduals than isolated plants. Thus, faunal diversity is reduced, not increased within the largest patches of P. fastigiata.     

Carbohydrate metabolism in the leg muscles of the fresh-water field crab Paratelphusa hydrodromous adapted to high salinities

Utilization of marine plants and their constituents by bacteria isolated from the guts of echinoids was investigated to determine the potential role bacteria may have in carbohydrate digestion in echinoids. Bacteria from the guts of the regular echinoids Lytechinus variegatus (Lamarck) and Arbacia punctulata (Lamarck) could physically degrade the marine plants Ulva lactuca and Eucheuma nudum, but not Caulerpa prolifera. Diplanthera wrightii and Thalassia testudinum were only slightly degraded by the gut bacteria. Bacteria from the guts of the irregular echinoids Mellita quinquiesperforata (Leske) and Encope aberrans (Martens) could not physically degrade any of these marine plants. Mixed and some isolated bacteria from the gut of L. variegatus could utilize xylose, rhamnose, glucose, galactose, laminarin, carageenan, starch and agar, but not cellulose or chitin. The results with isolates suggest that the bacteria of the echinoid gut are fairly non-selective. The bacteria of the guts of the two regular echinoids could utilize certain marine plants that the echinoids eat and certain of the plant constituents. The bacteria could not utilize plant fibers.     

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.     

Meiofauna on the seagrass Thalassia testudinum: population characteristics of harpacticoid copepods and associations with algal epiphytes

The composition and abundance of bladedwelling meiofauna was determined over a 15 mo period (1983–1984) from a Thalassia testudinum Banks ex König meadow near Egmont Key, Florida, USA. Harpacticoid copepods, copepod nauplii, and nematodes were the most abundant meiofaunal taxa on T. testudinum blades. Temporal patterns in species composition and population life-history stages were determined for harpacticoid copepods, the numerically predominant taxon. Sixteen species or species complexes of harpacticoid copepods were identified. Harpacticus sp., the most abundant harpacticoid, comprised 47.8% of the total copepods collected, and was present throughout the study. Copepodites dominated the population structures of the blade-dwelling harpacticoid species on most collection dates. Ovigerous females and/or copepodites were always present, indicating continuous reproductive activity. Results suggest that epiphytic algae influence meiofaunal abundance on seagrass blades, as densities of most meiofaunal taxa at Egmont Key were positively associated with percent cover of epiphytic algae throughout the study. The majority of significant correlations between meiofaunal density and cover of epiphytic algae involved filamentous algae, although encrusting algae dominated the epiphytic community. It appears that resources provided by epiphytic algae to seagrass meiofauna (additional food, habitat, and/or shelter from predation) may be associated with algal morphology.     

Action spectra and spectral quantum yield of photosynthesis in marine macroalgae with thin and thick thalli

Net photosynthesis at 10mol photons m^-2 s^-1 in each of 24 wavelengths was measured in absolute units by an O₂-electrode and corrected for dark respiration to construct action spectra for gross photosynthesis in nine species of algae, which included plants with thin and thick thalli from each of four major pigment groups. The photosynthesis of green and brown algae with thin thalli decreased in green light, but species with thick thalli from these two groups had action spectra which were almost flat, and matched the optical blackness of the thalli but did not reflect the pigment differences between the species. Among the red algae, on the other hand, there was little difference between the action spectra for thin and thick algae. Only wavelengths absorbed by the phycobilin pigments were effective in photosynthesis, even in species (e.g. Chondrus, Phyllophora) which absorbed all visible wavelengths strongly. Maximal quantum yields of 0.10 to 0.12 O₂ molecules per absorbed photon were recorded for thin green and brown algae, but thicker algae in these two groups had lower values. Red algae exhibited maximal values close to 0.10 O₂ molecules per absorbed photon, irrespective of thallus thickness or phycocyanin content, but the quantum yields of phycoerythrin-rich species in the 600 to 650 nm waveband were lower than those of phycocyanin-rich species.