Standing crop and aboveground biomass partitioning of a dwarf mangrove forest in Taylor River Slough,Florida

The structure and standing crop biomass of a dwarf mangrove forest, located in the salinity transition zone ofTaylor River Slough in the Everglades National Park, were studied. Although the four mangrove species reported for Florida occurred at the study site, dwarf Rhizophora mangle trees dominated the forest. The structural characteristics of the mangrove forest were relatively simple: tree height varied from 0.9 to 1.2 meters, and tree density ranged from 7062 to 23 778 stems ha^–1. An allometric relationship was developed to estimate leaf, branch, prop root, and total aboveground biomass of dwarf Rhizophora mangle trees. Total aboveground biomass and their components were best estimated as a power function of the crown area times number of prop roots as an independent variable (Y = B × X^–0.5083). The allometric equation for each tree component was highly significant (p<0.0001), with all r² values greater than 0.90. The allometric relationship was used to estimate total aboveground biomass that ranged from 7.9 to 23.2 ton ha^–1. Rhizophora mangle contributed 85% of total standing crop biomass. Conocarpus erectus, Laguncularia racemosa, and Avicennia germinans contributed the remaining biomass. Average aboveground biomass allocation was 69% for prop roots, 25% for stem and branches, and 6% for leaves. This aboveground biomass partitioning pattern, which gives a major role to prop roots that have the potential to produce an extensive root system, may be an important biological strategy in response to low phosphorus availability and relatively reduced soils that characterize mangrove forests in South Florida.     

Continental scale patterns in mangrove litter fall

Litter fall was monitored in stands of the mangrove species Rhizophora stylosa Griff., Ceriops tagal (Perr.) C. B. Robinson and Avicennia marina (Forsk.), Vierh. at approximately monthly intervals over a single annual cycle at selected locations around the coastline of Australia and throughout the distribution of each species. Concurrent data were obtained from a single location near Port Moresby in Papua New Guinea. The materials recovered in sub-canopy catchers were sorted into major categories and dried and weighed as leaves, petiolar stipules, twigs and other woody tissues, reproductive parts (flowers, flower buds, fruit and propagules) and residual detritus. This paper considers the principal findings of the study among which it may be reported that the highest total annual litter recoveries at individual catchers were 1598 g dry wt m^–2 for A. marina, 2369 g dry wt m^–2 for R. stylosa and 1290 g dry wt m^–2 for C. tagal. Significant regional differences in litter fall emerged when data from major climatic zones were compared. The outcome of this analysis is detailed in the body of the paper.     

Copepod nauplii from the barrier reef of Belize

A study of areal distribution of the copepod nauplii near mangrove covered cays on the barrier reef of Belize was begun in the summer of 1989 and continued in the spring of 1990 with the collection of twenty samples from four locations. At distances in the order of 100 of meters to 1000 from the mangrove prop roots there were thousands of nauplii m^–3. In the samples from Twin Cays where the plankton tows were made 15 m from the prop roots or less the average was 387 nauplii m^–3. Copepod nauplii compose 60% of the metazoan plankton abundance in all of the samples. Harpacticoid nauplii were the largest single component of the plankton in every sample; they account for 42% to 76% of the copepod nauplii. Cyclopoid nauplii represent 13 to 40% and are second to the harpacticoid nauplii. Calanoid nauplii account for 2% to 23% of the copepod nauplii.     

Life history and production of Neomysis mercedis in two British Columbia coastal lakes

The life history and production characteristics of Neomysis mercedis from two British Columbia lakes were examined for their potential influence on zooplankton and limnetic fish communities. During the day, mysids in shallow Muriel Lake (45 m) were on or near the bottom; in Kennedy Lake (> 100 m), mysids remained deeper than 50 m. In both lakes, mysids spent summer nights at 0–15 m depths despite > 20 °C temperatures. Mysid density was not strongly correlated with lake depth. Mysids generally displayed spring to early summer minima and late summer to fall maxima in numbers and biomass. Single peaks in gravid females and juvenile mysid abundance, and the absence of pronounced seasonal size changes of gravid females suggest that N. mercedis produced a single generation each year. Fecundities of study lake mysids are the lowest on record, and although size-dependent, exhibited unusually high variability. Annual productivity of mysids averaged 485 mg m^–2 y^–1 (range 205–690). Calculations indicate mysids consume several times more zooplankton per annum than limnetic fish do. N. mercedis is likely an important competitor of juvenile sockeye salmon (Oncorhynchus nerka) since: (i) sockeye exhibit food limited growth and survival patterns in coastal lakes, (ii) mysids and sockeye consume similar zooplankton prey and (iii) mysids do not contribute greatly to sockeye diet (i.e. < 26% of summer and fall diets by numbers or weight).     

Uptake of sulphate,taurine, cysteine and methionine by symbiotic and free-living dinoflagellates

Sulphate uptake by Amphidinium carterae, Amphidinium klebsii and Gymnodinium microadriaticum grown on artificial seawater medium with sulphate, cysteine, methionine or taurine as sulphur source occurred via an active transport system which conformed to Michaelis-Menten type saturation kinetics. Values for K _m ranged from 0.18–2.13 mM and V _max ranged from 0.2–24.2 nmol · 10⁵ cells^–1 · h^–1. K _m for symbiotic G. microadriaticum was 0.48 mM and V _max was 0.2 nmol · 10⁵ cells^–1 · h^–1. Sulphate uptake was slightly inhibited by chromate and selenate, but not by tungstate, molybdate, sulphite or thiosulphate. Cysteine and methionine (0.1 mM), but not taurine, inhibited sulphate uptake by symbiotic G. microadriaticum, but not by the two species of Amphidinium. Uptake was inhibited 45–97% under both light and dark conditions by carbonylcyanide 3-chlorophenylhydrazone (CCCP); under dark conditions sulphate uptake was 40–60% of that observed under light conditions and was little affected by 3-(3,4-dichlorophenyl) 1,1-dimethylurea (DCMU).The uptake of taurine, cysteine and methionine by A. carterae, A. klebsii, cultured and symbiotic G. microadriaticum conformed to Michaelis-Menten type saturation kinetics. K _m values of taurine uptake ranged from 1.9–10 mM; for cysteine uptake from 0.6–3.2 mM and methionine from 0.001–0.021 mM. Cysteine induced a taurine uptake system with a K _m of 0.3–0.7 mM. Cysteine and methionine uptake by all organisms was largely unaffected by darkness or by DCMU in light or darkness. CCCP significantly inhibited uptake of these amino acids. Thus energy for cysteine and methionine uptake was supplied mainly by respiration. Taurine uptake by A. carterae was independent of light but was inhibited by CCCP, whereas uptake by A. klebsii and symbiotic G. microadriaticum was partially dependent on photosynthetic energy. Taurine uptake by cultured G. microadriaticum was more dependent on photosynthetic energy and was more sensitive to CCCP. Cysteine inhibited uptake of methionine and taurine by cultured and symbiotic G. microadriaticum to a greater extent than in the Amphidinium species. Methionine did not greatly affect taurine uptake, but did inhibit cysteine uptake. Taurine did not affect the uptake of cysteine or methionine.     

Variation in δC values for the seagrass Thalassia testudinum and its relations to mangrove carbon

Carbon isotope ratios (¹³C/¹²C) were measured for the leaves of the seagrass Thalassia testudinum Banks ex König and carbonates of shells collected at the seagrass beds from seven sites along the coast of southern Florida, U.S.A. The δ¹³C values of seagrass leaves ranged from −7.3 to −16.3‰ among different study sites, with a significantly lower mean value for seagrass leaves from those sites near mangrove forests (−12.8 ± 1.1‰) than those far from mangrove forests (−8.3 ± 0.9‰; P < 0.05). Furthermore, seagrass leaves from a shallow water area had significantly lower δ¹³C values than those found in a deep water area (P < 0.01). There was no significant variation in δ¹³C values between young and mature leaves (P = 0.59) or between the tip and base of a leaf blade (P = 0.46). Carbonates of shells also showed a significantly lower mean δ¹³C value in the mangrove areas (−2.3 ± 0.6‰) than in the non-mangrove areas (0.6 ± 0.3‰; P <0.025). In addition, the δ¹³C values of seagrass leaves were significantly correlated with those of shell carbonates (δ¹³C seagrass leaf = −9.1 + 1.3δ¹³C shell carbonate (R² = 0.83, P < 0.01)). These results indicated that the input of carbon dioxide from the mineralization of mangrove detritus caused the variation in carbon isotope ratios of seagrass leaves among different sites in this study.     

Mechanism for the small-scale movement of carbon among estuarine habitats: organic matter transfer not crab movement

In theory, carbon is highly mobile in aquatic systems. Recent evidence from carbon stable isotopes of crabs (Parasesarma erythrodactyla and Australoplax tridentata), however, shows that in subtropical Australian waters, measurable carbon movement between adjacent mangrove and saltmarsh habitats is limited to no more than a few metres. We tested whether the pattern in crab δ¹³C values across mangrove and saltmarsh habitats was explained by crab movement, or the movement of particulate organic matter. We estimated crab movement in a mark–recapture program using an array of pitfall traps on 13 transects (a total of 65 traps) covering an area of 600 m² across the interface of these two habitats. Over a 19-day period, the majority of crabs (91% for P. erythrodactyla, 93% for A. tridentata) moved <2 m from the place of initial capture. Crab movement cannot, therefore, explain the patterns in δ¹³C values of crabs. δ¹³C values of detritus collected at 2-m intervals across the same habitat interface fitted a sigmoidal curve of a similar form to that fitting the δ¹³C values of crabs. δ¹³C values of detritus were 2–4‰ more depleted in saltmarsh (−18.5±0.6‰), and 4–7‰ more depleted in mangroves (−25.9±0.1‰) than δ¹³C values of crabs recorded previously in each habitat. Assimilation by crabs of very small detrital fragments or microphytobenthos, more enriched in ¹³C, may explain the disparity in δ¹³C values. Nevertheless, the pattern in δ¹³C values of detritus suggests that crabs obtain their carbon from up to several metres away, but without themselves foraging more then a metre or so from their burrow. Such detailed measurements of carbon movement in estuaries provide a spatially explicit understanding of the functioning of food webs in saltmarsh and mangrove habitats.     

Long- and short-term effects of crop residues on aluminum toxicity,phosphorus availability and growth of pearl millet in an acid sandy soil

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha^–1 yr^–1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha^–1 yr^–1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha^–1 yr^–1 by ¹⁵N dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a ¹⁵N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.     

Emergence of Chironomidae from the shallow eutrophic Lake Kasumigaura,Japan

Seasonal chironomid emergence was monitored in the shallow eutrophic Lake Kasumigaura and 18 species were collected with a battery-operated light trap fixed on a floating stage and with surface emergence traps. During October–December, samples in the light trap comprised exclusively Tokunagayusurika akamusi (Tokunaga) and small numbers of one or two other species. T. akamusi, Procladuis (Holotanypus) culiciformis (L.), and Chironomus plumosus (L.) constituted 91.6% of the annual catch of chironomid adults. The predominance of T. akamusi (75.3 % of chironomid catch) and the high ratio (13) of T. akamusi to C. plumosus was more marked in this lake than other Japanese eutrophic lakes. Glyptotendipes tokunagai Sasa and Dicrotendipes pelochloris (Kieffer) were also caught abundantly with the light trap, but not with surface traps, indicating these were littoral species. The dry weight of emerging adults during May–December 1982 was 2.87 g m^–2, of which 1.92 gm^–2 (67%) was T. akamusi and 0.67 gm^–2 (23%) C. plumosus and 0.23 g m^–2 (8%) Clinotanypus sugiyamai Tokunaga and 0.03 gm^–2 (1%) P. (H.) culiciformis. The weight of emerging Tanypodinae was much higher than the annual mean larval biomass or estimated larval production, which have been due to underestimating the population density using an Ekman-Birge dredge. High numbers of individuals and species of chironomids were caught during April–July, presumably as a result of the high food supply for chironomid communities.     

Effects of diet and heavy metals on growth rate and fertility in the deposit-feeding snail Potamopyrgus jenkinsi (Smith) (Gastropoda: Hydrobiidae)

Evidence for the influence of food type and heavy metals on shell growth and fertility is presented for a freshwater population of the snail P. jenkinsi. When fed an excess of lettuce or lamb heart (protein source), growth rates were higher for lettuce. Highest growth rates occurred at a diet of lettuce plus lamb heart. Fertility was favoured by a diet of lamb heart. When fed an excess of lettuce, the EC₅₀ growth values were 16 µg Cd l^–1, 13 µg Cu l^–1, and 103 µg Zn l^–1 in lake water; snail fertility was inhibited at 25 µg Cd l^–1 and 30 µg Cu l^–1. A diet of lake detritus spiked with Cd or Cu resulted in a decrease of approximately 50% in growth rates, when compared with growth on non-spiked detritus. Spiked detritus lost metals into lake water. Food type positively interacted with metal stress, both for growth rate and fertility. The assessment of inhibitory effects of detritus contaminated either in the field or, notably, by spiking, and serving as food source for deposit feeders is hampered by sampling problems in the field and by redistribution processes of pollutants between particles and water in laboratory-scale experiments.     

Gonadal neoplasms in wild carp-goldfish hybrids from the Welland River near Niagara Falls,Canada

An experimental approach was taken to examine the processes of detritus decomposition in river sediments.Addition of macrophyte detritus (Alternanthera philoxeroides (Mart.) Griseb) to river sediment resultedin an increase in carbon mineralization from a sbasal rate of 200 up to 500 mg C m^–2 d^–1.Carbon mineralization after addition of oak detritus was only slightly higher than mineralization in sediments thatreceived no addition ( 200 mg C m^–2 d^–1). Bacterial biomass and production in sediments \s+ alligator-weedwere higher than in sediments + oak or zero-addition. In these experiments the major fate of addedalligatorweed was mineralization. For alligatorweed detritus, microbial metabolism depletes organic carbonrather than leading to increases in food quality. Therefore, a pulse input of alligatorweed detritus would notbe available as a long-term source of organic carbon. Oak detritus was not rapidly decomposed, and sopersists in these sediments for a longer period.     

Plant regeneration via somatic embryogenesis from protoplasts of six plant species related to Citrus

Protoplasts isolated from embryogenic callus of Fortunella polyandra (Ridl.), Atalantia bilocularis (Pieree ex Guill.), Hesperethusa crenulata (Roxb.), Glycosmis pentaphylla (Retz.) Corr., Triphasia trifolia (Burm. f.) P. Wils. and Murraya koenigii (L.) Spreng. were cultured in MT (Murashige and Tucker 1969) basal medium containing 5% sucrose supplemented with 0.0, 0.001, 0.01, 0.1 or 1.0 mg l^–1 BA and 0.6 M sorbitol. The highest plating efficiencies for all species were obtained on MT basal medium containing 5% sucrose supplemented with 0.001 mg l^–1 BA. F. polyandra produced higher percentages of globular somatic embryo development, while A. bilocularis consistently showed a lower percentage of globular somatic embryo development in all 5 concentrations of BA. MT basal medium containing 5% sucrose and supplemented with 0.001 mg l^–1 BA was found to be a suitable medium for development of globular somatic embryos derived from protoplasts to form heart-shaped somatic embryos with cotyledon-like structures. The highest percentages of shoot formation for all 6 species were obtained using 0.1 mg l^–1 GA₃. A complete protoplast-to-plant system was developed for F. polyandra, A. bilocularis and T. trifolia, which could facilitate the transfer of nuclear and cytoplasmic genes from these species into cultivated Citrus through protoplast fusion.Abbreviations BA N⁶-benzyladenine - 2,4-D 2,4-dichlorophenoxyacetic acid - FDA fluorescein diacetate - GA₃ gibberellin A₃     

Late Holocene mangrove dynamics of Marajó Island in Amazonia,northern Brazil

Two sediment cores from the eastern coastal region of Marajó Island, Pará State, northern Brazil have been studied by pollen analysis to reconstruct late Holocene mangrove dynamics and environmental changes. Seven AMS radiocarbon dates provide time control. Mangrove vegetation became established at the Barra Velha site at about 2750 B.P. (2880 cal B.P.) and at the Praia do Pesqueiro site at about 650 B.P. (670 cal B.P.). Rhizophora was the dominant mangrove tree throughout the recorded period, while Avicennia and Laguncularia were rare. Existing remnants of the former coastal Amazon rain forest were replaced by mangrove in the Barra Velha area between about 2750 and 740 B.P. (2880–760 cal B.P.) and at Praia do Pesqueiro area between about 650 and 530 B.P. (670–540 cal B.P.), suggesting a rise in relative sea level or, alternatively, an increase in discharge from the river Amazon. Areas of coastal shrub and herb vegetation, the so-called restinga vegetation, also became slightly reduced during the late Holocene. The largest area of mangrove at the two sites suggests that the highest sea level was probably reached during the last 200–250 years. The only evidence of human activity at the two sites is an indication of cattle pastureland at the Barra Velha area during the last decades.     

Particulate organic matter in a mountain stream in the south-western Cape,South Africa

The quality and quantity of allochthonous inputs and of benthic organic matter were investigated in a second-order, perennial mountain stream in the south-west Cape, South Africa, between April 1983 and January 1986. Although the endemic, riparian vegetation is sclerophyllous, low and evergreen, inputs of allochthonous detritus to the stream (434 to 500 g m^–2y^–1) were similar to those recorded for riparian communities worldwide, as were calorific values of these inputs (9548 to 10 032 KJ m^–2y^–1). Leaf fall of the riparian vegetation is seasonal, occurring in spring (November) as discharge decreases, resulting in retention of benthic organic matter (BOM) on the stream bed during summer and early autumn (maximum 224 g m^–2). Early winter rains (May) scoured the stream almost clean of benthic detritus (winter minimum 8 g m^–2). Therefore, BOM was predictably plentiful for about half of each year and predictably scarce for the other half. Coarse BOM (CBOM) and fine BOM (FBOM) constituted 46–64% of BOM standing stock, ultra-fine BOM (UBOM) 16–33% and leaf packs 13–24%. The mean annual calorific value of total BOM standing stock was 1709 KJ m^–2. Both standing stocks and total calorific values of BOM were lower than those reported for streams in other biogeographical regions. Values of C:N ratios decreased with decrease in BOM particle size (CBOM 27–100; FBOM 25–27; UBOM 13–19) with no seasonal trends. The stream is erosive with a poor ability to retain organic detritus. Its character appears to be dictated by abiotic factors, the most important of which is winter spates.     

Selective inhibitors of germination of legume seeds in activated sludge compost

Water extracts of the compost produced from activated sludge and coffee residue were found to be selectively inhibitory to seed germination of some legumes. Germination rate of white clover (Trifolium repens L.), red clover (Trifolium pratense L.) and alfalfa (Medicago sativa L.) seeds were reduced to 2, 29 and 73% of the control, respectively, by water extracts of the compost (20 g l^–1). However, the extracts did not show any inhibition to seed germination of sorghum (Sorghum bicolor Moench), African millet (Eleusine coracana Gaertn.), and Komatsuna (Brassica rapa L.) at the same concentration. The inhibitors in the compost extracts were separated by ion-exchange chromatography and reverse-phase high performance liquid chromatography (HPLC) and the inhibitory activities of seed germination were tested with white clover seeds. Five inhibitors were isolated and identified as 3,4-dichlorophenylacetic acid (3,4-DCP), 3,4-dichlorobenzoic acid (3,4-DCB), 3,4,5-trichlorophenylacetic acid, 3,4,5-trichlorobenzoic acid and mono-2-ethylhexylphthalate by ¹H-, ¹³C-NMR spectroscopy and mass spectrometry. The inhibitory activities of some authentic chemicals of the inhibitors and the related compounds were compared. The results indicated that the main inhibitor in the compost could be 3,4-DCB, which was contained at the concentration of 6.58 mg kg^–1 compost and showed the strongest inhibitory effect on seed germination of white clover among the tested compounds.     

Primary production and nutrient budgets of Sarcocornia perennis ssp. alpini (Lag.) Castroviejo in the salt marsh of the Palmones River estuary (Southern Spain)

Biomass, primary production and nutrient budgets associated to Sarcocornia perennis subspecies (ssp.) alpini were studied in the Palmones River estuary salt marsh (Southern Spain) to evaluate the nutrient sequestration capacity of the low marsh. Above- and belowground living and dead biomass, as well as carbon, nitrogen and phosphorus content were monitored during 1 year. Additionally, the fate of aboveground detritus was evaluated in an experiment on litter decomposition. The detritus production of S. perennis ssp. alpini was almost equivalent to its annual primary production indicating a rapid turnover of biomass. We calculated that only 12% of the aboveground detritus was exported out of the low marsh while the rest was decomposed in the sediment with a rate of 0.8 year⁻¹. Changes in concentrations of total carbon, nitrogen and phosphorus in the sediment showed patterns related to S. perennis ssp. alpini belowground biomass. Our results suggested that the sediment functions as a net sink for nutrients accumulating 550 g C m⁻² year⁻¹, 55 g N m⁻² year⁻¹, and 13 g P m⁻² year⁻¹.     

Meiobenthos in mangrove areas in eastern Africa with emphasis on assemblage structure of free-living marine nematodes

A survey was conducted to examine spatial variations in the population density of major meiofaunal taxa and the assemblage structure of free-living marine nematodes within 5 mangrove areas on the west and east coast of Zanzibar. Meiofauna densities in surface sediments (0–5 cm) ranged from 205 to 5263 ind. 10 cm², being on average 1493 ind. 10 cm². Of the 17 major taxa recorded, nematodes dominated (64–99%) in all samples while harpacticoid copepods were usually second most abundant. Within all areas the numbers of meiofauna were very variable and significant differences among areas were only detected for oligochaetes and turbellarians. Densities of nematodes, harpacticoids, polychaetes and turbellarians were, however, significantly (P<0.001) higher at low water stations compared with mid and high water stations. Harpacticoids were negatively correlated with the numbers of fiddler crab (Uca spp.) burrows. Other correlations between environmental factors (grain size, temperature, salinity, oxygen tension, prop root density, fiddler crab burrows) and major meiofaunal taxa were non-significant. A total of 94 nematode genera were recorded from four mangrove areas. The most abundant and frequent genera were Microlaimus and Spirinia, followed by Desmodora and Metachromadora. Representatives of the genera most common in current study are found all over the globe. There was a high variation in nematode assemblage structure within and between sampling areas indicating the absence of a well defined nematode assemblage confined to mangrove areas. In a hypersaline area diversity was much reduced and where salinity was over 100%. the fauna was restricted to 3 nematode genera, Microlaimus, Theristus and Bathylaimus. Multidimensional scaling ordination (MDS) of the nematode genera separated samples taken from low water stations from other stations, the assemblage structure being significantly different at the low water stations. Numbers of selective deposit feeders were negatively correlated with average grain size and positively correlated with silt content.     

The use of carbon isotope ratios to evaluate legume contribution to soil enhancement in tropical pastures

Soil carbon distribution with depth, stable carbon isotope ratios in soil organic matter and their changes as a consequence of the presence of legume were studied in three 12-year-old tropical pastures (grass alone —Brachiaria decumbens (C₄), legume alone —Pueraria phaseoloides (C₃) and grass + legume) on an Oxisol in Colombia. The objective of this study was to determine the changes that occurred in the¹³C isotope composition of soil from a grass + legume pasture that was established by cultivation of a native savanna dominated by C₄ vegetation. The¹³C natural abundance technique was used to estimate the amount of soil organic carbon originating from the legume. Up to 29% of the organic carbon in soil of the grass + legume pasture was estimated to be derived from legume residues in the top 0–2-cm soil depth, which decreased to 7% at 8–10 cm depth. Improvements in soil fertility resulting from the soil organic carbon originated from legume residues were measured as increased potential rates of nitrogen mineralization and increased yields of rice in a subsequent crop after the grass + legume pasture compared with the grass-only pasture. We conclude that the¹³C natural abundance technique may help to predict the improvements in soil quality in terms of fertility resulting from the presence of a forage legume (C₃) in a predominantly C₄ grass pasture.     

Contributions of three subsystems of a freshwater marsh to total bacterial secondary productivity

Rates of bacterial production were measured in the water column, on the surface of plant detritus, and in the surface sediments of a freshwater marsh in the Okefenokee Swamp, Georgia, USA. Bacterioplankton production rates were not correlated with several measures of quantity and quality of dissolved organic matter, including an index of the relative importance of vascular plant derivatives. Bacterioplankton productivity was high (mean: 63 g C liter^–1 day^–1) compared with rates reported for other aquatic ecosystems. Somewhat paradoxically, bacterial productivity on plant detritus (mean: 13 g C g^–1 day^–1) and sediments (mean: 15 g C g^–1 day^–1) was low relative to other locations. On an a real basis, total bacterial productivity in this marsh ecosystem averaged 22 mg C m^–2 day^–1, based on sample dates in May 1990 and February 1991. Marsh sediments supported the bulk of the production, accounting for 46% (May) and 88% (February) of the total. The remainder was contributed approximately equally by bacteria in the water column and on accumulated stores of plant detritus. Send offprint requests to: M. A. Moran.     

Food web structure in exotic and native mangroves: a Hawaii–Puerto Rico comparison

Plant invasions can fundamentally alter detrital inputs and the structure of detritus-based food webs. We examined the detrital pathways in mangrove food webs in native (Puerto Rican) and introduced (Hawaiian) Rhizophora mangle forests using a dual isotope approach and a mixing model. Based on trophic-level fractionation of 0–1‰ for δ ¹³C and 2–3‰ for δ ¹⁵N, among the invertebrates, only nematodes, oligochaetes, and nereid polychaetes from native mangroves exhibited stable isotopes consistent with a mangrove-derived diet. Certain fauna, in particular tubificid oligochaetes, had δ ¹³C values consistent with the consumption of mangrove leaves, but they were depleted in ¹⁵N, suggesting their primary nitrogen source was low in ¹⁵N, and was possibly N₂-fixing bacteria. In introduced mangroves, all feeding groups appeared to rely heavily on non-mangrove sources, especially phytoplankton inputs. Mixing model results and discriminant analysis showed clear separation of introduced and native mangrove sites based on differential food source utilization within feeding groups, with stronger and more diverse use of benthic foods observed in native forests. Observed differences between native and invasive mangrove food webs may be due to Hawaiian detritivores being poorly adapted to utilizing the tannin-rich, nitrogen-poor mangrove detritus. In addition, differential utilization of mangrove detritus between native and introduced mangroves may be a consequence of forest age. We postulate that increasing mangrove forest age may promote diversification of bacterial food webs important in N and S cycling. Our results also suggest a potentially important role for sulfur bacteria in supporting the most abundant infaunal consumers, nematodes, in the most mature systems. Electronic supplementary material The online version of this article (doi:) contains supplementary material that is available to authorized users.