Bare Sediment Research Articles

Biogeochemistry of billabong sediments. I. The effect of macrophytes

SUMMARY. We examined the effects of an emergent macrophyte (Eleocharis sphacelata R. Br., Cyperaceae) and a submerged macrophyte (Vallisneria gigantea Graeb., Hydrocharitaceae) on the biogeochemistry of the sediments of a billabong in south‐eastern Australia. Sediments from an E. sphacelata bed had significantly lower concentrations of exchangeable phosphorus than did sediments from a nearby bare area or a V. gigantea bed, but neither macrophyte had a measureable effect on their sediment's exchangeable ammonium content. The redox potential in the upper 10cm of E. sphacelata sediments was about 100 mV higher than that of bare sediments, or of sediments colonized by V. gigantea. There were few consistent differences between vegetated and bare sediments in terms of the activity of extracellular enzymes, such as α‐amylase, protease, β‐d glucosidase, lipase or alkaline phosphatase. Rates of alkaline phosphatase activity (235–306μmol (g dry wt)−1 day−1) were markedly higher than those commonly reported for sediments or soils. Rates of gas release were higher from bare sediments (21–93 ml m−2 h−1) than from E. sphacelata or V. gigantea sediments (17–23 and 21‐24ml m−2 h−1, respectively). Gas bubbles consisted mainly of methane (26–66%) and nitrogen (15–68%). Rates of methane ebullition varied from 5 to 60ml m−2 h−1. In‐vitro methanogenesis was most rapid in samples of the upper flocculent sediment. Methanogenesis was slower in V. gigantea sediments than in bare area or E. sphacelata sediments, but was markedly accelerated by additions of acetate and/or H2/CO2 in all sites. Profiles of total extractable fatty acids and phospholipid fatty acids demonstrated that material derived from higher plants dominated the sediment organic matter in all sites. Bacteria were also a significant component of sediment organic matter, as fatty acids for which bacteria can be assumed the sole source accounted for 18–30% of total fatty acid content. Biomarkers for sulphate‐reducing bacteria (Desulfobacter spp.) were detected, and for type II methanotrophic bacteria.

The relationship between nitrogen fixation and tidal exports of nitrogen in a tropical mangrove system

Various components (sediments, algal mats, decomposing logs and algal-covered prop roots) of a tropical mangrove forest showed low to moderate nitrogen fixation (acetylene reduction) rates. Measurements carried out in March, May and October 1989 revealed negligible or inconsistent seasonal variation in activity for all components. Prop roots with their associated algae showed much greater activity during night-time, typical of many substrates with associated cyanobacteria, whereas all other components had constant activity over 24 h periods. Extrapolation of the nitrogen fixation rates for each component to a ‘whole-forest’ basis, using field estimates of the densities of each component, indicated that sediments, prop roots and decomposing logs contributed approximately 3·5, 1·6 and 1·0%, respectively, of the nitrogen requirements for forest net primary production (FPPN). Blue-green algal mats on bare saltpan areas showed no significant nitrogen-fixing activity (in excess of that for the bare sediments in the same area). The total nitrogen contribution of the various components (6% of FPPN) closely matches previous esimates of the net nitrogen loss from the system through tidal fluxes of particulate and dissolved materials. The possible relative importance of other nitrogen input and loss mechanisms for this mangrove system is also discussed.

Nitrogen cycling in sediments with estuarine populations of Potamogeton perfoliatus and Zostera marina

Rates of nitrogen transformations and concentrations of extractable NH,+ and NOs(plus NO2-) were measured in estuarine sediments vegetated with the submersed macrophytes Potarnogeton perfoliatus and Zostera manna, and in adjacent bare sediments, 3 or 4 times during the growing season. Nitrification and denitrification potentials were measured in substrate-amended sediment slurries at 5 depth intervals to provide a measure of bacterial activity. In general, rates were significantly higher in vegetated compared to bare sediments. It appears that both plant species affected nitrogen transformations through several similar mechanisms, while the microbial community, in turn, regulated nitrogen available for plant growth. In P, perfoliatus beds, ammonification and potential nitrification rates were correlated. Both exhibited summer maxima coinciding with peak plant biomass and productivity. Although vertically integrated (0-12 cm) amrnonification rates were about t w c e as high in vegetated than in bare sediments, NH4+ pools were significantly lower, probably due to high plant nitrogen demand. In contrast, denitrification rates were highest in spring when NO3concentrations peaked, and were significantly correlated to nitrification rates in both spring and fall. Denitrification was only about 20% of total NO3reduction, suggesting that NH4+ production from NO3may be important in conserving nitrogen within the grassbed. In sediments with Z. marina, rates of ammonification, and nitrification and denitrificabon potentials each exhibited a distinct seasonal cycle, indicating that rates were not as tightly coupled as in P. perfoljatus beds. High amrnonification rates exceeded plant demand leading to NH,' accumulation. Potential nitrification rates were highest in vegetated sediments during fall. Denitrification rates, which were also greater in vegetated than in bare sediments, were highest in spring when NO3concentrations were high. Potential denitrification rates comprised about 10 of total NO3reduction, indicating that NO,reduction to NH,+ dominated. The microbial communities responsible for key nitrogen transformations in the sediments were enhanced by both P. perfoliatus and Z. manna ammonificatlon by inputs of organic nitrogen; nitrification by release of O2 by plant roots; and denitnfication by production of NO3-

Pyrite accumulation in salt marshes in the Eastern Scheldt, southwest Netherlands

Pore water composition, pyrite distribution and pyrite crystal morphology of sediments from salt marshes in the Eastern Scheldt, southwestern Netherlands, were examined from July 1984 to October 1986. Hydrology and marsh vegetation were the chief determinants of pyrite accumulation. In the bare sediments of pans in the low marsh, highly reducing conditions prevailed just below the surface. At these sites, practically all the incoming detrital pyrite (0.5–1% FeS2) was preserved. The in-situ formation of pyrites was negligible in these anoxic sediments. All incoming detrital pyrite was oxidized in the surface layers (0–10 cm) of the medium-high marsh overgrown withSpartina anglica. Pyrite was formed at a rate of 2.6–3.8 mol S-FeS2m−2yr−1 in a narrow range of depths (15–20cm), at the interface of the oxidizing and underlying reducing sediment. At this interface the concentration profiles of Fe2+ and dissolved ΣS intersected. The role of the rhizosphere is discussed in connection with pyrite formation. No further pyrite formation occurred deeper in the sediment. This resulted in the build up of high concentrations of dissolved ΣS and acid volatile sulfides (AVS). The decrease with depth in oxalate-extractable Fe indicated that most of the iron oxyhydroxides (70–80%) had been transformed to pyrite. Another 10–20% of oxalate-extractable Fe was present as AVS. The abundance of framboidal pyrite particles and the high concentrations of AVS and dissolved ΣS indicated that the formation of pyrite occurred via iron monosulfide intermediates There was a linear relationship between the organic carbon and the S-FeS2 content in theSpartina overgrown reducing sediment. The mean C/S ratio was 4.2.

The effects of nutrient additions on mixtures of Typha latifolia L. and Typha domingensis pers. along a water-depth gradient

Over a 2-year period, nutrients (N, P and K) were added to mixtures of Typha latifolia L. and T. domingensis Pers. growing across a water-depth gradient in order to determine: (1) the relationship between nutrient limitation and water depth; (2) the differential effect of nutrient addition on species performance. In addition, sediment characteristics were determined for untreated mixtures, monocultures and bare sediment. Additions of nutrients resulted in a rapid increase in total shoot density compared to controls. Increases in total shoot biomass, however, occurred gradually over time in response to nutrient additions. Nutrients enhanced T. latifolia in shallow to medium depths while T. domingensis was enhanced in medium to deep waters. Compared to bare sediment, pH was not affected by the presence of either species. Redox potential, however, was increased by T. domingensis in medium depths while unaffected by T. latifolia. Coupled with the observation that T. domingensis is capable of growing in deeper water than T. latifolia, the E h data suggest that T. domingensis may have a greater capacity to aerate its roots and rhizosphere. Sediment ammonium concentrations were reduced by c. 90–95% by plant growth while potassium was reduced by c. 35–55%. Phosphate was unchanged compared to bare sediment. These results suggest that sediment nitrogen was the most limiting nutrient.

Emission of Gaseous Carbon Dioxide from Salt-Marsh Sediments and Its Relation to Other Carbon Losses

Rates of CO2 emission from bare salt-marsh sediments in areas of short and tall formSpartina alterniflora were measured monthly for 1 yr. Maximum emission rates, as high as 325 ml CO2m−2h−1, were observed during summer months, while minimum rates, 10.2 ml CO2 m−2h−1, were observed during the winter. An exponential function of inverse soil temperature explained most of the seasonal variability, but other factors are involved in regulating CO2 emissions as demonstrated by rates that were higher in spring than in late summer at equivalent temperatures. Annual CO2 emissions from bare sediments were 27.3 and 18.6 mol C m−2 yr−1 in communities of short and tallS. alterniflora, respectively. It was estimated that losses of dissolved inorganic carbon from the turnover of pore water, up to 14.6 mol C m−2 yr−1 at the creek bank (tall,S. alterniflora) site, and diffusion of CO2 from the root system ofS. alterniflora through the culms, 12.3 to 16.2 mol C m−2 yr−1, could also be important pathways of carbon loss from marsh sediments. If the internal flux of CO2 from the root system through the culm is refixed within the leaves, then the observed rate of 9.8 μI CO2 min−1 cm−2 of culm cross sectional area appears to make a small but significant contribution to total photosynthesis.

Effects of Predatory Sculpin on the Chironomid Communities in an Arctic Lake

To determine the importance of fish predation and habitat heterogeneity in structuring the chironomid community of an arctic lake, I used a combination of field and laboratory experiments in conjunction with field sampling data. Chironomid density was consistently °1.5 times as high in macrophyte—covered as in bare sediments in the soft—sediment littoral zone of arctic Toolik Lake. In laboratory experiments, most chironomid taxa showed no preference for one habitat over the other, but schulpins, the major predators on larval chironomids, were more efficient foragers in bare sediments than in macrophytes. Field exclosure experiments in macrophyte and bare—sediments habitats indicated that macrophytes afforded chironomids a refuge from their sculpin predators. At the end of a 21—d experiment, chironomid density in unenclosed areas significantly lower than in fish exclosures in bare sediments, but this was not so in macrophytes. This difference was due to an effect of sculpin on chironomids that are small rather than on large chironomids or on any single chironomid taxon or broad trophic group (predaceous vs. nonpredaceous). Small chironomids were probably more available as prey because of their tube—dwelling and sediment—penetrating behaviors.

Littoral chironomid communities in an arctic Alaskan lake

The soft‐sediment littoral zone of arctic Toolik Lake (68°N) is composed of patches of macrophytes and bare sediments, which support distinct communities of chironomid larvae. Macrophytes were typically dominated by Stictochironomus rosenschoeldi (Zett.) but also had persistent populations of Heterotrissocladius maeaeri Brund., Paratanytarsus spp., Ablabesmyia sp., as well as several other less common taxa. Bare sediments were dominated by H. maeaeri, Parakiefferiella sp., and Zalutschia zalutschicola Lip. S. rosenschoeldi was common in bare sediments hut far less so than in macrophytes. Few taxa were restricted to either habitat. Life cycle lengths, estimated for some species, ranged from 1 to 4 yr. Analysis of indicator species suggested that Toolik is oligotrophic to ultra‐oligotrophic, which is consistent with the available primary production estimate for Toolik Lake. Littoral chironomid communities, in addition to profundal communities, may be useful in lake typology.

Report: London International Food and Drink Exhibition

The Banc d'Arguin in Mauritania is an area characterised by shallow waters and tidal flats with extensive seagrass beds. The aim of this study was to determine the role of the Banc d'Arguin tidal flats as a habitat and a nursery for shrimps. Shrimps were sampled in the Baie d'Aouatif in the intertidal (ponds, Zostera noltii seagrass beds and bare sediment) with a push net during high and low tide, a beam trawl during high tide and in the subtidal (Cymodocea nodosa and Zostera noltii seagrass beds) with a beam trawl during low and high tide from January to March 2002. In total, eleven shrimp species (9349 individuals) were found, the most abundant of which were Palaemon elegans, Hippolyte longirostris, H. inermis (in subtidal) and Peneaus spp., whereas Athanas nitescens, Leander tenuicornis, Palaemon adspersus, Pontophilus fasciatus, Sicyonia carinata and Processa edulis subsp. crassipes were less abundant. At high tide in the intertidal all species were mainly found in sheltered areas (ponds and seagrass) rather than in unvegetated habitats. Mark-recapture experiments showed that shrimps migrate between intertidal ponds during high tide. Densities in the subtidal were similar during high and low tide, suggesting that there is no substantial tidal migration to and from the tidal flats for any of the species. Two subtidal shrimp species seemed to prefer Cymodocea nodosa beds. A large fraction of the individuals of five shrimp species carried eggs. Ten of the eleven species caught were also found as juveniles. The results of this study, and earlier studies, indicate that many shrimp species reproduce in or near the tidal-flat area of the Banc d'Arguin. Some shrimp species, notably Penaeus kerathurus and P. notialis, are likely to use the area as a nursery, migrating offshore as adults. This suggests that the Banc d'Arguin is an important resource for commercial fishery.

Foraging Efficiency and Body Size: A Study of Optimal Diet and Habitat Use by Bluegills

A foraging model was developed to predict the optimal diet breadth and maximum energetic intake of a given—sized fish foraging in each of three aquatic habitats: the open water, vegetation, and bare sediments. Model parameters of prey encounter rates and prey handling times were quantified as functions of fish size, prey density, and prey size through a series of laboratory feeding experiments using the bluegill sunfish (Lepomis macrochirus). Results of these experiments show both searching ability and prey handling efficiency to increase with increasing fish size. Predictions of prey size selection and optimal habitat use based upon maximizing energetic gain were then examined in a small Michigan lake for three size classes of bluegills. Bluegills > 100 mm standard length were highly size selective in their feeding and their diets closely matched predictions of an optimal diet model. From two estimates of relative prey visibilities I show that these fish selected larger prey items than would be predicted if prey were consumed “as encountered.” Habitat use of large bluegills was also shown to maximize foraging return as fish switched from utilizing vegetation—living prey to utilizing open—water zooplankton as relative foraging profitabilities in the two habitats changed across the summer. Bluegills <100 mm standard length were restricted in their habitat use, remaining in or near the vegetation despite demonstrated increases in foraging return available in the open—water habitat. Size—related predation risk apparently accounts for differences in habitat use between bluegill size classes, and its consequences for intra— and interspecific competition between size classes are discussed.

Patterns of Invertebrate Size and Abundance in Aquatic Habitats

The size distributions of invertebrate prey found in the vegetation, bare sediment, and open water habitats of a small, Michigan lake were quantified for the months of May through August, 1979. The distribution of prey body size in each habitat generally conformed to a lognormal distribution, allowing a simple characterization of prey size and abundance in each habitat. Seasonally, the abundance of large littoral invertebrates declined markedly, shifting the distribution of prey size in the vegetation and bare sediment habitats towards smaller prey. The total biomass of vegetation-dwelling prey (exclusive of gastropods) also declined dramatically from May to August. These seasonal changes in prey size and abundance were correlated with a reduction in foraging intake for the dominant fish species in the lake, the bluegill sunfish (Lepomis macrochirus). Bluegills whose diet was > 80% vegetation prey showed a > 3-fold decline in stomach contents (prey weight/fish weight) from May to Ausut, during which time prey biomass in the vegetation declined 2.5-fold.Key words: body size, lognormal distribution, benthos, foraging, Lepomis, Michigan