Vascular Plant Detritus Research Articles

The effect of Hurricane Lili on the distribution of organic matter along the inner Louisiana shelf (Gulf of Mexico, USA)

On October 3, 2002 Hurricane Lili made landfall on a previously studied region of the inner Louisiana shelf as a Category 2 storm with winds over 160 km/h. A week after the hurricane, major impacts of the storm were not evident in the water column except for the lower than expected inshore salinities (∼12 psu) for this time of year, which was characterized by low river discharge. Turbidity profiles were typical of those measured during previous investigations with suspended sediment concentrations >75 mg/L at inshore stations and <50 mg/L in surface waters and offshore. The implication is that the sediments resuspended during the hurricane settled soon after the storm passage. Water column particulate organic carbon (POC) concentrations ranged from 0.1 to over 2.0 mg/L, with the highest concentrations measured near the seabed and in the inshore portions of the study area. Suspended particles were characterized by low organic matter content (%POC of 0.5–2 wt%), low chlorophyll:POC ratios (Chl:POC<4 mg/g) and moderately elevated POC:particulate nitrogen ratios (POC:PN of 10–14 mol/mol), all suggesting their source was locally resuspended seabed sediment rather than from algal biomass or land-derived vascular plant detritus. Post hurricane sediment deposition throughout the study area resulted in a storm layer that ranged from <0.5 to 20 cm in thickness. In most locations sediment accumulation ranged from 3 to 10 cm. The storm deposits were generally composed of silty clays with a coarser, somewhat sandy 1–2 cm basal layer. Surface sediments from the storm layer were characterized by relatively high mineral surface areas (SA of 30–50 m 2/g) and elevated OC contents (%OC of 1.0–2.0%). The dispersal of fine sediments following the hurricane resulted in marked changes in the SA and %OC values of surface sediments from offshore locations, which prior to the storm contained coarser, organic-poor particles (SA of 5–15 m 2/g and %OC of 0.2–0.6%). The OC:SA and OC:N ratios of storm layer sediments ranged from 0.4 to 0.6 mg OC/m 2 and from 10 to 12 mol/mol, respectively, and were comparable to those measured in surface sediments prior to the hurricane. Such similarities in the composition of the organic matter reinforce the idea that the source of the storm deposits was the finer fraction of resuspended seabed sediments, with little evidence for inputs from local land-derived sources or autochthonous algal production. Overall, the magnitude of sediment and organic matter deposition on the seabed after the storm greatly exceeded the annual inputs from the Atchafalaya River and coastal primary production. The combined effects of hurricane-driven erosion and post-storm deposition represent a major perturbation to the benthic community of the region, which is already subject to these types of disturbances due to the combined effects of peaks in river discharge and the passage of storm fronts.

Functional and taxonomic diversity of saprobic filamentous fungi from Typha latifolia from central Alberta, Canada

The fate of vascular plant detritus and the microbial communities and processes involved during the decomposition of litter are important aspects in elucidating energy flow and nutrient cycling in wetlands. Therefore, we collected and identified conspicuous fungal sporocarps in situ and isolated microfungi from living and dead Typha latifolia (cattail) leaf tissues. Cattail is a dominant plant species in southern boreal and temperate marshes and abundant in the Low Boreal Mixedwood ecoregion in central Alberta, Canada. Following two successive field collections in early and late summer 2001, 45 different fungal taxa were identified. There were 26 ascomycetes, five basidiomycetes, and 14 anamorphic taxa, most of them with putative ascomycetous affinities. Twenty-four taxa represented new records for T. latifolia, 12 were new to Canada, and seven were new to North America. Also, five taxa were new reports outside of the country of the type locality. To elucidate their roles in the decomposition of T. latifolia leaves, we examined 33 taxa for their ability to use cellulose, gelatin, starch, tannic acid, and lignin as carbon sources (based on calorimetric tests), as well as to cause mass losses of sterile T. latifolia leaves. The number of fungi using cellulose and gelatin as carbon sources was significantly greater than those using starch, tannic acid, or lignin. Mass losses of T. latifolia leaf tissues by ascomycetes and basidiomycetes ranged from 1.3 to 54.6% and −0.4 to 52.1%, respectively. There was a positive relationship between mass loss of T. latifolia leaves and cellulose degradation but not between mass loss and any of the other carbon sources. Our data showed that a taxonomically diverse suite of fungi effectively degrades this plant material; however, additional studies examining the decomposer communities of other dominant wetland plants are necessary to gain a better understanding of nutrient and energy dynamics in wetlands at the ecosystem level.

Food selection in freshwater omnivores: a case study of crayfish Austropotamobius pallipes

The omnivorous habit and food selection in freshwater macroinvertebrates was investigated in the white-clawed crayfish, Austropotamobius pallipes, a threatened species. First, during an annual cycle we analysed gut contents for vascular plant detritus, moss, amorphous plant material, and animal remains, and evaluated their organic content. Second, we ran preference experiments in the laboratory, in which crayfish had to choose, in the first trial among three vegetal items, in the second trial among three animal prey, and in the third trial between the two preferred items of the previous two trials (i.e. moss and insect larvae). Third, we analysed whether the assimilation efficiencies of different food items affected crayfish food choice. Our results revealed that this species mostly acted as a detritus consumer, obtaining nutrition from the associated microbes, fungi, proto- and metazoans, but also showed herbivorous and carnivorous habits. However, at least when food resources were not limited, crayfish displayed distinct feeding preferences, often ruled by factors other than the nutritional quality and the assimilation efficiency of the selected food.

Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA)

The sources and distribution of organic matter (OM) in surface waters and sediments from Winyah Bay (South Carolina, USA) were investigated using a variety of analytical techniques, including elemental, stable isotope and organic biomarker analyses. Several locations along the estuary salinity gradient were sampled during four different periods of contrasting river discharge and tidal range. The dissolved organic carbon (DOC) concentrations of surface waters ranged from 7 mg l −1 in the lower bay stations closest to the ocean to 20 mg l −1 in the river and upper bay samples. There was a general linear relationship between DOC concentrations and salinity in three of the four sampling periods. In contrast, particulate organic carbon (POC) concentrations were significantly lower (0.1–3 mg l −1) and showed no relationship with salinity. The high molecular weight dissolved OM (HMW DOM) isolated from selected water samples collected along the bay displayed atomic carbon:nitrogen ratios ([C/N]a) and stable carbon isotopic compositions of organic carbon ( δ 13C OC) that ranged from 10 to 30 and from −28 to −25‰, respectively. Combined, such compositions indicate that in most HMW DOM samples, the majority of the OM originates from terrigenous sources, with smaller contributions from riverine and estuarine phytoplankton. In contrast, the [C/N]a ratios of particulate OM (POM) samples varied significantly among the collection periods, ranging from low values of ∼5 to high values of >20. Overall, the trends in [C/N]a ratios indicated that algal sources of POM were most important during the early and late summer, whereas terrigenous sources dominated in the winter and early spring. In Winyah Bay bottom sediments, the concentrations of the mineral-associated OM were positively correlated with sediment surface area. The [C/N]a ratios and δ 13C OC compositions of the bulk sedimentary OM ranged from 5 to 45 and from −28 to −23‰, respectively. These compositions were consistent with predominant contributions of terrigenous sources and lesser (but significant) inputs of freshwater, estuarine and marine phytoplankton. The highest terrigenous contents were found in sediments from the river and upper bay sites, with smaller contributions to the lower parts of the estuary. The yields of lignin-derived CuO oxidation products from Winyah Bay sediments indicated that the terrigenous OM in these samples was composed of variable mixtures of relatively fresh vascular plant detritus and moderately altered soil OM. Based on the lignin phenol compositions, most of this material appeared to be derived from angiosperm and gymnosperm vascular plant sources similar to those found in the upland coastal forests in this region. A few samples displayed lignin compositions that suggested a more significant contribution from marsh C 3 grasses. However, there was no evidence of inputs of Spartina alterniflora (a C 4 grass) remains from the salt marshes that surround the lower sections of Winyah Bay.

Seasonal utilization of different seston carbon sources by the ribbed mussel, Geukensia demissa (Dillwyn) in a mid-Atlantic salt marsh.

Seston in salt marshes contains a temporally and spatially complex mixture of natural microparticulate organic material, including phytoplankton, vascular plant detritus, bacteria, heterotrophic nanoflagellates and benthic diatoms. Quantitative information is available concerning how suspension-feeding consumers, such as the ribbed mussel, Geukensia demissa (Dillwyn), utilize some of these components to satisfy their carbon demands. Despite this information there is still a limited understanding of how the relative nutritive contribution of these different dietary items may shift during the year associated with variations in both seston composition and the mussel's physiological condition. To investigate if the mussel's ability to use specific constituents of natural seston varies seasonally, we ran a series of pulse-chase 14C feeding experiments under ambient conditions in March, May, August and November 1996. Phytoplankton, cellulosic detritus, bacteria, heterotrophic nanoflagellates and benthic diatoms were radiolabeled and supplemented in small amounts to natural marsh water for feeding to mussels. The fate of 14C in mussel tissues, feces, respiration and excretion was quantified and contrasted among the different diet types and seasons. Microcapsules containing radiolabeled carbohydrate and protein were used as standards to differentiate possible between-experiment variations in seston composition from seasonal changes in the mussel's feeding and digestive physiology. Mussel clearance rates for all diets were highest in summer and autumn and lowest in winter and spring. In contrast, seasonal shifts in digestive physiology were only found for certain diets. The seasonal range of assimilation efficiencies for microcapsule standards (18-29%) and field-collected microheterotrophs (bacteria 76-93% and heterotrophic nanoflagellates 87-94%) did not differ significantly during the year, whereas summer and autumn assimilation efficiencies for cellulosic detritus (22-24%), phytoplankton (71-79%) and benthic diatoms (89-93%) were up to twofold greater than those in winter and spring (13%, 40-59% and 45-81%, respectively). We conclude that the digestive physiology (e.g., digestive enzyme production) of mussels responds to shifts in dietary components during the year.

Decreased bacterial growth on vascular plant detritus due to photochemical modification

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 17:159-165 (1999) - doi:10.3354/ame017159 Decreased bacterial growth on vascular plant detritus due to photochemical modification Alexandre M. Anesio*, C. Måns T. Denward, Lars J. Tranvik**, Wilhelm Granéli Department of Ecology/Limnology, Lund University, S-22362 Lund, Sweden *E-mail: alexandre.anesio@limnol.lu.se **Present address: Department of Limnology, Evolutionary Biology Centre, Norbyvägen 20, S-75236 Uppsala, Sweden ABSTRACT: We investigated the effects of UV radiation on abiotic decomposition and dissolution of leaf litter from the aquatic macrophyte Phragmites australis. Dead leaves were autoclaved and incubated in quartz tubes with autoclaved Milli-Q water, in darkness, under UVA or under UVA+UVB radiation, using fluorescent tubes with intensities similar to solar radiation. More DOM was produced in irradiated samples compared to dark ones. After exposure, the water was inoculated with a natural assemblage of bacteria and incubated in the dark. Free bacteria reached higher densities in cultures with detritus pre-treated in darkness than in cultures with irradiated substrates. No significant differences in numbers of attached bacteria were found. We suggest that UV-irradiated detritus from P. australis has a negative effect on the growth of free living bacteria, possibly due to the leaching of inhibitory substances from the leaf litter into the water and/or a decrease in DOM bioavailability. Similar experiments, using natural solar radiation, indicated that visible light has only a minor effect on leaching of inhibitory substances. KEY WORDS: UV radiation · Leaf litter · Bacteria · Bioavailability · DOM · Solar radiation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 17, No. 2. Publication date: May 28, 1999 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1999 Inter-Research.

Estimating the assimilation of mangrove detritus by fiddler crabs in Laguna Joyuda, Puerto Rico, using dual stable isotopes

Dual stable isotope analyses (δ13C and δ15N) of fiddler crabs from a forest-fringed, land-locked lagoon in Puerto Rico indicated the differential assimilation of material from ingested sediments. Fiddler crabs preferentially selected for nitrogen-fixing benthic microalgae (cyanobacteria) over vascular plant detritus. These results question the assumption that mangrove detritus is always the principal source of energy to estuarine consumers. Previous research from this lagoon as well as from Amazonia suggests that the magnitude of land-water ecotonal coupling may be low for these particular tropical systems where benthic algal productivity is presumably high.

Intraguild Predation and Cannibalism among Larvae of Detritivorous Caddisflies in Subalpine Wetlands

Comparative data from subalpine wetlands in Colorado indicate that larvae of the limnephilid caddisflies, Asynarchus nigriculus and Limnephilus externus, are reciprocally abundant among habitats–Limnephilus larvae dominate in permanent waters, whereas Asynarchus larvae dominate in temporary basins. The purpose of this paper is to report on field and laboratory experiments that link this pattern of abundance to biotic interactions among larvae. In the first field experiment, growth and survival were compared in single and mixed species treatments in littoral enclosures. Larvae, which eat mainly vascular plant detritus, grew at similar rates among treatments in both temporary and permanent habitats suggesting that exploitative competition is not important under natural food levels and caddisfly densities. However, the survival of Limnephilus larvae was reduced in the presence of Asynarchus larvae. Subsequent behavioral studies in laboratory arenas revealed that Asynarchus larvae are extremely aggressive predators on Limnephilus larvae. In a second field experiment we manipulated the relative sizes of larvae and found that Limnephilus larvae were preyed on only when Asynarchus larvae had the same size advantage observed in natural populations. Our data suggest that the dominance of Asynarchus larvae in temporary habitats is due to asymmetric intraguild predation (IGP) facilitated by a phenological head start in development. These data do not explain the dominance of Limnephilus larvae in permanent basins, which we show elsewhere to be an indirect effect of salamander predation. Behavioral observations also revealed that Asynarchus larvae are cannibalistic. In contrast to the IGP on Limnephilus larvae, Asynarchus cannibalism occurs among same—sized larvae and often involves the mobbing of one victim by several conspecifics. In a third field experiment, we found that Asynarchus cannibalism was not density—dependent and occurred even at low larval densities. We hypothesize that Asynarchus IGP and cannibalism provide a dietary supplement to detritus that may be necessary for the timely completion of development in these nutrient—poor, high—elevation wetlands.

Dissolved humic substances of vascular plant origin in a coastal marine environment

Bacterial decomposition of vascular plant detritus in coastal wetlands results in the conversion of particulate organic matter to dissolved form and causes the release of humic substances into the bulk dissolved organic carbon (DOC) pool. We found that 34% of the DOC accumulating during degradation of salt marsh grass (Spartina alterniflora) from coastal wetlands of the southeastern U.S. fits the definition of humic substances and that lignin is the primary source of the dissolved humic substances (66% of the total). Although marine bacterioplankton used the lignin‐rich humic substances more slowly and less efficiently than other components of the DOC pool, a significant fraction (24%) of these substances was mineralized within 7 weeks. Concentrations of vascular plant biomarkers (lignin phenols) indicate that 11–75% of the dissolved humic substances on the southeastern U.S. continental shelf is from vascular plant‐dominated environments. Calculations indicate that about half this material is contributed by the coastal salt marshes and half by river export. Vascular plant influence was lower in the bulk DOC pool (2–38%), indicating that terrestrially derived material is harbored preferentially in the humic substances subcomponent of marine DOC.

An evaluation of the efficiencies of several ergosterol extraction techniques

Axenic cultures representing ten species of filamentous higher marine fungi were analyzed for ergosterol content to determine the relative efficiencies of five extraction protocols. Methanol refluxing resulted in consistently higher yields than did ethanol treatment. In every case treatment of mycelia with alcoholic KOH solution substantially enhanced ergosterol recovery. Experimental results relative to fungal biomass quantification in vascular plant detritus are discussed.

Early Diagenesis of Plant Pigments in Hudson River Sediments

Plant pigment concentrations were measured using high performance liquid chromatography in Hudson River sediments. Sedimentation rates and mixing characteristics were determined from depth profiles of the naturally occurring radionuclides 7Be, 210Pb. Previous estimates of the mean-lives (1/λ) of plant pigments (chlorophylls a and b, fucoxanthin) from laboratory experiments, indicate a range of c. 20-40 days. However, in the field, we observed that these pigments decreased less rapidly with depth and penetrated deeper than 7Be (mean-life 77 days) in Hudson sediments 95% of the time. Assuming similar mixing processes for particles carrying 7Be and pigments, this indicates that pigment decay rates in the field are slower than the 0·013 day -1 decay rate of 7Be, hence, more than 2-fold slower than the derived laboratory rates. We believe that high inputs of vascular plant detritus in the Hudson may increase the complexation of humic substances with pigments resulting in slower decay rates. While most pigments showed an exponential decrease with sediment depth, lutein concentrations generally increased with depth. This pattern of decay resistance is in agreement with laboratory experiments which show that lutein is the most decay resistant among dominant pigments. These data along with other studies demonstrate that the carotenoids lacking the 5,6-epoxide group (i.e., lutein) are more decay resistant than pigments that contain it (i.e., fucoxanthin).

Organic Matter Sources in the Water Column and Sediments of the Hudson River Estuary: the Use of Plant Pigments as Tracers

The purpose of this study was to document inputs of organic matter into the Hudson River Estuary using plant pigments as tracers. Plant pigments (carotenoids and chloropigments) were determined using high pressure liquid chromatography (HPLC). Water column and sediment samples were collected in 1988 and 1989, from stations along a 165 km transect in the Hudson River Estuary. In the water column, high concentrations of lutein were found at stations adjacent to wetland areas indicating inputs of vascular plant detritus, particularly during late fall. Chlorophyll b/lutein ratios at these stations were 2-4 which are similar to that found in fresh vascular plant material collected from the Hudson. Low chlorophyll b/lutein ratios at Haverstraw Bay, a shallow and turbid area of the estuary, indicate high levels of sediment resuspension. Cyanophyte blooms, as indicated by high concentrations of myxoxanthophyll, reach their peak in late summer at the Hudson and Kingston stations. The chlorophyll a from these algae may comprise as much as 78% of the total chlorophyll a pool during these periods. Concentrations of total phaeophorbide, an indicator of grazing activity, were not correlated with chlorophyll a but were correlated with fucoxanthin and/or fucoxanthinol. This correlation suggests that much of the zooplankton grazing activity is associated with high quality food resources such as diatoms and not with cyanophytes. Surface sediments from a coarse-grained sandy habitat had significantly lower amounts of total organic matter than muddy habitats. However, the sandy sediment had higher grazing activity per gram organic matter, as indicated by total phaeophorbides. Higher concentrations of chlorophyll a and fucoxanthin at the sandy habitat indicate the presence of benthic diatoms which are high quality (low C/N ratio) living resources in contrast to the detrital sources (high C/N, mostly vascular plant) at the muddy stations. High concentrations of chlorophyllide a at some muddy stations, near macrophyte beds, suggest inputs of epiphytic diatom detritus. Overall, sediment pigments of the Hudson estuary are derived from vascular plant material that does not reflect the pigment signature of the water column.

Bathymetric patterns of deep-sea benthic communities from bathyal to abyssal depths in the western South Pacific (Solomon and Coral Seas)

Ten stations were sampled from bathyal to abyssal depths (695–4350 m) in the Solomon and Coral Seas to determine variations in densities of Protozoa, meiofauna and macroinfauna in relation to environmental conditions and bacterial densities and productivity. Densities of large flagellates and amoebae did not change with water depth, whereas the abundance of other microbenthic groups either decreased (Foraminifera, ciliates) or increased (yeasts) significantly with increasing ocean depth. Numerically, the microbenthos was dominated by Foraminifera, followed in decreasing order of abundance by yeasts, large flagellates, amoebae and ciliates. Densities of total metazoan meiofauna and macroinfauna were low at most stations compared to communities of other regions at equivalent depths, and declined significantly with ocean depth, although many taxa and total macroinfaunal biomass did not. Differences in the rate of decline between total metazoan meiofaunal and macroinfaunal densities with depth were not significant. When the effect of water depth was held constant, only a few protozoan, meiofaunal and macroinfaunal taxa correlated with bacteria and sediment characteristics. Depth-related variables (e.g. low temperatures) appear to be the dominant factors regulating bathymetric patterns. However, there is some evidence that rapid transport of turbidites and other shallow-water materials (wood, vascular plant and fresh algal detritus) lead to episodes of erosion and deposition that may influence the distribution and standing crop of these deep-sea benthic communities. These events may be influential in other tectonically active basins within tropical oceans bordered by steep continental margins and dominated by extensive reefs and large rivers.

Detrital processing in seagrass systems: A review of factors affecting decay rates, remineralization and detritivory

The literature on three related aspects of the ecological role of seagrass detritus was analysed for clues to the factors that determine the rates of decomposition, the amount of remineralization vs. immobilization of nitrogen and the nutritional value of the detritus to animals. Rates of decomposition (usually < 1% of dry wt. day −1) are generally low compared with other vascular macrophyte sources of detritus, but are influenced by many variables, e.g., chemical composition and growth state of the plant when decay begins, pre-treatments given the plant material (drying, grinding), methods of incubating (litterbags, enclosed microcosms), and the physical and chemical environment in which decay occurs. Standard techniques are lacking, making comparisons difficult. Seagrass detritus undergoes an initial period of leaching, leaving a poor substrate for bacteria because what soluble material remains is deficient in inorganic nutrients, contains inhibitory phenolic compounds, and is protected by cellulose and lignin. During decay, there is little evidence for the net immobilization of nitrogen on seagrass detritus that has been reported for other types of vascular plant detritus; instead, nitrogen and other elements are gradually released. The difference may arise from the quantity and nature of the lignin and phenolic materials in different plants, since those compounds are the source of some of the molecules that form resistant complexes with microbial proteins and lead to a build-up of nitrogen. Since fungi seem to be unimportant in seagrass decay, the development of anaerobic conditions (usually through burial of detritus in sediments) promotes decay by anaerobic cellulolytic bacteria. Fragmentation by detritivores also increases decay rates in the egested detritus by reducing particle sizes, increasing surfaces available for microbial attack and (sometimes) adding amino-nitrogen to the detritus. Some animals can assimilate the cell contents in seagrass leaves while others exhibit cellulolysis in their guts (probably as a result of the gut microflora) and are efficient assimilators of seagrass structural carbohydrates. Compared to other vascular plants, the pattern of detrital processing for seagrasses exhibits several unusual features that require further study: (1) slow rates of loss of biomass despite relatively low initial phenolic contents; (2) low rates of microbial metabolism despite relatively high initial nitrogen contents; (3) no build-up of nitrogen.

Bacterial secondary production on vascular plant detritus: relationships to detritus composition and degradation rate.

Bacterial production at the expense of vascular plant detritus was measured for three emergent plant species (Juncus effusus, Panicum hemitomon, and Typha latifolia) degrading in the littoral zone of a thermally impacted lake. Bacterial secondary production, measured as tritiated thymidine incorporation into DNA, ranged from 0.01 to 0.81 microgram of bacterial C mg of detritus-1 day-1. The three plant species differed with respect to the amount of bacterial productivity they supported per milligram of detritus, in accordance with the predicted biodegradability of the plant material based on initial nitrogen content, lignin content, and C/N ratio. Bacterial production also varied throughout the 22 weeks of in situ decomposition and was positively related to the nitrogen content and lignin content of the remaining detritus, as well as to the temperature of the lake water. Over time, production was negatively related to the C/N ratio and cellulose content of the degrading plant material. Bacterial production on degrading plant material was also calculated on the basis of plant surface area and ranged from 0.17 to 1.98 micrograms of bacterial C cm-2 day-1. Surface area-based calculations did not correlate well with either initial plant composition or changing composition of the remaining detritus during decomposition. The rate of bacterial detritus degradation, calculated from measured production of surface-attached bacteria, was much lower than the actual rate of weight loss of plant material. This discrepancy may be attributable to the importance of nonbacterial organisms in the degradation and loss of plant material from litterbags or to the microbially mediated solubilization of particulate material prior to bacterial utilization, or both.

Formation and bacterial utilization of dissolved organic carbon derived from detrital lignocellulose

In various wetland ecosystems, lignocellulose from aquatic vascular plants is the most abundant reservoir of organic material. During microbial degradation of lignocellulose, soluble decomposition products are released into the environment. The importance of lignocellulose‐derived dissolved organic carbon (LC‐DOC) to ecological and geochemical processes depends on the rate of formation of these compounds and their subsequent fates. In laboratory microcosms, rates of formation of LC‐DOC from the salt marsh plant Spartina alterniflora varied from 0.7 to 1.0% of particulate lignocellulose per day and decreased as decomposition progressed. Rates of bacterial utilization of LC‐DOC for pools formed at 1, 2, 7, and 13 weeks into decomposition indicated that soluble intermediates produced throughout the first 3 months of lignocellulose degradation, although gradually decreasing in amount, were of similar composition. Bacterial mineralization of each pool was initially very rapid (12% in 16 h), indicating the presence of labile compounds in LC‐DOC. Rates slowed after the first day, however, resulting in mineralization of 30% of the original pool after 30 d. LC‐DOC formation and its subsequent bacterial utilization were also documented for the dominant vascular plant in two other aquatic ecosystems—a freshwater swamp and a mangrove swamp. Up to 40% of total lignocellulose mineralization in these marine and freshwater systems can be traced to use of LC‐DOC by both free‐living and attached bacteria. However, some components of LC‐DOC, presumably those compounds recalcitrant to microbial action, accumulated during lignocellulose decomposition. This unused LC‐DOC may play a role in the formation of humic substances in wetland environments or other aquatic ecosystems with substantial inputs of vascular plant detritus.

Kinetics of microbial degradation of vascular plant material in two wetland ecosystems.

Vascular plant decomposition was followed during two different years in one freshwater and one marine wetland in southeastern Georgia, USA, using a modified litterbag technique. Chemical analysis of plant material revealed different rates of decomposition for different components of the plant material (soluble components, α-cellulose, hemicellulose, and lignin) and, further, that rates of decomposition of each component changed over time, such that the specific rate of decay for each fraction decreased as decomposition proceeded. Three mathematical models which differen in their treatment of the biochemical heterogeneity of vascular plant detritus were investigated with regard to their relative abilities to describe decomposition kinetics from the field incubations as well as from laboratory microcosm studies with radiolabeled plant material. A decaying coefficient model, which treats plant detritus as a single component but allows for a decreasing specific decomposition rate as material ages, was most successful in describing kinetics of decomposition. This model accomodates the changes in quality of vascular plant detritus resulting from preferential decomposition of more labile components (e.g., non-lignocellulosic material and holocellulose) and the relative accumulation of more refractory components (e.g., lignin) observed with time. The model also accomodates the potential transformation of various plant components into more refractory compounds (humification) during the decomposition process.

Production and use of detritus in various freshwater, estuarine, and coastal marine ecosystems

In both freshwater and marine habitats, vascular marine plants arc little used by animals that graze directly on them, because they have a relatively high content of indigestible fiber and a low content of nitrogen. The chief emphasis of detritus research in the 1970s was to show how microorganisms progressively reduce the content of fiber and increase the content of nitrogren in vascular plant detritus, rendering it nutritious for animals. Algal (seaweed, diatom, etc.) detritus starts with a lower fiber content and a higher nitrogen content. Many animals can use it directly, and a very short period of microbial colonization renders it highly nutritious. As a result, a high proportion of the algal carbon originally produced passes into animals via detrital food webs, while a low proportion of vascular plant carbon does so. Much more of the latter simply supports microbial respiration.In the 1980s it was shown, particularly for freshwater habitats, that the dissolved organic matter (DOM) released by plants while living or in the early stages of decomposition readily precipitates on surfaces and forms amorphous particulate matter with a low content of refractory material. These particles are highly nutritious for animals and are used directly by freshwater fish such as Sarotherodon (= Tilapia), which is commercially important, especially in Africa and South America. It is suggested that the DOM pathway may be ecologically more significant than the POM (particulate organic matter) pathway and that processes analogous to those shown for lakes and rivers probably occur in estuarine and coastal waters.There is much circumstantial evidence to suggest that planktonic food webs based on DOM are much more important than previously thought. The conversion of DOM to POM through the “microbial loop” and its utilization in higher trophic levels is an urgent topic for further study.

Dependence of Fishery Species on Salt Marshes: The Role of Food and Refuge

Salt marshes are widely believed to serve as nurseries for many fishes and crustaceans of fishery value as a result of the high production of vascular plant detritus and the protection from predation offered by shallow, spatially complex habitats. Comparisons of the yields of species which reside in salt marsh habitats during critical life history stages (such as penaeid shrimp) with the area of such habitats and their greater densities in flooded marshes and associated tidal creeks support the premise that marshes enhance the yield of such species. A range of evidence, including the amount of detrital export from marshes, the poor nutritive value of vascular plant detritus, and natural diets, casts doubt on the notion that production of fishery species is based on the direct consumption of marsh grass detritus or predominantly on food chains based on this detritus. Vascular plants and associated algae may, however, contribute to the production of prey species. The limited observations available support the hypothesis that salt marshes offer significant escape from mortality due to predation, but there have been yet few experimental tests of this hypothesis. Knowledge of relative importance of the food and refuge functions in support of living resources is of practical value in marsh and fisheries management. Better understanding of these roles is important to the effective evaluation of the effects of coastal habitat modifications on fisheries resources and design of alterations to minimize the losses of these values.

Organic nitrogen and caloric content of detritus III. Effect on growth of a deposit-feeding polychaete, Capitella capitata

Laboratory experiments showed that individual growth rate of the deposit-feeding polychaete, Capitella capitata (type 1) can be predicted by rate of organic nitrogen supply (ration) of detritus to the animals. The rate at which detritus is available to a deposit-feeder, whether by sedimentation to the bottom, or decomposition of otherwise undigestable components via microbial activity, or production rate of microbes themselves, is important and not just the concentrations of limiting substrate of different detritus sources. Even so, the portion of total caloric content of the detritus that can be utilized by a deposit-feeder—approximated by the ‘available’ caloric content—also contributes to limiting growth. However, available caloric content usually is low when nitrogen content is low, as in the case of vascular plant detritus. Trophic transfer efficiency (net production/food supplied) is a measurement of food chain transfer that is usually calculated in carbon or caloric units. If one is interested in comparing energy or carbon flow in food chains, then use of these parameters is appropriate; if one is interested in comparing the efficiencies of different species, then calculations must be made using nutritional factors limiting growth.