Bacterial--Invertebrate Interactions in Uptake of Dissolved Organic Matter

SYNOPSIS. AS compared to integumentary uptake systems of soft-bodiedmarine invertebrates, bacterial systems, in terms of transportconstants, are much better adapted to the low concentrationsof dissolved organic nutrients encountered in coastal and offshorewaters. Bacteria respond to the presence of suitable dissolvedorganic substrates with induction, uptake and multiplication,maintaining the concentrations of dissolved organic matter (DOM)permanently low. At realistic in situ concentrations, epidermaluptake by pelagic and epibenthic animals proceeds at such lowrates that scarcely a substantial proportion of their metabolicneeds is provided by absorption. In marine sediments, wherethe life processes of bacteria and animals are closely interrelated,the macrofauna is sheltered by shells, firm tubes and burrows,which are irrigated by means of overlying water of the watercolumn. Hence, interstitial water with its sometimes higherconcentrations of DOM is scarcely available to sediment-dwellinglarger metazoans. The meiofauna mainly inhabits the few millimetersof the upper sediment layers and the thin halos surroundingirrigated macrofaunal burrows, where sufficient oxygen is available.Unless the magnitude of horizontal water movement, the amountsof diffusional nutrient supply and the percentages, by whichnutrient concentrations are reduced by meiofaunal uptake, areknown, estimations of nutritional benefits from uptake of DOMby meiofauna cannot be made. For all infaunal taxa, bacteriaappear to represent a major food supply.     

Meiofaunal prominence and benthic seasonality in a coastal marine ecosystem

Summary The muds of a shallow (7 m) site in Narragansett Bay, Rhode Island contained higher abundances of meiofauna (averaging 17×10⁶ individuals per m² and ash free dry weight of 2.9 g/m² during a 3 year period) than have been found in any other sediment. The majority of sublittoral muds, worldwide, have been reported to contain about 10⁶ individuals per m². This difference is attributed primarily to differences in sampling techniques and laboratory processing.Extremely high meiofaunal abundances may have also occurred because Narragansett Bay sediments were a foodrich environment. While the quantity of organic deposition in the bay is not unusually high for coastal waters, this input, primarily composed of diatom detritus, may contain an unusually high proportion of labile organics. Furthermore, meiofauna could have thrived because of spatial segregation of meiofauna and macrofauna. While meiofauna were concentrated at the sediment-water interface, most macrofauna were subsurface deposit feeders. Macrofaunal competition with, and ingestion of meiofauna may thus have been minimized.The seasonal cycles of meiofauna and macrofauna were similar. Highest abundances and biomass were observed in May and June and lowest values in the late summer and fall. Springtime increases of meiofaunal abundance were observed in all depth horizons, to 10 cm. We hypothesize that phytoplankton detritus accumulated in the sediment during the winter and early spring, and that the benthos responded to this store of food when temperatures rose rapidly in the late spring. By late summer, the stored detritus was exhausted and the benthos declined.     

Role of meiofauna in estuarine soft-bottom habitats*

Meiofauna are ubiquitous in estuaries worldwide averaging 10⁶ m^?2. Abundance and species composition are controlled primarily by three physical factors: sediment particle size, temperature and salinity. While meiofauna are integral parts of estuarine food webs, the evidence that they are biologically controlled is equivocal. Top down (predation) control clearly does not regulate meiofaunal assemblages. Meiofauna reproduce so rapidly and are so abundant that predators cannot significantly reduce population size. Food quantity (bottom up control) also does not appear to limit meiofaunal abundance; there is little data on the effect of food quality. In estuarine sediments meiofauna: (i) facilitate biomineralization of organic matter and enhance nutrient regeneration; (ii) serve as food for a variety of higher trophic levels; and (iii) exhibit high sensitivity to anthropogenic inputs, making them excellent sentinels of estuarine pollution. Generally mineralization of organic matter is enhanced and bacterial production stimulated in the presence of meiofauna. Tannins from mangrove detritus in northern Queensland appear to inhibit meiofaunal abundance and therefore the role of meiofauna in breakdown of the leaves. Meiofauna, particularly copepods, are known foods for a variety of predators especially juvenile fish and the meiofaunal copepods are high in the essential fatty acids required by fish. The copepod’s fatty acid composition is like that of the microphytobenthos they eat; bacterial eaters (nematodes?) do not have the essential fatty acids necessary for fish. Most contaminants in estuaries reside in sediments, and meiofauna are intimately associated with sediments over their entire life-cycle, thus they are increasingly being used as pollution sentinels. Australian estuarine meiofauna research has been concentrated in Queensland, the Hunter River estuarine system in New South Wales, and Victoria’s coastal lagoons. Studies in northern Queensland have primarily concentrated on the role of nematodes in mineralization of organic matter, whereas those from southern Queensland have concentrated on the role of meiofauna as food for fish and as bacterial grazers. The New South Wales studies have concentrated on the Hunter River estuary and on the structure and function of marine nematode communities. In Victoria, several fish have been shown to eat meiofauna. The Australian world of meiofaunal research has hardly been touched; there are innumerable opportunities for meiofaunal studies. In contaminated estuarine sediments reduced trophic coupling between meiofauna and juvenile fish is a basic ecological question of habitat suitability, but also a question with relevance to management of estuarine resources. Because meiofauna have short lifecycles, the effects of a contaminant on the entire life-history can be assessed within a relatively short time. The use of modern molecular biology techniques to assess genetic diversity of meiofauna in contaminated vs uncontaminated sediments is a promising avenue for future research. Much of the important meiofaunal functions take place in very muddy substrata; thus, it is imperative to retain mudflats in estuaries.     

On the Vertical Distribution of the Metazoan Meiofauna in Shelf Break and Upper Slope Habitats of the NE Atlantic

The metazoan meiofauna of nine stations in shelf break and upper slope areas (70 to 1500 m water depth) of the N.E. Atlantic were investigated in order to assess which environmental factors are important in the control of densities and sediment profiles. Total meiofaunal densities (ranging between 368 and 1523 ind/10 cm²) were correlated with bacterial densities, an important food source for meiofauna. However, considering sediment vertical distribution profiles, the relative importance of both food and oxygen on the meiofauna became obvious. A combination of both bacterial densities and oxygen supply could explain about 95% of the variability in the vertical profiles of the meiofauna densities. Meiofauna numbers increase in proportion to food availability in the surface sediment layers, but this relationship breaks down in deeper sediment layers where the oxygen supply is often limiting, particularly in fine sediments.     

Meiofauna sediment relations in leeward slope turf algae of Heron Island reef

As part of studies investigating the influence of grazers on reef meiofauna, we assessed the density, composition and richness of meiofauna (retained on a 100-μm sieve) on the leeward reef slope of Heron Reef, GBR, Australia using an airlift vacuum sampling device. Estimates of meiofauna densities ranged between 40 individuals 10 cm⁻² and 290 individuals 10 cm⁻², which is considerably lower than many estimates from carbonate sediments and hard substrates from other reefs and marine habitats. The 17 taxa of meiofauna were dominated by harpacticoid copepods (40%) and nematodes (32%). Varying sediment load within algal turfs explained 37% of variation of meiofauna density. A model is proposed in which increased shelter afforded by high living coral cover reduces meiofaunal losses from grazing and increases sediment loads, balanced by areas of low coral cover in which sedimentation rates are lower and grazing rates higher. At none of the four sites did major differences in abundance occur between November and March sampling events. Together these observations suggest that epilithic meiofaunal communities are generally spatially and temporally predictable at small scales in this reef system, indicating that their ecological services are similarly conservative. Handling editor: I. Nagelkerken     

Meiobenthic stocks and benthic activity on the NE-Svalbard shelf and in the Nansen Basin

Summary High Arctic meiofaunal distribution, standing stock, sediment chemistry and benthic respiratory activity (determined by sediment oxygen consumption using a shipboard technique) were studied in summer 1980 on the NE Svalbard shelf (northern Barents Sea) and along a transect into the Nansen Basin, over a depth range of 240–3920 m. Particulate sediment proteins, carbohydrates and adenylates were measured as additional measures of benthic biomass. To estimate the sedimentation potential of primary organic matter, sediment bound chloroplastic pigments (chlorophylls, pheopigments) were assayed. Pigment concentrations were found comparable to values in sediments from the boreal and temperate N-Atlantic. Meiofauna, which was abundant on the shelf, decreased in numbers and biomasses with increasing depth, as did sediment proteins, carbohydrates, adenylates and sediment oxygen consumption. Meiofaunal abundances and biomasses within the Nansen Basin were comparable with those observed in abyssal sediments of the North Atlantic. Nematodes clearly dominated in metazoan meiofauna. Protozoans were abundant in shelf sediments. Probably in response to the sedimentation of the plankton bloom, meiofauna abundance and biomass as well as sediment proteins, carbohydrates and adenylates were significantly correlated to the amount of sediment bound chloroplastic pigments, stressing the importance of food quantity to determine benthic stocks. Ninety-four percent of the variance in sediment oxygen consumption were caused by chloroplastic pigments. Benthic respiration, calculated per unit biomass, was 3–10 times lower than in the East Atlantic, suggesting low turnover rates in combination with a high standing stocks for the high Arctic benthos.     

Microbial–meiofaunal interrelationships in coastal sediments of the Red Sea

Population density and biomass of bacteria and meiofauna were investigated seasonally in the sediments of the north-western bank of Red Sea. Samples of sediments were collected seasonally from three different stations to determine microphytobenthic biomass (chlorophyll a), protein, lipid, carbohydrate, and total organic matter concentrations. These investigations revealed that microbial components tended to increase their dominancy, whereas sensitive meiofauna were extremely reduced during the entire study period. Thus a very low density of the total meiofauna (with an annual average of 109 ± 26 ind./10 cm²) was recorded whilst the benthic microbial population densities exhibited higher values (ranging from 0.31 ± 0.02 × 10⁸ to 43.67 ± 18.62 × 10⁸/g dry sediment). These changes in the relative importance analysis of benthic microbial components versus meiofaunal ones seem to be based on the impact of organic matter accumulation on the function and structure of these benthic communities. Proteins, lipids and carbohydrates showed very low concentration values, and the organic matter mostly consisted of carbohydrates, reflecting lower nutritional values for benthic fauna in general and meiofauna in particular. The distribution of microbial and meiofaunal communities seems to be dependent on the quality of the organic matter rather than on its quantity. Total organic matter concentrations varied between 5.8 and 7.6 mg/g, with organic carbon accounting for only 32% of the total organic matter. Chlorophyll a attained very low values, fluctuating between 0.11 and 0.56 μg/g, indicating the oligotrophy of the studied area. The very low concentration of chlorophyll a in the Red Sea sediment suggests that the sedimentary organic matter, heterotrophic bacteria and/or protozoa constitute an alternative resource that is consumed by meiofauna when algae are less abundant. Protozoa, therefore, represent the “missing link in bacteria–meiofauna interaction in the Red Sea marine sediment ecosystem.     

Distribution of bathyal meiofauna in the region of the Subtropical Front, Chatham Rise, south-west Pacific

Metazoan meiofauna was sampled along a temperate latitude transect (41-47°S 178°30′E) across the Chatham Rise, south-west Pacific, from water depths of 350-2600 m on three occasions (austral autumn 1997, spring 1997, summer 2000). Samples were collected using a multicorer and extracted on a 63-μm mesh. Meiofaunal density and biomass were negatively correlated with water depth, median grain-size and sediment calcium carbonate content, and positively correlated with sediment chlorophyll a. There were no significant relationships between meiofaunal abundance and biomass and measures of sediment organic matter and bacterial productivity and biomass, except in summer when bacterial and meiofaunal biomass were positively correlated. Vertical penetration of meiofauna into the sediment was related to sediment organic matter and sediment chloroplastic pigments. Variation in meiofaunal abundance at different spatial scales was examined at two sites: at 450 and 2300 m on the northern slope of the rise. At both sites, variation between subcores of a multicore (< 10-cm scale) was lower than that of multicores within a deployment (< 1-m scale). However, whilst the highest variance component at the shallow site was for multicores within a deployment, it was between multicore deployments (< 1-km scale) for the deep site. Meiofaunal density and biomass were generally higher on the southern slope of the rise than on the northern slope, a trend probably attributable to high productivity of the overlying Subtropical Front and the flux of more nutritious organic material to the sea floor.     

Meiofauna as food source for small-sized demersal fish in the southern North Sea

Meiofauna play an essential role in the diet of small and juvenile fish. However, it is less well documented which meiofaunal prey groups in the sediment are eaten by fish. Trophic relationships between five demersal fish species (solenette, goby, scaldfish, dab <20 cm and plaice <20 cm) and meiofaunal prey were investigated by means of comparing sediment samples and fish stomach contents collected seasonally between January 2009 and January 2010 in the German Bight. In all seasons, meiofauna in the sediment was numerically dominated by nematodes, whereas harpacticoids dominated in terms of occurrence and biomass. Between autumn and spring, the harpacticoid community was characterized by Pseudobradya minor and Halectinosoma canaliculatum, and in summer by Longipedia coronata. Meiofaunal prey dominated the diets of solenette and gobies in all seasons, occurred only seasonally in the diet of scaldfish and dab, and was completely absent in the diet of plaice. For all fish species (excluding plaice) and in each season, harpacticoids were the most important meiofauna prey group in terms of occurrence, abundance and biomass. High values of Ivlev’s index of selectivity for Pseudobradya spp. in winter and Longipedia spp. in summer provided evidence that predation on harpacticoids was species-selective, even though both harpacticoids co-occurred in high densities in the sediments. Most surficial feeding strategies of the studied fish species and emergent behaviours of Pseudobradya spp. and Longipedia spp. might have caused this prey selection. With increasing fish sizes, harpacticoid prey densities decreased in the fish stomachs, indicating a diet change towards larger benthic prey during the ontogeny of all fish species investigated.     

Meiofauna distribution and mesoscale variability in two sites of the Ross Sea (Antarctica) with contrasting food supply

Meiofauna abundance, biomass and community structure were investigated in two comparable deep sites of the Ross Sea (Antarctica) characterized by different trophic and sediment characteristics. Site B (567 m depth, dominated by muddy sediments) and site C (439 m depth, characterized by the presence of calcareous debris and coarse sand) were located at increasing distance from the polynyas and were subject to different inputs of organic material to the seabed. Total meiofauna abundance ranged from 192.0 to 1191.2 ind. 10 cm⁻², and total biomass varied between 9.5 and 50.3 μgC 10 cm⁻². Meiofauna densities from the Ross Sea are, on average, 2–7 times lower than those reported from other similar deep polar regions and displayed significant differences between the sites. Nematodes dominated the samples at both sites, but their relative significance changed between the sites (80% at site B and 56% at site C), followed by copepods (1.6% and 35% at sites B and C, respectively). Meiofauna composition at site B appeared similar to that reported for deep-sea antarctic or temperate sediments, whereas the composition at site C was similar to that of coastal areas. On a macroscale, the different inputs of utilizable organic material at the two sites were reflected in meiofaunal distribution patterns, indicating that meiofaunal communities from the Ross Sea are dependent on particulate organic matter fluxes from the photic layer and are coupled to pelagic phenomena. Very low microscale variations (i.e. between replicates) in meiofauna density contrasted with large mesoscale variability, which was related to the concentration of the main food indicators (phytopigments, proteins, carbohydrates and lipids). Accepted: 18 February 1999     

2008年浒苔大暴发末期黄海小型底栖动物现存量及空间分布

在2008年夏季浒苔暴发末期,对黄海33个站位(其中冷水团中22个,浒苔暴发区域9个)进行了小型底栖动物现存量、分布及沉积环境研究.结果表明:在浒苔灾害严重的南黄海区域,沉积物中的粉砂-粘土含量较往年(2007年)有明显增加;沉积物中叶绿素a和脱镁叶绿素a含量在大多区域较2007年变化不大,但在苏南至长江口近岸海域的含量有明显降低.所获的16个小型底栖动物类群的垂直分布及各类群对现存量的贡献较往年未见明显差异.小型底栖动物平均丰度为(1375±793)ind·10 cm-2,生物量为(1203±707)μg·10cm-2.黄海小型底栖动物总体现存量较2007年低约1/3,并在浒苔暴发区域下降尤为明显,而南黄海冷水团海域则略有上升,由此导致南黄海的小型底栖动物现存量分布呈现从中央冷水区向外围减小的反常现象;相较于此,在浒苔灾害影响较小的北黄海则呈从冷水区向外围增加的趋势.统计分析显示,浒苔暴发区域的小型底栖动物丰度与盐度呈正相关,与叶绿素a含量不相关.浒苔的暴发对近岸的小型底栖生物现存量产生了明显的抑制,但导致小型底栖动物现存量降低的主要因素并非缘于饵料匮乏,而更可能是浒苔暴发后的沉积与降解过程产生的影响所致.     

Influence of Dissolved Organic Matter and Invertebrates on the Function of Microbial Films in Groundwater

Microbial films play a central role in mediating energy flux in groundwater ecosystems. The activity of these microbes is likely to be influenced by the availability of resources, especially dissolved organic matter (DOM), and also by consumers, such as invertebrates that feed on microbial films. We used microcosm experiments to examine how bacterial production and extracellular enzyme activity on rocks and fine sediments from cave streams responded to amendments of DOM of varying form and to cave amphipods (Gammarus minus) that feed on microbial films. Glucose and mixtures of DOM extracted from soils and leaves stimulated bacterial production on rocks by 89–166% relative to unamended controls. In contrast, tannic acid amendment did not influence production. Microbial films on fine sediment were not consistently responsive to DOM amendment. Glucose amendment led to increased activity of enzymes associated with C acquisition, but other forms of DOM generally did not alter enzyme activity. DOM amendment led to removal of nitrate and this was correlated with bacterial production, suggesting microbes can link carbon and nitrogen cycling in groundwater as is the case in surface systems. Amphipods reduced bacterial production on rocks, but not fine sediments. The reduction caused by amphipods offset the stimulatory effect of glucose amendment, but there was no interactive effect of DOM and grazing on bacterial production or enzyme activity. Both resources and consumers play important roles in regulating microbial activity in groundwater with important implications for higher trophic levels that use microbes for food.     

Meiofaunal colonization of azoic estuarine sediment in Louisiana: Mechanisms of dispersal1

Two mechanisms of muddy-bottom meiofaunal dispersal, waterborne suspended transport and holobenthic infaunal immigration, were compared as to their rate and effectiveness in mediating community reestablishment after small-scale defaunation. Colonizing meiofauna were quantitatively sampled in winter and summer from 16 replicates of two azoic sediment chamber designs on 2 and 29 days postplacement. The chambers were ≈ 3750 cm³; one design allowed colonization via suspended movement through an open top, while the other design permitted entry only by infaunal crawling through subsurface open sides. After 48 h, mean harpacticoid copepod and naupliar densities in sediment chambers open to colonization exclusively by meiofauna in suspended transport were not significantly different from background sediment densities. Sediment chambers allowing colonization exclusively via infaunal immigration through the sediment, however, contained copepod and naupliar densities that were significantly less than densities in background sediments and suspension-colonized chambers. In contrast, nematode densities in both suspension- and infaunally colonized chambers were significantly less than in background sediments, but densities were not significantly different between the chamber treatments. Thus for a small-scale defaunation, copepods most rapidly and completely recolonize sediments via suspended transport. Nematode dispersal occurs equally well via suspended or infaunal movement; however nematodes never seemed to utilize the chambers fully because densities did not reach background levels even after 29 days.     

Benthic seasonality in an intertidal mud flat at Kerguelen Islands (Austral ocean). The relationships between meiofaunal abundance and their potential microbial food

Summary Samples were taken weekly for one year at an intertidal mudflat at the Halage des Swains, Morbihan Sound, Kerguelen Islands, for meiofauna, their suspected microbial food (bacteria and diatoms) and associated chemical and physical factors. Organic carbon and nitrogen content, bacterial abundance and biomass, pigment content and daily primary production, were significantly correlated (Spearman rank) to the temperature. Meiofauna exhibited very high abundances (up to 14 000 ind./10 cm^–2) without seasonal trend but with distinct short term oscillations of population densities. No direct correlation occurred between meiofauna (85.9% nematodes and 10.8% copepods) and temperature. Total meiofauna abundance was positively correlated to bacterial biomass in the oxidized layer, to organic content below redox potential discontinuity layer, and negatively correlated to the hourly primary production. The data suggest that nematodes are correlated to bacterial biomass and organic content in the sediment. Effect of ambient temperature on development time of nematofauna could be described by a Belehradek function. Even though some correlations existed, this study shows that peaks of meiofaunal abundance are not correlated to potential food abundance variability. Thus, the limitation of meiofauna community and its annual pattern is reasonably governed by the development time and reproductive strategy of the few co-dominant species of the main taxa.     

Food-chain relationships in subtidal silty sand marine sediments and the role of meiofauna in stimulating bacterial productivity

Summary From bibliographic data the biomass correlations (organic dry weight) are constructed for the subsurface layer of a hypothetical 30 m deep silty sand station: 200 g/ml macrofauna (including 120 g/ml subsurface deposit feeders), 50 g/ml meiofauna, 20 g/ml Foraminifera, 1 g/ml Ciliata and Flagellata, and 100 g/ml bacteria. ATP-biomass is discussed.Meiofauna and Foraminifera contribute with 30 and 12% to the living biomass in the sediment, and it is assumed that their contribution to the food of deposit-feeding macrofauna is of a similar percentage. This is corroborated by productivity estimations.Bacteria are the main food of deposit feeding macrofauna, meiofauna, and microfauna. From different calculations it becomes evident that the productivity of bacteria in the sediment is far below figures achieved in experimental cultures: the conclusion is that sediment bacteria, in general, do not live under good environmental conditions.A rather large part of the bacterial population in the sediment seems to be in the stationary phase of life, and only a fraction of the total population exhibits high metabolic rates and rapid duplications. Only these active bacteria are of importance for the breakdown of relatively refractive organic matter in the sediment.In soft bottom marine sediments where the input of organic matter is higher than the remineralization rate, benthic animals stimulate by their activities and by nutrient cycling the decomposition of detritus via bacteria. Though meiofauna, in principle, feeds upon the same food resource as macrofauna, there is no real competition for food, because meiofaunal animals by their activities and by excreting metabolic end products induce a bacterial productivity which would not be there without them, and feed on it. There are a few examples where more specialized interactions exist between benthic animals and bacteria; these interactions have been termed gardening. They could be highly important in the benthic ecosystem.     

Response of bacteria and phytoplankton to contaminated sediments from Trenton Channel,Detroit River

Several types of bioassays were used in 1986 and 1987 to investigate the effect of contaminated sediments on natural populations of bacteria and phytoplankton from the Trenton Channel, Detroit River. The approach included the measurement of uptake of ³H-glucose or ³H-adenine by bacteria and ¹⁴C-bicarbonate by phytoplankton in the presence of different amounts of Trenton Channel and Lake Michigan (control) sediments. Trenton Channel sediments are contaminated by high levels of toxic organic compounds and metals, especially zinc, lead, and copper. Because levels of biomass of bacteria and phytoplankton varied widely among the different bioassays, it was necessary to adjust uptake rates for biomass. Biomass adjustments were made using acridine orange counts for bacteria and chlorophyll measurements for phytoplankton. The results show a statistically significant suppression of uptake of substrates for both bacteria and phytoplankton with increasing amounts of sediment. Uptake was suppressed as much as 90 percent for bacteria and 93 percent for phytoplankton at 1200 mg l^-1 of Trenton Channel sediments compared to bioassays without sediment. Uncontaminated Lake Michigan sediment suppressed uptake much less than Detroit River sediment; the difference in suppression of uptake between the two types of sediment was statistically significant for both bacteria and phytoplankton.Contribution No. 518 of the Center for Great Lakes and Aquatic Sciences of the University of Michigan.     

Population fluctuation and vertical distribution of meiofauna in the Red Sea interstitial environment

The composition and distribution of the benthic meiofauna assemblages of the Egyptian coasts along the Red Sea are described in relation to abiotic variables. Sediment samples were collected seasonally from three stations chosen along the Red Sea to observe the meiofaunal community structure, its temporal distribution and vertical fluctuation in relation to environmental conditions of the Red Sea marine ecosystem. The temperature, salinity, pH, dissolved oxygen, and redox potential were measured at the time of collection. The water content of the sediments, total organic matters and chlorophyll a values were determined, and sediment samples were subjected to granulometric analysis. A total of 10 meiofauna taxa were identified, with the meiofauna being primarily represented by nematodes (on annual average from 42% to 84%), harpacticoids, polycheates and ostracodes; and the meiofauna abundances ranging from 41 to 167 ind./10 cm². The meiofaunal population density fluctuated seasonally with a peak of 192.52 ind./10 cm² during summer at station II. The vertical zonation in the distribution of meiofaunal community was significantly correlated with interstitial water, chlorophyll a and total organic matter values. The present study indicates the existence of the well diversified meiofaunal group which can serve as food for higher trophic levels in the Red Sea interstitial environment.     

Diversity of larger consumers enhances interference competition effects on smaller competitors

Competition between large and small species for the same food is common in a number of ecosystems including aquatic ones. How diversity of larger consumers affects the access of smaller competitors to a limiting resource is not well understood. We tested experimentally how species richness (0–3 spp.) of benthic deposit-feeding macrofauna changes meiofaunal ostracods’ incorporation of fresh organic matter from a stable-isotope-labeled cyanobacterial bloom, using fauna from the species-poor Baltic Sea. Presence of macrofauna mostly decreased meiofaunal incorporation of bloom material, depending on the macrofauna species present. As expected, the species identity of macrofauna influenced the incorporation of organic matter by meiofauna. Interestingly, our results show that, in addition, species richness of the macrofauna significantly reduced meiofauna incorporation of freshly settled nitrogen and carbon. With more than one macrofauna species, the reduction was always greater than expected from the single-species treatments. Field data from the Baltic Sea showed a negative correlation between macrofauna diversity and meiofaunal ostracod abundance, as expected from the experimental results. We argue that this is caused by interference competition, due to spatial niche differentiation between macrofauna species reducing the sediment volume in which ostracods can feed undisturbed by larger competitors. Interference from macrofauna significantly reduces organic matter incorporation by meiofauna, indicating that diversity of larger consumers is an important factor controlling the access of smaller competitors to a limiting food resource.     

The use of meiofauna in freshwater sediment assessments: Structural and functional responses of meiobenthic communities to metal and organics contamination

Because soft sediments are often hotspots of chemical contamination, their assessment can aid in identifying the causes of environmental stress and the implementation of measures to improve the health of the respective ecosystems. Achieving a “good ecological status” of surface waters, as required by the European Water Framework Directive, strongly depends on recognition of the chemical status of sediments. Meiobenthic organisms are important, but widely neglected components of the ecologically relevant fauna of a wide variety of ecosystems. In the present study, microcosms containing freshwater sediments were used to investigate the effects of eight different metals and polycyclic aromatic hydrocarbons (PAHs), in single and mixed applications, on natural meiofaunal assemblages. Structural (abundance and biomass) and functional (secondary production) parameters of the investigated assemblages were measured as ecologically relevant endpoints. Their sensitivity in revealing both the differential effects and the responses of meiofaunal taxa was evaluated to assess the general suitability of meiofauna and, in particular, of individual tested taxa, as bioindicators of soft sediment contamination. Structural parameters were found to be more valuable indicators than functional measurements, with more pronounced effects observed on the taxon level than on total meiofauna. Among the meiofaunal taxa considered in this study, nematodes were of particular utility as early indicators of chemical stress in freshwater soft sediments. Overall, this study provides new insights into the impact of toxicants on soft freshwater sediments and demonstrates the suitability of meiofaunal communities, especially nematodes, in assessing contamination of this ecosystem.     

Composition and distribution of meiofauna,including nematode genera,in two contrasting Arctic beaches

The meiofauna of two tidal beaches, one exposed and one more sheltered, on Bjornoya (Bear Island) was investigated in summer 2000. Both meiofaunal densities and composition seem to be controlled by physical properties of the sediment, which in turn are controlled by exposure. The moderately and poorly sorted sediments in the sheltered beach were more abundant in terms of meiofaunal densities than the well sorted sediments in the exposed beach (254–481 individuals in 10 cm² vs 7–269 individuals in 10 cm², respectively). In total, seven higher meiofaunal taxa were found. Turbellaria were the numerically dominant taxon in the exposed beach. In the sheltered beach, Turbellaria also dominated, followed by Nematoda and Harpacticoida. The vertical distribution of the meiofauna was in accordance with what has been reported from other intertidal beaches. Nematoda were studied in detail and their densities ranged over 0.7–7.7 individuals in 10 cm² in the exposed beach and 2.7–186.0 individuals in 10 cm² in the sheltered beach. Nematodes were identified to genus level and a total of eight nematode genera were found. Sediment community respiration, measured as oxygen consumption, ranged between 2.3 cm³ O₂ m^–2 h^–1 in the exposed beach and 7.3 cm³ O₂ m^–2 h^–1 in the sheltered beach (respectively, the equivalent of 24 mg and 75 mg of organic carbon metabolised per day). Values from the sheltered site are within the range of results registered in much warmer localities.