The modified niche model: Including detritus in simple structural food web models

Food webs are constructed as structural directed graphs that describe “who eats whom,” but it is common to interpret them as energy flow diagrams where predation represents an energy transfer from the prey to the predator. It is the aim of this work to demonstrate that food webs are incomplete as energy flow diagrams if they ignore passive flows to detritus (dead organic material). While many ecologists do include detritus in conceptual and mathematical models, the detrital omission is still commonly found. Often detritus is either ignored or treated as an unlimited energy source, yet all organisms contribute to the detritus pool, which can be an energy source for other species in the system. This feedback loop is of high importance, since it increases the number of pathways available for energy flows, revealing the significance of indirect effects, and making the functional role of the top predators less clear. In this work we propose the modified niche model by adding a detritus compartment to the niche model. We demonstrate the effect of structural loops that result from feeding on detritus, by comparing empirical data sets to five different assembly models: (1) cascade, (2) constant connectance, (3) niche, (4) modified niche (original in this work), and (5) cyber-ecosystem. Of these models, only the last two explicitly include detritus. We show that when passive flows to detritus are included in the food web structure, the structure becomes more robust to the removal of individual nodes or connections. In addition, we show that food web models that include the detritus feedback loop perform better with respect to several structural network metrics.     

Mineralization of particulate organic matter derived from coral-reef organisms in reef sediments of the Gulf of Aqaba

In situ and laboratory incubation experiments in a fringing reef in the Gulf of Aqaba were performed to study degradation rates of particulate organic matter in reef sediments. Coral mucus, clam eggs, and zooxanthellae were used as model particulate organic compounds for these experiments. Aerobic and anaerobic mineralization rates were calculated by dissolved inorganic carbon (DIC) and O₂ fluxes from the sediments under different particulate organic matter additions. Fast enhancement (approximately twofold) of O₂ and DIC fluxes were found with the addition of coral mucus and clam eggs compared with control incubations without addition. Most of the degradation is believed to have occurred anaerobically rather than aerobically (DIC:O₂ ratios were 4.3–28.1). Higher degradation rates of coral mucus and clam eggs were estimated in carbonate sediment than in silicate sediment (1.2–1.6-fold), which was attributed to the different physical and chemical properties of both sediments. Our study shows the significance of the reef sediment as a suitable site for microbial degradation of particulate organic material excreted from different reef community organisms. This may increase the regeneration of nutrients in the reef environment necessary to sustain high biological productivity.     

Mineralization of particulate organic matter derived from coral-reef organisms in reef sediments of the Gulf of Aqaba

In situ and laboratory incubation experiments in a fringing reef in the Gulf of Aqaba were performed to study degradation rates of particulate organic matter in reef sediments. Coral mucus, clam eggs, and zooxanthellae were used as model particulate organic compounds for these experiments. Aerobic and anaerobic mineralization rates were calculated by dissolved inorganic carbon (DIC) and O₂ fluxes from the sediments under different particulate organic matter additions. Fast enhancement (approximately twofold) of O₂ and DIC fluxes were found with the addition of coral mucus and clam eggs compared with control incubations without addition. Most of the degradation is believed to have occurred anaerobically rather than aerobically (DIC:O₂ ratios were 4.3-28.1). Higher degradation rates of coral mucus and clam eggs were estimated in carbonate sediment than in silicate sediment (1.2-1.6-fold), which was attributed to the different physical and chemical properties of both sediments. Our study shows the significance of the reef sediment as a suitable site for microbial degradation of particulate organic material excreted from different reef community organisms. This may increase the regeneration of nutrients in the reef environment necessary to sustain high biological productivity.     

Response of the microbial food web to manipulation of nutrients and grazers in the oligotrophic Gulf of Aqaba and northern Red Sea

The control mechanisms within the pelagic microbial food web of the oligotrophic Gulf of Aqaba and the northern Red Sea were investigated in the spring of 1999. Nutrient conditions and potential grazer impact were manipulated in a series of dilution experiments. Ambient nutrient concentrations and autotrophic biomass were very low (0.23–1.21 µmol NO₃ l^–1, 0.06–0.98 µmol NH₄ l^–1, 1.08–1.17 µmol Si l^–1, 0.08–0.12 µmol P l^–1, 0.15–0.36 µg chlorophyll a l^–1). The planktonic community was characterized by low abundances [3.0–5.5×10⁵ heterotrophic bacteria ml^–1, 0.58–7.2×10³ ultraphytoplankton <8 µm ml^–1 (small eukaryotic photoautotrophs and Prochlorococcus sp., excluding Synechococcus sp.), 0.45–4.4×10⁴ Synechococcus sp. ml^–1, 0.32–1.2×10³ heterotrophic nanoflagellates ml^–1, 1.3–3.8×10³ phytoplankton >8 µm l^–1, 0.93–5.4×10² microzooplankton l^–1] and dominated by small forms (0.2–8 µm). Dinoflagellates and oligotrichous ciliates were the most common groups in initial samples among the phytoplankton >8 µm and microzooplankton, respectively. Results show that bottom-up and top-down control mechanisms operated simultaneously. Small organisms were vulnerable to grazing, with maximum grazing rates of 1.1 day^–1 on heterotrophic bacteria and 1.3 day^–1 on ultraphytoplankton. In contrast, algae >8 µm showed stronger signs of nutrient limitation, especially when the final assemblages were dominated by diatoms. Synechococcus sp. were not grazed and only showed moderate to no response to nutrient additions. The high spatial and temporal variation of our results indicates that the composition of the planktonic community determines the prevailing control mechanisms. It further implies that, at this transitional time of the year (onset of summer stratification), the populations fluctuate about an equilibrium between growth and grazing.Communicated by O. Kinne, Oldendorf/Luhe     

Sedimentation and benthic mineralization of organic detritus in a Norwegian fjord

The supply of particulate material to the sea-bed as well as the oxygen consumption and the redox potential of the sea-bed were measured during a one-year period (1979/1980) at 60 and 90 m depth in the inner part of a west Norwegian fjord, Fanafjorden. At both sites, uniform sedimentation rates of total particulate material (825 and 885 g m^-2 yr^-1, respectively) and particulate inorganic material (576 and 616 g m^-2 yr^-1, respectively) were found. The sedimentation rates of particulate organic carbon (96 and 107 g m^-2 yr^-1, respectively) and particulate organic nitrogen (10 and 12 g m^-2 yr^-1, respectively) were low in winter, higher in summer and autumn, with maxima in May/June, reflecting similar maxima in the phytoplankton biomass in the area, with 6 to 8 wk delay. The oxygen consumption of the sea-floor was lowest in winter/spring and highest in summer. Thirtytwo and 38 g C m^-2 yr^-1 (respiration quotient=0.85) were metabolized by the sediment at 60 and 90 m, respectively. The simultaneous measurements of sedimentation rates and sediment oxygen uptake throughout a whole year demonstrated that the benthic mineralization is governed by the sedimentation over a longer time-scale, but that seasonal imbalances do occur. A box-model of the flux of particulate organic carbon to the sediment surface is presented, and includes the relevant processes and some quantitative estimates.     

Interspecific territoriality and competition for food between the reef fishes Forsterygion varium and Pseudolabrus celidotus

Long-term behavioural studies were made of the blennioid fish Forsterygion varium and the wrasse Pseudolabrus celidotus in the Cape Rodney-Okakari Point Marine Reserve in New Zealand. Both male and female F. varium defended small 1 to 2 m² territories from the wrasse all year round, althoughl aggressive encounters were most frequent from December to April. Only Large P. celidotus (≧180 mm standard length) were observed to dominate the blenny in interactions. Defence of nests, which were found only on the territories of males between May and October, did not explain the distribution of F. varium aggression between the sexes and seasons. An extensive overlap in the categories and size of prey eaten suggested that protection of food was a major function of F. varium territoriality. F. varium was most frequently aggressive toward P. celidotus of an intermediate size (110 to 160 mm SL) and exhibited greatest overlap in diet with wrasses in this size lange. This dnected the study toward examining the effect of F. varium on the feeding activity of P. celidotus. Within a shallow, Leterogeneous reef habitat, feeding pressure by P. celidotus (bites/are/time) was inversely related to the density of F. varium. Removal of F. varium from small 5x5 m quadrats resulted in large increases in wrasse feeding pressure, suggesting that the blenny influences the foraging of wrasses within their home ranges. When F. varium were removed from the entire home ranges of five 1+yr (110 to 130 mm SL) P. celidotus, these fish exhibited relative increases in both feeding rate and foraging time. The results suggest that P. celidotus grow through a period during which individuals compete with F. varium for food, adding support to the contention that territoriality in F. varium functions to preserve its food supply.     

A technique for collection of particulate organic matter in situ

A technique is described for concentrating and collecting particulate organic matter from water with minimal disturbance. This involves exposing in situ the surface of a Millipore^® filter monitor mounted on a simple vacuum flask assembly.     

A numerical model of wave- and current-driven nutrient uptake by coral reef communities

We developed a numerical model capable of simulating the spatial zonation of nutrient uptake in coral reef systems driven by hydrodynamic forcing (both from waves and currents). Relationships between nutrient uptake and bed stress derived from flume and field studies were added to a four-component biogeochemical model embedded within a three-dimensional (3-D) hydrodynamic ocean model coupled to a numerical wave model. The performance of the resulting coupled physical-biogeochemical model was first evaluated in an idealized one-dimensional (1-D) channel for both a pure current and a combined wave-current flow. Waves in the channel were represented by an oscillatory flow with constant amplitude and frequency. The simulated nutrient concentrations were in good agreement with the analytical solution for nutrient depletion along a uniform channel, as well as with existing observations of phosphate uptake across a real reef flat. We then applied this integrated model to investigate more complex two-dimensional (2-D) nutrient dynamics, firstly to an idealized coral reef-lagoon morphology, and secondly to a realistic section of Ningaloo Reef in Western Australia, where nutrients were advected into the domain via alongshore coastal currents. Both the idealized reef and Ningaloo Reef simulations showed similar patterns of maximum uptake rates on the shallow forereef and reef crest, and with nutrient concentration decreasing as water flowed over the reef flat. As a result of the cumulative outflow of nutrient-depleted water exiting the reef channels and then being advected down the coast by alongshore currents, both reef simulations exhibited substantial alongshore variation in nutrient concentrations. The coupled models successfully reproduced the observed spatial-variability in nitrate concentration across the Ningaloo Reef system.     

Sedimentation of organic detritus in Lochs Etive and Creran,Argyll, Scotland

The rates of sedimentation of organic detritus were measured at 3 stations in two Scottish sea lochs, Loch Etive and Loch Creran, using sedimentation jars exposed at various depths. Details are given of the seasonal and depth distribution of sedimenting material, and its composition for a 1-year period. Differences in the pattern of sedimenting material collected at different depths and in the seasonal patterns of sedimentation at the different stations suggested that, in each case there were differences in the relative importance of detritus from various sources. At all stations, phytoplankton production made a relatively small contribution to the total detritus collected, either directly as dead cells, or indirectly as the faeces of zooplankton organisms. Near the head of Loch Etive there were contributions by filamentous algae and Enteromorpha sp., but a major source of detritus was terrestrial debris, mainly carried into the loch in the waters of the River Etive. In the lower basin of Loch Etive, terrestrial detritus also contributed to the total sedimenting near the surface, but at greater depths much of the material collected in the sedimentation jars probably resulted from short-term resuspension and re-deposition of bottom material, reflecting a net transport of fine sediment from the shallower to the deeper areas of the loch. Secondarily resuspended material was also a major source of material collected in the jars exposed in Loch Creran.     

Arm regeneration in the field in Ophiocoma echinata (Echinodermata: Ophiuroidea): effects on body composition and its potential role in a reef food web

Eighteen percent and 47% of two populations of Ophiocoma echinata in the Florida Keys were undergoing regeneration. An individual would take approximately 720 days to completely regenerate three arms. Regeneration of three arms had a greater effect on gonad production in females than in males, and reduced storage material in the stomach of both. Regeneration in O. echinata was estimated to be 0.07 kJ m^-2 day^-1, which could provide the equivalent of 0.07% of primary production on a reef to higher trophic levels per day.     

Food web structure affects the extinction risk of species in ecological communities

This paper studies the effect of food web structure on the extinction risk of species. We examine 793 different six-species food web structures with different number, position and strength of trophic links and expose them to stochasticity in a model with Lotka–Volterra predator–prey dynamics. The characteristics of species (intrinsic rates of increase as well as intraspecific density dependence) are held constant, but the interactions with other species and characteristics of the food web are varied.     

Variability of particulate organic carbon and nitrogen in the Namibian upwelling system

Particulate organic carbon (POC) and nitrogen (PON) were analyzed during cruises undertaken in September 1985 and April 1986 along the Namibian upwelling system. The main objectives were to provide estimates of standing stocks of particulate organic matter (POM) and analyze its temporal and spatial variability. Average estimates of total carbon standing stock (0 to 100 m depth) accounted for 1.2 g-at C m^–2 during active and 1.32 g-at C m^–2 during abated upwelling. Zooplankton carbon was estimated as 0.22 and 0.27 g-at C m^–2, respectively, for both periods. POM was generally concentrated near the surface, especially during abated upwelling. During abated upwelling, POM was not only abundant inshore but also over the shelf, as a response of a more diffuse frontal system and a more strongly stratified water column. Cross-shelf gradients were more significant during active upwelling, while alongshore gradients accounted for most of the variance of particulate organic matter during abated periods. This result was a consequence of the seasonal intrusion of warm, Angolan water from the north during the period of minimum upwelling, and resulted in poorer POM concentrations and higher consumer: producer ratios (24.4%). Nevertheless, this last conclusion should be regarded with caution due to the lack of comparative interannual variability. A 48 h study at a fixed station permitted analysis of the daily variability in POM during the intrusion process. Changes in the thickness of the surface mixed layer due to irregular time-spaced pulses of non-homogeneous water masses resulted in sudden enrichments and renewals of phytoplankton and zooplankton populations in a matter of hours.     

Evidence supporting the importance of terrestrial carbon in a large-river food web

Algal carbon has been increasingly recognized as the primary carbon source supporting large-river food webs; however, many of the studies that support this contention have focused on lotic main channels during low-flow periods. The flow variability and habitat-heterogeneity characteristic of these systems has the potential to significantly influence food web structure and must be integrated into models of large-river webs. We used stable-isotope analysis and IsoSource software to model terrestrial and algal sources of organic carbon supporting consumer taxa in the main channel and oxbow lakes of the Brazos River, Texas, USA, during a period of frequent hydrologic connectivity between these habitat types. Standardized sampling was conducted monthly to collect production sources and consumer species used in isotopic analysis. Predictability of hydrologic connections between habitat types was based on the previous 30 years of flow data. IsoSource mixing models identified terrestrial C3 macrophytes (riparian origin) as the primary carbon source supporting virtually all consumers in the main channel and most consumers in oxbow lakes. Small-bodied consumers (<100 mm) in oxbow lakes assimilated large fractions of algal carbon whereas this pattern was not apparent in the main channel. Estimates of detritivore trophic positions based on delta15N values indicated that terrestrial material was likely assimilated via invertebrates rather than directly from detritus. High flows in the river channel influenced algal standing stock, and differences in the importance of terrestrial and algal production sources among consumers in channel vs. oxbow habitats were associated with patterns of flooding. The importance of terrestrial material contradicts the findings of recent studies of large-river food webs that have emphasized the importance of algal carbon and indicates that there can be significant spatial, temporal, and taxonomic variation in carbon sources supporting consumers in large rivers.     

Pyruvate-utilizing bacteria as potential contributors to the food web in the Arabian Gulf

Water samples from the Kuwaiti coasts of the Arabian Gulf were used for counting and isolating bacteria capable of growth on low molecular weight organic compounds known to be released by picocyanobacteria. The compounds tested were potassium acetate, sodium pyruvate, fumaric acid, succinic acid, sodium citrate and glycerol. For comparison, the bacterial numbers on glucose (a conventional carbon source) and Tween 80 and crude oil (unconventional carbon sources), as sole sources of carbon and energy were also determined. Sodium pyruvate was, in most cases, the carbon and energy source most commonly utilized by the cultivable surface water bacteria. The most common cultivable bacterial genera on the test carbon sources were Pseudoalteromonas, Vibrio, Cobetia and Roseobacter. Less common genera were Rhodococcus, Pseudomonas and Bacillus. Quantification of heterotrophic bacteria associated with cultures of local picocyanobacterial strains, originally isolated from the Gulf surface water, also revealed that the carbon source most commonly utilized by cultivable bacteria was sodium pyruvate. However, a different bacterial composition was identified, with Alcanivorax, Stappia and Marinobacter as the major heterotrophic genera. All heterotrophic bacteria that grew on sodium pyruvate could also grow on β-alanine, as sole sources of carbon and energy. From this study we suggest that the Arabian Gulf bacteria utilizing picocyanobacterial low molecular weight compounds, particularly pyruvate may potentially contribute to the food web in this aquatic system. The experiments comply with the current laws of Kuwait.     

A seasonal stable isotope survey of the food web associated to a peri-urban rocky shore

We characterized the structure and functioning of the food web associated to an intertidal rocky shore in the harbour of Brest (Western Brittany, France), through a seasonal δ¹³C and δ¹⁵N survey. The dominance of benthic-derived organic matter, and particularly resuspended sediment, in the local pool of suspended organic matter, is highlighted. This benthic-derived organic matter dominates the diet of filter-feeders, including a certain degree of selectivity displayed by various taxa belonging to this trophic group. The food web structure appeared very stable temporally, which suggested the dominance of mixed diets, rather than the consumption of pure sources. It was constituted of 4 trophic levels, including a large diversity of predators, likely favoured by the diversity of microhabitats that characterize this intertidal habitat. From our results, the trophic functioning of this peri-urban rocky shore community does not appear functionally simplified as previously reported for other anthropized and/or artificial rocky ecosystems. We suggest that the topological complexity of rocky habitats, including a diversity of microhabitats, might be responsible of the important animal diversity, in spite of the anthropogenic disturbances characterizing this peri-urban area.     

Phase-shift in coral reef communities in the Florida Keys National Marine Sanctuary (FKNMS), USA

Characterizing the Florida Keys National Marine Sanctuary (FKNMS), USA, has gained much attention over the past several decades because of apparent changes in the benthic community structure over space and time representative of patterns occurring in the Caribbean region. We used a 5-year dataset (1996–2000) of macroalgal and sponge cover and water quality measurements as predictor variables of hard coral community structure in the FKNMS. The 16 water quality variables were summarized into 4 groups by principal component analysis (PCA). Hierarchical agglomerative cluster analysis of the mean and standard deviation (SD) of the principal component scores of water quality variables separated the reef sites into two main groups (and five sub-groups), referred to as reefs of similar influence (RSI). The main groups corresponded with their geographical locations within the Florida Keys: the reefs in the Upper and Middle Keys being homogeneous and collectively, having lower water quality scores relative to reefs in the Lower Keys. Canonical correspondence analysis (CCA) between hard coral cover and key predictor variables (i.e., water quality, macroalgal cover and sponge cover) also separated the reefs in the Lower Keys from reefs in the Upper–Middle Keys, consistent with results of the cluster analysis, which categorized reefs based on RSI. These results suggest that the prevailing gradient of predictor variables may have influenced the structuring of coral reef communities at a spatial scale larger than the individual reef. Furthermore, it is conceivable that these predictor variables exerted influence for a long time rather than being a recent event. Results also revealed a pattern showing reduction in hard coral cover and species richness, and subsequent proliferation of macroalgae and sponges during the study period. Our analyses of the Florida Keys present a pattern that is consistent with the characteristics of a reef that has undergone a “phase-shift,” a phenomenon that is widely reported in the Caribbean region.     

A link between ecological diversity indices and measures of biodiversity

The practice of environmental planning and protection frequently necessitates the quantification of ecological diversity. Traditional ‘ecological diversity indices’ are based on the abundances of species present. However, such indices are insensitive to taxonomic or similar differences. With equal species abundances they measure the species richness (species number) only. Conversely, so-called ‘biodiversity indices’ are based on species differences, but are insensitive to the abundance conditions. The quadratic entropy index is the only ecological diversity index, the value of which reflects both the differences ‘and’ abundances of the species. When a species list is given without abundance data, then, using the quadratic entropy index and postulating equal abundances, one gets the only biodiversity index derived from a traditional ecological index of diversity. Its extensive form is identical with the sum of differences or distances between the species present. This index trivially satisfies set monotonicity, an important property for biodiversity indices.     

Macroinvertebrate production and food web energetics in an industrially contaminated stream.

This study examines secondary production and periphyton-invertebrate food web energetics at two sites in an industrially contaminated, nutrient-enriched stream. Secondary production data and data from the literature were used to calculate potential amounts of mercury transferred from periphyton to chironomid larvae and into terrestrial food webs with emerging adults. The nutritional quality of periphyton was characterized using energy content, chlorophyll a, protein, ash-free dry mass (AFDM), and percentage of organic matter. Chironomid larvae (Orthocladiinae: Cricotopus spp.) comprised 96% of all macroinvertebrates collected from stones at the two sites. Cricotopus production was extremely high: production was 59.5 g AFDM x m(-2) x yr(-1) at the site upstream of a 1-ha settling basin and 32.4 g AFDM x m(-2) x yr(-1) at the site below the basin. Apparent differences in annual secondary production were associated with reduced organic content (i.e., nutritional quality) of the periphyton matrix under different loading of total suspended solids. The periphyton matrix at both sites was contaminated with inorganic (Hg(II)) and methyl (MeHg) mercury. The amount of Hg(II) potentially ingested by Cricotopus was calculated to be 49 mg Hg(II) x m(-1) x yr(-1) at the upstream site and 19 mg Hg(II)x m(-2) x yr(-1) at the downstream site. Mercury ingestion by Cricotopus at the downstream site was calculated to be 2% of the estimated annual deposition of particulate-bound Hg(II) to the stream bed. Emergence of adult Cricotopus was calculated to remove 563 microg Hg(II)x m(-2) x yr(-1) from the stream at the upstream site and 117 microg Hg(II) x m(-2) x yr(-1) at the downstream site, which amounted to 4.1 g Hg(II)/yr for the 2.1-km reach of stream included in this study. The ratio of metal export in emergence production to surface area for the study stream was 10 to 10(3) times higher than ratios calculated for lakes using data from the literature. This study is the first well-documented example of extremely high aquatic insect production in an industrially contaminated, nutrient-enriched stream, and it highlights the application of production measurements to examine the role of aquatic insect production in the trophic transfer of energy and persistent contaminants in aquatic food webs and into terrestrial food webs.     

Some biologically important low molecular weight organic acids in the sediments of Loch Eil

The isolation of biologically important low molecular weight organic acids from organically enriched sediments in Loch Eil, Scotland, was carried out by extraction of pore water with acidified ethyl acetate. High concentrations of acetic acid, up to 1.8 mg g^-1 dry weight of sediment were found at Station E-24. Propionate, butyrate, valerate, lactate and traces of succinate were also found. Succinate was present in significant amounts, 42.2 g g^-1 dry weight of sediment at Station E-70, which received a higher input of organic matter than E-24. Both propionate and succinate were absent from a control station in the Lynn of Lorne where the sediment was low in carbon compared with Loch Eil. In experimental tanks, acetate levels increased as the input of organic carbon (as cellulose) was increased up to a load level of 1.5 g m^-2 day^-1. Above this, acetate decreased and succinate appeared. Succinate was not detected in low-loaded tanks. Experiments with sieved mud showed a vertical distribution of the different acids with depth. Lactate and succinate reached highest concentration in the 0 to 3 cm layer, acetate at 3 to 6 cm and propionate at 6 to 9 cm. The results are discussed in relation to the role of these acids as food sources and as indicators of biochemical pathways taking place in sediments with different carbon input levels.