Detrital Aggregates Research Articles

Functional responses of aggregate‐colonizing copepods

AbstractZooplankton consumption of aggregates, such as marine snow, is an important factor in determining the efficiency of the biological carbon pump. However, the feeding rates of aggregate‐associated small harpacticoid and poecilostomatoid copepods are largely unknown, as are the factors that influence these rates. We measured the functional responses of pellet production (PP) of aggregate‐feeding copepods on appendicularian houses, algal and detrital aggregates, and on Trichodesmium spp. tufts. The PP rates of all copepods increased with increasing aggregate concentrations, but the response varied depending on the aggregate type. The pelagic harpacticoid copepod Microsetella norvegica had the highest PP rates on algal aggregates, while the poecilostomatoid copepod Oncaea spp. and the benthic harpacticoid copepod Amonardia normanni PP rates were highest on appendicularian houses. The ingestion rates of M. norvegica and Oncaea spp. were typically 0.04–0.13 μg C ind.−1 d−1, with the exception of ca. three times higher rates of Oncaea spp. on appendicularian houses and ca. 10 times higher rates of M. norvegica on algal aggregates. The ingestion rates of the larger species, A. normanni, were generally higher, 0.3–1.3 μg C ind.−1 d−1 on algal aggregates, and even ca. 10‐fold higher on appendicularian houses. Our results suggest that the aggregate degradation rates by copepods can vary many‐fold depending on the quality (origin) of the aggregate and the copepod species. This can have large consequences for the attenuation of vertical carbon flux.

Model study of organic carbon attenuation and oxygen mass transfer in persistent aggregate layers in the deep sea

A time series study from 1989 to 2017 indicates that, increasingly, carbon export to abyssal sediments in the California Current Ecosystem (CCE), 220 km west of the central California coast (Sta. M), occurs as rapidly sinking pulses of particulate organic carbon (POC). Nearly continuous sediment trap collections confirm that POC export to 3400 m increased significantly after 2011 with carbon attenuation (as fraction of POC remineralized) ranging from a low of 57% to a high of 77% during periods of pulsed flux. All of the major pulse events for the period resulted in the delivery of detrital aggregates that covered part or all of the sediment surface as an organic carbon rich layer at ~4000 m depth. However, the magnitude of the measured Sediment Community Oxygen Consumption (SCOC) did not increase proportionally to the organic carbon inventory change. Here, a model of oxygen consumption, informed by the time series data at Sta. M in the CCE, suggests that aggregate flocs constitute a major source for benthic carbon and a barrier to mass transport of oxygen, leading to reduced carbon attenuation in surface sediments. The correlation of POC delivery with aggregate coverage shows that the majority of POC delivery during pulse events is likely to consume all of the oxygen within the aggregates. Results indicate that an increasing fraction of POC reaches the abyssal sediments at Sta. M under conditions that support increased net burial of carbon.This finding is significant in the context of the fraction of the total POC flux that occurs as pulse events in this important upwelling region. During the period from 2011 to 2017, a third of the total POC flux resulted in aggregate flocs that covered a significant portion of the sediment surface (>47%). Following the 2015–2016 El Nino period the flocs became a dominant feature of the sediment surface. More than 80% of the POC flux to near bottom sediment traps in 2017 occurred coincident with aggregate floc cover of more than 90% of the sediments. The resultant physical barrier to oxygen transport and relatively low consumption of newly delivered carbon indicate conditions that are more typical of environments having low oxygen bottom waters.

Marine particles in the Gulf of Alaska shelf system: Spatial patterns and size distributions from in situ optics

The Gulf of Alaska is a biologically productive ocean region surrounded by coastal mountains with high seasonal runoff from rivers and glaciers. In this complex environment, we measured the concentrations and size distributions of 2.5µm–27mm marine particles using the Laser in situ Scattering and Transmissometry device (LISST-DEEP) and the Underwater Vision Profiler 5 (UVP) during summer 2015. We analyzed the spatial distribution of particles across a wide range of size classes to determine the probable drivers. Spatially, total particle concentrations surpassed 1000µl/l nearshore in the northeasternmost entrances and in the outflow of Cook Inlet, as well as offshore past the shelf break. These dual maxima suggest high lithogenic inputs nearshore and high biological production at and beyond the shelf break. Most large particles (> 0.5mm) imaged by the UVP were detrital aggregates. In nearshore surface waters near river inputs, size distributions revealed small size classes (< 100µm) to be most influential. At the shelf break, size distributions revealed a dual peak in both small (< 100µm) and very large (> 2mm) size classes. This study highlights the importance of lithogenic inputs from a mountainous margin to the coastal ocean and their potential to enhance sinking of biological material produced at the shelf break.

Clarifying functional roles: algal removal by the surgeonfishes Ctenochaetus striatus and Acanthurus nigrofuscus

The lined bristletooth, Ctenochaetus striatus, and the brown surgeonfish, Acanthurus nigrofuscus, are among the most abundant surgeonfishes on Indo-Pacific coral reefs. Yet, the functional role of these species has been the focus of an ongoing debate lasting at least six decades. Specifically, to what extent are C. striatus herbivorous like the visually similar A. nigrofuscus? To address this question, we used natural feeding surfaces, covered with late successional stage reef-grown algal turfs, to examine turf algal removal by the two species. Surfaces exposed to C. striatus in laboratory experiments exhibited no significant reductions in turf length or area covered by turfing algae. In marked contrast, A. nigrofuscus reduced turf length by 51% and area covered by turfing algae by 15% in 1 h. The gut contents of specimens from the reef revealed that A. nigrofuscus predominantly ingests algae (the dominant item in 79.6–94.7% of gut content quadrats), while C. striatus ingests detritus and sediments (dominant in 99.6–100% of quadrats). Therefore, C. striatus ingests detritus and sediment, leaving mature algal turfs relatively intact, while A. nigrofuscus directly removes and ingests turf algae. The function of C. striatus differs from cropping herbivorous surgeonfishes such as A. nigrofuscus. On coral reefs, C. striatus brush detrital aggregates from algal turfs, removing microorganisms, organic detritus and inorganic sediment. Confusion over the functional role of C. striatus may stem from an inability to fit it into a single functional category.

The Likelihood of Extinction of Iconic and Dominant Herbivores and Detritivores of Coral Reefs: The Parrotfishes and Surgeonfishes

Parrotfishes and surgeonfishes perform important functional roles in the dynamics of coral reef systems. This is a consequence of their varied feeding behaviors ranging from targeted consumption of living plant material (primarily surgeonfishes) to feeding on detrital aggregates that are either scraped from the reef surface or excavated from the deeper reef substratum (primarily parrotfishes). Increased fishing pressure and widespread habitat destruction have led to population declines for several species of these two groups. Species-specific data on global distribution, population status, life history characteristics, and major threats were compiled for each of the 179 known species of parrotfishes and surgeonfishes to determine the likelihood of extinction of each species under the Categories and Criteria of the IUCN Red List of Threatened Species. Due in part to the extensive distributions of most species and the life history traits exhibited in these two families, only three (1.7%) of the species are listed at an elevated risk of global extinction. The majority of the parrotfishes and surgeonfishes (86%) are listed as Least Concern, 10% are listed as Data Deficient and 1% are listed as Near Threatened. The risk of localized extinction, however, is higher in some areas, particularly in the Coral Triangle region. The relatively low proportion of species globally listed in threatened Categories is highly encouraging, and some conservation successes are attributed to concentrated conservation efforts. However, with the growing realization of man's profound impact on the planet, conservation actions such as improved marine reserve networks, more stringent fishing regulations, and continued monitoring of the population status at the species and community levels are imperative for the prevention of species loss in these groups of important and iconic coral reef fishes.

Carbon export associated with free-drifting icebergs in the Southern Ocean

Enrichment of the pelagic ecosystem associated with the proliferation of free-drifting icebergs prompts questions about increased productivity and the export flux of organic carbon to the deep ocean with continued climate warming. Lagrangian Sediment Traps (LST) were deployed autonomously beneath a large tabular, free-drifting iceberg (C-18a) in the NW Weddell Sea during March and April 2009 to collect sinking particles at a depth of 600 m. Three LST deployments associated with C-18a, within a 30-km radius, collected sinking diatom frustules, dominated by Corethron pennatum and Fragilariopsis nana, euphausiid fragments, crustacean and fish fecal material, detrital aggregates and mineral grains. One LST deployment at a “control” site 74 km away in open water devoid of icebergs collected diatom frustules, euphausiid molts, crustacean fecal material and detrital aggregates. Phytoplankton abundance, microbial abundance and biomass were significantly higher in the LST samples than in open-water collections at 500 m depth. The mean mass flux and organic carbon flux associated with iceberg C-18a were twice as high, 124 mg m −2 d −1 and 5.6 mg C org m −2 d −1, respectively, than at the control site. A similar trend was observed in C org/ 234Th activity, being highest near C-18a and lowest at the control site. Extrapolation of the area of enrichment to 30 km radius around C-18a, 2826 km 2, produces an estimated mass flux of 350 tons d −1 and carbon flux of 15.8 tons C org d −1. Five similar sized icebergs to C-18a were identified in satellite images in a surrounding 47,636 km 2 area at the same time of sampling. Assuming a 30-km radius as the area of influence around each of these five icebergs, 46% of the total area would be enhanced with an export flux at 600 m of 122.4 tons C org d −1. The large numbers of smaller icebergs identified visually in this area would only increase this area of influence. Icebergs serve as areas of local enrichment and with increased proliferation, must be considered in the cycling of carbon in the Southern Ocean.

Autotrophic picoplankton in mesozooplankton guts: evidence of aggregate feeding in the mesopelagic zone and export of small phytoplankton

Zooplankton play a key role in affecting the efficiency by which organic matter is exported to depth. Mesozooplankton consumption of detrital aggregates has been hypothesized as a mechanism for enhancing the export of picoplankton from surface layers. We analyzed the gut con- tents of mesopelagic copepods and ostracods using light and epifluorescence microscopy to deter- mine if cyanobacteria and eukaryotic phytoplankton too small to be ingested individually were pre- sent. Hind-guts were dissected from multiple species collected in discrete depth intervals between 0 and 1000 m during the day and night, at contrasting sites in the subtropical (Hawaii Ocean Time- series site ALOHA) and subarctic (Japanese time-series site K2) Pacific Ocean. Autofluorescing cyanobacteria and small eukaryotic phytoplankton were found in the guts of nearly all species sam- pled from all depths, indicating consumption of aggregates. Some of the cyanobacteria and other small cells ingested may have originated from inside the guts, or as symbionts, of microzooplankton, which were also common in the guts of many of these species. At both sites, most species' guts con- tained higher concentrations of cyanobacteria and small phytoplankton at night than during the day. Ostracod guts at ALOHA contained higher densities of picoplankton than those at K2, reflecting the predominance of smaller cells at ALOHA. Guts of diel vertical migrators still contained picoplankton at their deep, daytime residence depths, indicating active export of these cells. Our results indicate mesozooplankton grazing on aggregates is a pathway by which flux of picoplankton can be enhanced.

Importance of diatoms for Oithona in Antarctic waters

Fatty acid biomarker analysis of the Cyclopoid copepods Oithona similis and Oithona frigida sampled from the Southern Ocean during the austral summer has indicated differences in diet between the two species. Oithona similis contained higher proportions of protozoan and bacterial fatty acids markers, indicative of microbial foodwebs involved in the recycling of detrital material and faecal pellets. In contrast, the fatty acid composition of O. frigida was characterized by a strong diatom signature. Despite these species-specific differences, the fatty acid biomarker composition of both species at each of the stations sampled primarily reflected the species composition of the microplankton in their environment. Comparison of an index of fatty acid composition with nutritional condition indicated that those CV and female O. similis containing the highest levels of diatom biomarkers fatty acids were in the best condition. These findings suggest that diatoms are more important for Oithona spp. in the Southern Ocean than for other oceanic regions and are consistent view that O. similis is an integral component of food webs associated with the recycling of detrital aggregates and faecal material.

Estimating ecosystem function: contrasting roles of closely related herbivorous rabbitfishes (Siganidae) on coral reefs

We explored the role of behaviour and trophic ecology in driving differences in ecosys- tem function between 2 closely related species. We examined the relationships between diet, feeding rate, alimentary tract structure and patterns of digestion for 2 reef herbivores commonly found on the Great Barrier Reef, Siganus doliatus and S. lineatus. Despite their similar morphology, the 2 species exhibited distinct feeding behaviours and significantly different feeding rates, diets and movements of digesta through the alimentary tract. S. doliatus displayed a typical herbivore diurnal feeding pattern, taking an average 9.7 bites min -1 over the main part of the feeding day and with a diet dom- inated by red thallate algae (primarily Laurencia spp., Eucheuma sp., Halymenia sp. and Graci- laria sp.) and red and green filamentous algae. S. lineatus was not observed taking a single bite from the reef substratum in >100 h of underwater observations. The stomach contents of S. lineatus were dominated by amorphous organic matter (detritus). Gut passage rates suggest that S. lineatus is feed- ing nocturnally or during crepuscular periods. We suggest that these 2 species have distinct func- tional roles, with S. doliatus being a grazer of reef turf algae and S. lineatus primarily a grazer of off- reef detrital aggregates. This versatility of ecosystem function in closely related species provides further evidence that functional roles do not necessarily divide along taxonomic lines. The results highlight the importance of validating ecosystem function on a species-by-species basis.

Tracing abyssal food supply back to upper‐ocean processes over a 17‐year time series in the northeast Pacific

Detrital aggregates episodically deposited on the seafloor represent an underestimated food source to deep‐sea communities. A 17‐yr time‐series study was conducted from 1990 to 2006 in the abyssal northeast Pacific (Sta. M, 4100 m in depth) to evaluate the importance of this food source and its temporal relationship to water column and surface ocean processes. Detrital aggregates appeared on the seafloor from June through December, with the highest peaks in 1990, 1994, 2001, and 2002 reaching a maximum density of 23 m−2 in fall 2001. A total of 15,816 aggregates were measured, most less than 20 cm2 in area and with a mode of 9 cm2. Density of detrital aggregates was highly correlated with particulate organic carbon (POC) flux at 600 and 50 m above the bottom (p &lt; 0.001) with no time lag. Export flux of organic carbon from the euphotic zone was significantly correlated with aggregate density, lagged earlier by 1‐4 months (p ≤ 0.001). Zooplankton displacement volume was significantly correlated with POC flux (p = 0.023) and with detrital aggregate density (p = 0.028) on the seafloor when lagged earlier by ≤1 month. The Bakun upwelling index computed for the region around Sta. M was significantly correlated with detrital aggregate density when lagged earlier by 2‐5 months (p &lt; 0.001). A strong correlation exists between surface ocean processes and abyssal food supply, including POC flux and detrital aggregates. This direct coupling through the entire water column must be considered in resolving the marine carbon cycle.

Organic and siliceous protistan scales in north-east Atlantic abyssal sediments

We report the occurrence of a high diversity of minute (∼1 μm diameter) organic and siliceous protistan scales in small samples (total volume ∼35 μl) of superficial sediment from the Porcupine Abyssal Plain (PAP), north-east Atlantic (4850 m water depth). Many exhibit characters by which they can be identified to species. The organic scales belong to the haptophyte genera Chrysochromulina (8–9 species), Chrysocampanula and Dolichomastix (1 species each). The siliceous scales belong to the chrysophytes Paraphysomonas vestita and Meringosphaera sp. and to the heterotrophic flagellate genus Thaumatomastix (T. dybsoeana, T. formosa, Thaumatomastix sp.). As far as we are aware, this is the first time that non-calcareous protistan scales have been observed in deep-sea sediments (although siliceous skeletal plates and cysts are reported). All scales probably originated from the upper water column and were delivered to the deep-sea floor on rapidly sinking detrital aggregates. However, naked heterotrophic flagellates are known to thrive in abyssal sediment habitats and so the possibility that some scale-bearing protists also live in benthic deep-sea environments cannot be eliminated. Many species identified at the PAP site are common in coastal marine waters around Europe; some occur as far afield as Tasmania and New Zealand. Five Chrysochromulina species are known from central oceanic areas, including parts of the North Atlantic, while another species, C. pringsheimii, is reported from a British freshwater lake. We retrieved ∼15% of the 55 named Chrysochromulina species (∼8% of the estimated total number of species in this diverse group) in the ∼35 μl of abyssal sediment. Because the scales can persist and be identified after cell death, they may provide useful time- and space-averaged information about the distribution of protist species in marine habitats. The long-term fate of the scales on the sea-floor is unknown. It is possible that at least some of the organic scales are preserved as microfossils in deep-sea sediments.

Turbulence and feeding behaviour affect the vertical distributions of Oithona similis and Microsetella norwegica

The small copepods Oithona similis and Microsetella norwegica are often numerically abundant and widely distributed, but the factors controlling their vertical distributions and role in carbon cycling are yet unknown. Here we examined the vertical distributions of copepods during spring and summer in the Skagerrak and during autumn in the North Sea with respect to different physiochemical factors including turbulent dissipation rate. The ambush feeder O. similis numeri- cally dominated the copepod community; they were located in the layers with high microzooplank- ton abundance. M. norwegica were dominant in the Skagerrak and were observed within or just below the pycnocline; they are assumed to feed on sinking detrital aggregates. Both copepods use remote detection of either hydromechanical (O. similis) or chemical signals (M. norwegica) generated by the prey and both species migrated to deeper depths in response to elevated surface turbulence. The potential effect of turbulence on both types of feeding is theoretically shown to be negative and we suggest a turbulent dissipation rate in the range 10 -7 to 10 -6 m 2 s -3 as a threshold triggering the observed avoidance responses.

An investigation of diel synchronicity between water column marine snow concentration and the flux of organic matter in the Santa Barbara Channel, California

Diel variation in the concentration of marine snow (detrital aggregates >0.5 mm) in the surface ocean has been documented at several locations, but it is not clear whether this water column signal translates into a diel pulse in particle flux out of the upper mixed layer. In this field study we investigated the temporal relationship between the concentration of marine snow in the upper water column and carbon (C) flux as measured by a sediment trap at 100 m in the Santa Barbara Channel, CA. Camera profiles of marine snow displayed two opposing patterns: (1) higher nighttime total (i.e. cumulative) aggregate volume and (2) higher midday total aggregate volume. Increased nighttime total aggregate volume was only observed during a brief study in 1999 and was associated with increased daytime C flux. For the remaining deployments midday increases in total aggregate volume were observed but, depending on the deployment, were associated with (a) higher nighttime C flux, (b) higher daytime C flux, or (c) no diel pattern. Correspondence between water column aggregate concentration and sediment trap flux increased when average aggregate size exceeded a threshold volume of 0.5 mm 3 (1.0 mm in diameter). Particles caught in sediment traps generally accounted for a small percentage of decreased marine snow particulate organic carbon (POC) in the upper 100 m. Other aggregate loss terms such as macrozooplankton grazing may dominate. When diel patterns in particle flux did occur, changes between day and night samples ranged from small (14%) to large (>200%). Diel variations in particle flux may impact mid-water and benthic ecology particularly animal grazing strategies, waste production, and reproductive cycles. Pulsed sedimentation may also create patchy vertical distributions of particle-associated biota and remineralization products and pulsed food delivery to the benthos.

Mn–Ba–Hg mineralization at shallow submarine hydrothermal vents in Bahía Concepción, Baja California Sur, Mexico

Coastal submarine hydrothermal venting occurs on the west shore of the fault-bounded bay of Bahía Concepción, along a stretch of about 700 m of rocky shoreline. Diffuse hydrothermal seepage of water and continuous gas bubbling (mainly CO 2 and N 2) take place through the sediment-covered seafloor at shallow depth (5 to 15 m). In addition, at about 500 m SE, a cluster of hot springs occurs within the intertidal zone. The temperature and pH of the hydrothermal fluids varies from 72 to 87 °C and 5.95 to 6.02 in the submarine diffuse venting area and reaches 62 °C and pH of 6.68 in the intertidal hot springs. Mn–Ba–Hg mineralization is presently forming as a result of the hydrothermal venting. At the intertidal hot springs, moss-like crusts of manganese oxides and structureless detrital aggregates cemented by opal-A, barite and calcite occur around the main discharge conduits. These aggregates are overlaid by silica-carbonate fine-layered stromatolitic aggregates that extend some meters from the hot springs forming crusts and coalescing rims over volcanic pebbles and boulders. Manganese crusts are made up of barium rich, X-ray-amorphous todorokite-like and romanechite phases, and represent a rare example of a manganese deposit formed in a shallow submarine hydrothermal environment. In the submarine diffuse venting area the hydrothermal precipitates are less conspicuous and consist in millimeter-thick iron oxyhydroxide coatings on volcanic cobbles and boulders. These coatings are composed of poorly crystallized six-line ferrihydrite, accompanied by minor cinnabar and fine-grained colloform pyrite. Textures and fabrics of the mineral assemblages suggest microbial mediation for mineral deposition. The vent precipitates display a significant enrichment of Hg, As and Sb. Arsenic is probably adsorbed onto Mn- and Fe-oxide substrates. The NASC (North American Shale Composite)-normalized REE patterns of the vent precipitates show a pronounced positive Eu anomaly that is probably inherited from the hydrothermal fluid.

The trophic status of herbivorous fishes on coral reefs

Estimates of feeding rates, alimentary tract structure and temporal patterns of food processing obtained from twelve species of nominally herbivorous fishes on the northern Great Barrier Reef were compared. These included members of the families Acanthuridae, Scaridae and Kyphosidae. Based on an analysis of diet and short-chain fatty acid (SCFA) profiles from a previous study we initially partitioned the twelve species into four dietary categories, as follows: (a) Category 1: herbivores with a diet of macroscopic brown algae and high SCFA profiles in the hindgut region (Naso unicornis, Kyphosus vaigiensis); (b) Category 2: herbivores feeding on turfing and filamentous red and green algae with moderate SCFA profiles in the hindgut region (N. tonganus, K. cinerascens, Zebrasoma scopas, Acanthurus lineatus); (c) Category 3: zooplankton feeders with moderate SCFA profiles (N. vlamingii, N. brevirostris); (d) Category 4: species feeding on detrital and sedimentary materials with low levels of SCFA (Chlorurus microrhinos, Scarus schlegeli, Ctenochaetus striatus, A. olivaceus). The purpose of this comparison was to determine whether measures of feeding activity, alimentary tract structure, and food processing were concordant with diet. A dichotomy in feeding rates was observed. Species with a diet of algae and zooplankton (categories 1–3) had slower feeding rates than those feeding on detrital aggregates and sediment (category 4). The pattern of food processing also followed the same dichotomy with species of categories 1–3 retaining food in the alimentary tract overnight and commencing the feeding day with substantial amounts of food in the intestine and hindgut. Category-4 species commenced the feeding day with empty alimentary tracts suggesting a rapid turnover of gut contents. Within the herbivorous and zooplankton-feeding species neither alimentary tract structure nor food processing mode were predicted by diet or SCFA profiles. A hindgut fermentation chamber was present in K. vaigiensis but not in N. unicornis, a species with high levels of SCFA in the hindgut region and a diet of brown macroscopic algae. In contrast N. vlamingii, with a diet dominated by animal matter, retained large amounts of food material in a hindgut chamber over the entire feeding cycle. In tropical perciform fishes, herbivory and fermentation are not associated with the alimentary tract structures that characterise herbivorous terrestrial vertebrates. Estimates of the abundance of the different groupings of nominally herbivorous fishes indicated that the dominant elements in the reef grazing and browsing fauna were consumers of detrital and sedimentary materials. These could not be classified as herbivores. Members of this group were dominant in all habitats investigated. Explicitly herbivorous taxa were a minority component in all habitats investigated.

Nutritional value of detritus and algae in blenny territories on the Great Barrier Reef

The salariin blenny, Salarias patzneri, primarily ingests detrital aggregates and inorganic sediment, with small quantities of filamentous algae. Samples of particulate matter <125 μm and filamentous algae were collected from the territories of S. patzneri and the nutritional value of these resources assessed using organic content and protein/energy ratios calculated from protein, carbohydrate and lipid content. Samples were collected throughout the day, during both summer and winter, to enable temporal comparisons of nutritional value. The particulates <125 μm, which were predominantly amorphic detritus, accounted for 41±2% and 44±3% of the organic matter in the epilithic algal matrix (EAM) during the summer and winter, respectively, whilst filamentous algae accounted for 37±3% and 41±3% of the organic matter in the summer and winter, respectively. Carbohydrate and lipid concentration in the algal samples was significantly greater than in particulate matter <125 μm, although no significant difference was detected in protein levels of algal samples and particulates <125 μm. Consequently, the protein/energy ratio of particulates <125 μm in the summer (11±1 mg kJ −1) and winter (10±1 mg kJ −1) was similar to that of filamentous algae in the summer (9±1 mg kJ −1) and winter (8±1 mg kJ −1). These results suggest detrital aggregates are at least comparable to filamentous algae as a nutritional resource. The large contribution of high quality detritus to organic matter in the epilithic algal matrix collected from S. patzneri territories throughout the day and between seasons provides strong evidence that detritus is a valuable component of small territorial fish diets and is an integral part of coral reef food webs.

Regulation of bacterial biomass and community structure by metazoan and protozoan predation

We performed food web manipulation experiments in three eutrophic Daphnia‐dominated ponds, to compare the predation impact on planktonic bacteria exerted by metazoan and protozoan bacterial consumers. We analyzed the bacterial morphological composition by image analysis and the taxonomic composition by fluorescent in situ hybridization with group‐specific oligonucleotide probes. The removal of Daphnia always resulted in a microbial succession in which first free‐living bacteria and then heterotrophic nanoflagellates (HNFs) increased. Distinct stages of bacterial grazing either exclusively by Daphnia or by HNFs allowed a quantitative and qualitative comparison of their predation impact on the bacterial assemblage. Both bacterial consumers showed a strong size‐selective impact that shifted the free‐living bacterial community structure toward small cells. Suppression of bacterial biomass below the carrying capacity was similar with Daphnia or HNFs in one experiment and was significantly stronger with Daphnia as bacterial consumer in two experiments. This was due to the fact that bacteria that were resistant to protozoan predation partially compensated grazing mortality. Bacteria attached to aggregates and detrital particles were more important as grazing‐resistant forms than bacterial filaments and constituted up to 50% of total bacterial biomass at the end of the experiments. Changes in predation pressure were also associated with shifts in bacterial community composition. Bacteria belonging to the beta subclass of the class Proteobacteria and to the Cytophaga/Flavobacterium group dominated originally. The latter were most strongly reduced by HNF grazing, whereas other groups, in two experiments the alpha sublass (ALF) of Proteobacteria and in one experiment bacteria hybridizing with the probe for Archaea, even increased during HNF grazing. The composition also differed between bacteria associated with aggregates and freely suspended bacteria, the most obvious being the dominance of ALF growing on detrital aggregates. The experiments demonstrated that predation is a major structuring force for planktonic bacterial communities and that changes in predation regime probably have a much stronger impact on the structure of the bacterial community than on bacterial abundance and biomass.

Sources of dietary lipids in the coral reef blenny Salarias patzneri

The composition of essential fatty acids and biomarkers in the tissues of the blenny Salarias patzneri were compared to those in detritus and filamentous algae collected from S. patzneri territories in order to assess the relative contribution of these 2 resources to the diet. The ratio of 16:1 ω7/16:0 and percentage of 18:2 ω6 in S. patzneri body tissue and detritus was found to be significantly different to filamentous algae samples, suggesting that detritus was the major source of dietary lipids assimilated by S. patzneri. High levels of 16:1 ω7 in S. patzneri tissue and detritus relative to filamentous algae suggest that diatoms are an important component of detrital aggregates and S. patzneri diet. These results demonstrate that coral reef fishes assimilate detrital aggregates and confirm that detritus is a valuable dietary resource on coral reefs.

Trophic status and feeding selectivity of blennies (Blenniidae: Salariini)

An evaluation of the dietary resources available to, and selected by, blennies of the tribe Salariini was used to assess their trophic status on the Great Barrier Reef. Gut-content analysis of nine species of blennies found detrital aggregates to be the dominant item ingested. Samples of the dietary resources available to one of these species, Salarias patzneri, were compared biochemically. Of the two main dietary categories (detrital aggregates and filamentous algae), detrital aggregates were the major resource available, accounting for 53 ± 4.6% (SE) of the organic matter present. The mean C:N values for detrital aggregates (17.2 ± 0.8) and filamentous algae (20.0 ± 1.8) were very similar, as were protein concentrations (1.8 ± 0.1 and 2.1 ± 0.1 μg mg−1, respectively). However, mean carbohydrate concentrations were much lower in the aggregates than the filamentous algae (19 ± 2 and 76 ± 13 μg mg−1, respectively). Comparison of the inorganic particles ingested to those available indicated that S. patzneri selectively fed on particles 250 μm, (predominantly algal filaments). The patterns of resource availability, nutritional quality and selectivity suggest that salariin blennies utilise detrital aggregates as their primary dietary resource.

Field calibration of the Optical Plankton Counter with respect to Calanus finmarchicus

Zooplankton in the Faroe–Shetland Channel and north‐western North Sea were sampled concurrently with a profiling serial net system and an Optical Plankton Counter (OPC) during January and March 1995. Calanus finmarchicus, together with a small proportion of Calanus helgolandicus (&lt; 3% overall), made up &gt; 75% of the zooplankton in the region, with C. finmarchicus being predominant below 500 m depth. Sensitivity analysis of the OPC data was carried out to determine the particle size range providing the highest correlation between the concentrations of stage CIV–CV and stage CVI C. finmarchicus estimated from net catches, and particle concentrations from the OPC. Significant correlations were obtained for particle size ranges that corresponded closely with microscopic measurements of the dimensions of copepodites. The highest correlations were obtained for stages CIV–CV in January.According to the net sampling, the concentration of C. finmarchicus in January varied by at least four orders of magnitude over the survey region (&lt; 0.05 to &gt; 500 m−3), although only 4% of the samples contained &gt; 100 m−3. When C. finmarchicus was present at concentrations of around 120 m−3, similar estimates of its abundance were obtained from the OPC data as from the net samples. At lower concentrations, the OPC increasingly overestimated the net catches, and the overestimate reached 40:1 at the detection limit of the nets (0.05 m−3). The average overestimate, taking into account the distribution of concentrations, was 2.5:1. This over‐estimate could not be wholly accounted for by the occurrence of other zooplankton species or potential errors associated with the OPC or net system, and the conclusion was that it arose mainly from the detection of detrital aggregates by the OPC. Thus, the OPC was effective at resolving the peak concentrations (&gt; 100 m−3) of C. finmarchicus in the survey region, but was less effective at delineating the marginal areas of the distribution where C. finmarchicus was outnumbered by other similarly sized particles. Nevertheless, the results indicated that, with the application of calibration data such as described here, the OPC is a valuable tool for broad scale surveys of the spatial distribution of C. finmarchicus during the winter, when the animals are concentrated at depths &gt; 500 m in areas such as the Faroe–Shetland Channel.