Number Of Grazers Research Articles

Production and downward flux of organic matter and calcite in a North Sea bloom of the coccolithophore Emiliania huxleyi

In July 1993, an extensive study was made of a large bloom of the coccolithophorid Emiliania huxleyi in the North Sea halfway between the Shetland Islands and Norway. Here we report on the hydrography, production and sedimentation of particulate organic carbon (POC) and calcite carbon (calcite-C) at 4 stations occupied for 24 h, 2 inside the bloom and 2 just outside. The coccolithophorid bloom was confined to North Sea waters, where a stable shallow mixed layer had been formed. Bloom development had entered the decaying phase, judged by the relatively low living cell number (maximally 1200 cells cm-3), the high number of loose coccoliths (up to 350000 coccoliths cm-3), and the fact that sedimentation of calcite-C exceeded production. In the top 15 m at the bloom stations, the mean daily production of coccoliths was 17 per cell. At the 2 stations outside the bloom, the dominant coccolithophore was a holococcolithophorid (up to 1400 cells cn1r3), wlth insignificant amounts of calcite produced per cell. At these stations, nutrients were present in non-lim~ting concentrations and production of POC was twice as high as at the bloom stations. In the bloom, mixed layer nitrate levels were below 0.2 PM. Faecal pellets collected in the sediment traps contained large numbers of coccoliths of E, huxleyi. Although the numbers of grazers at the 2 stations outside the bloom were not lower than those in the bloom, the volume of faecal matter sedimenting at 50 m was about 70 times lower. It is hypothesized that faecal pellets outside the bloom were so light in weight that they did not sink very far before degradation, whereas the pellets produced in the E. huxleyi bloom in general were exported rapidly due to their heavy load of calcite. This implies that recycling of materials in the mixed layer of this bloom is relatively low due to high downward flux rate. The ratio at which POC and calcite-C were sedimenting amounted to 1.3 on average for the 2 bloom stations at 50 m water depth.

Experimental field study of the effects of crude oil, drill cuttings and natural biodeposits on microphyto- and macrozoobenthic communities in a Mediterranean area

Effects of an experimental pollution by neogenous and fossil organic matter on microphyto- and macrozoobenthic communities were studied in situ over a 1 yr period (July 1989 to May 1990) in a shallow microtidal bay (Gulf of Fos, south coast of France). Three experimental enclosures of 1 m2 non-defaunated sediments were covered with 1 cm of polluted defaunated sediments. The sediment in one enclosure contained natural biodeposits with a high organic matter content (BD), that in a second enclosure contained Arabian light crude oil (BAL), and the sediment in the third enclosure contained diesel oil-based cuttings (CUT). Pollution by contaminants did not prevent microphytobenthos from colouizing sediments. Population changes over time were quite similar in all enclosures, except in CUT, where a four times higher chlorophyll a content appeared to be related to a decreased number of grazers and consequently lower grazing rates of animals. Toxicity to the fauna was immediate in the case of BAL and occurred within 3 mo at CUT. Opportunistic species settled in all contaminated sediments; this occurred quite rapidly in BD and BAL which recovered within 3 mo to levels comparable with control sediments. In CUT, natural populations had not recovered after 1 yr, whereas a quasi-monospecific population of Capitella capitata was still present. During the first 3 mo, the oxygen demand of the sediment was higher in oil-contaminated sediments than in controls. On the whole, the changes in fluxes and organism assemblages in our weakly tidal area appear to be consistent with other findings in macrotidal seas.

The influence of copepod and krill grazing on the species composition of phytoplankton communities from the Scotia Weddell sea

The influence of copepods (mainly Oithona sim-ilis) and krill (Euphausia superba) grazing on the species composition of plankton communities in ship board con tainers was investigated during the spring and post spring period in the Scotia Weddell Sea in the Antarctic ocean. Numbers of grazers were experimentally manipulated in containers with natural phytoplankton assemblages. With ratural levels of copepods but no krill a high (700–950 μg C·l1, ca 30 μg chl a·.l1) phytoplankton biomass developed. In these cultures large diatoms, e.g. Corethron criophilum and chains of Thalassiosira sp., made up 80% of total phytoplankton cell carbon at the end of the experiment. In cultures with elevated numbers of copepods (5X or 10X the natural level) phytoplankton biomass was somewhat reduced (ca 23 μg chl a · l1) compared to cultures with natural copepod abundance, but still high. Phytoplankton species composition was on the other hand greatly influenced. Instead of large diatoms these cultures were dominated by Phaeocystis pouchetii (70%) together with small Nitszchia sp. and Chaetoceros neogracile (20%). In containers with krill (both juveniles and adults), but without elevated numbers of copepods, phytoplankton biomass rapidly approached zero. With 10X the in situ level of copepods, krill first preyed on these before Corethron criophilum and Thalassiosira sp. were grazed. When krill were removed a plankton community dominated by flagellates (60–90%), e.g. Pyramimonas sp. and a Cryptophycean species, grazed by an unidentified droplet-shaped heterothrophic flagellate, developed. These flagellates were the same as those which dominated the plankton community in the Weddell Sea after the ‘spring bloom’. A similar succession was observed in situ when a krill swarm grazed down a phytoplankton ‘bloom’ in a few hours. Our experiments show that copepods cannot control phytoplankton biomass in shipboard cultures even at artificially elevated numbers. Krill at concentrations similar to those in natural swarms have a great impact on both phytoplankton biomass and species composition in shipboard cultures. Both copepods and krill may have an impact on phytoplankton species composition and biomass in situ since the rates of phytoplankton cell division were probably artificially increased in shipboard cultures compared to natural conditions, where lower growth rates make phytoplankton more vulnerable to grazing. A similarity between phytoplankton successions in containers and in situ, especially with respect to krill grazing, supports the conclusion that grazing may structure phytoplankton communities in the Scotia-Weddell Sea.

The role of epiphytic periphyton and macroinvertebrate grazers in the trophic flux of a tropical seagrass community

Biomass and production of epiphytic periphyton, and the abundance, distribution and grazing rate of epifauna were measured in tropical seagrass beds in the Philippines. Periphyton comprised mainly detritus, diatoms and filamentous algae ( Polysiphonia sp. 1, Centroceras clavulatum (C. Agardh) Montagne, Ceramium gracillimum Harvey and Cladophora sp.). Mean biomass of periphyton was 0.16 mg ash-free dry weight (AFDW) cm −2 frond of Enhalus acoroides (L.f.) Royle and 0.24 mg AFDW cm −2 frond of Cymodocea serrulata (R. Br.) Aschers. and Magnus. Total periphyton biomass per unit area (m 2) of seagrass bed varied between habitats because of differing densities of seagrass, and ranged from 598 to 1061 mg AFDW or (24–646 mg C). Maximum (midday, summer) in situ rates of photosynthesis and respiration by epiphytes colonising artificial seagrass material averaged 11.6 μg O 2 cm −2 h −1 and 2.0 μg O 2 cm −2 h −1, respectively. Daily net productivity was 14 μg C cm −2 frond. Productivity of epiphytes per area of seagrass bed varied with site (36–77 mg C m −2 day −1). Relative to biomass, these data show that epiphytes are highly productive, with turnover times of 6–8 days, compared with known values of 30–100 + days for tropical seagrass fronds. The epifaunal grazer community was dominated by a few species of gastropod molluscs (especially Strombus mutabilis Swainson and Cerithium tenellum (Sowerby)). Within habitats, numbers of grazers on particular seagrass species were directly related to their available surface. Three groups of grazers were identified: those occurring on fronds day and night (e.g. S. mutabilis); those foraging over sediment during the day and fronds at night (e.g. Cerithium tenellum); those mainly confined to sediments (e.g. Strombus urceus L.). All epifaunal grazers exhibited upward movement into the seagrass canopy at night. Grazing was non-selective, removing the periphyton, except for the unutilised encrusting coralline algae, in proportion to abundance. Epifaunal grazers consumed between 20 and 62% of periphyton net production and, as in temperate systems, must therefore play a major role in the trophic flux of this tropical seagrass community.

Invertebrate grazer‐periphyton interactions in a eutrophic marsh pond

SUMMARY. 1. The effects of two different invertebrate grazer guilds on the periphytic algal community were studied in a marsh pond in Manitoba, Canada over one entire open‐water season.2. Microcrustaceans (cladocerans, copepods, ostracods), which were numerically dominant in the unperturbed grazer community, effectively reduced the periphytic algal biomass to a persistent, depauperate residual assemblage throughout the season.3. The perturbed grazer community, enclosed and treated with ro‐tenone, was composed primarily of chironomids and oligochaetes. This community permitted a more diverse algal community to develop except when grazer numbers peaked.4. When grazer numbers in the perturbed community were at a maximum, the algal assemblage appeared to be ‘overgrazed’. Algal biomass was reduced below the minimum characteristic of the unperturbed community.5. There was an inverse relationship between algal biomass and grazer numbers in both perturbed and unperturbed communities, but variation in degree of response was much greater in the perturbed system than in the unperturbed one.

Field measurement of specific growth rate, biomass, and primary production of benthic diatoms of Savin Hill Cove, Boston

14C‐labeling of chlorophyll a and subsequent extraction by high‐pressure liquid chromatography (HPLC) was adapted for field measurement of both specific growth rate and biomass of diatom films found on muddy intertidal sediments of Savin Hill Cove, a small embayment of northwest Dorchester Bay, Boston Harbor. Primary production was measured concurrently via 14C uptake into total cellular C. An incubation apparatus that incorporates stirring to enhance sediment‐water gas exchange was used at both low and high tides. Over the year‐long study period (August 1987–August 1988) measured specific growth rates were low (0.06–0.27 d−1) while biomass estimates were high (2.21 × 10−4 to 1.54 × 10−3 g C cm−2). C : Chl ratios varied from 18.7 to 60.4. The highest biomass values and C : Chl ratios were found during the 1988 spring bloom. The very low specific growth rates measured during the spring bloom (0.06 and 0.09 d−1) coupled with spring increases in the numbers of grazers may explain the steep decline in biomass that followed the spring bloom in 1986–1988.

DIGESTION SURVIVAL OF ALGAE: SOME ECOLOGICAL COMPARISIONS BETWEEN FREE SPORES AND PROPAGULES IN FECAL PELLETS1

ABSTRACTBenthic algae survive passage through the digestive tract of different types of grazers, either regenerating new tissues or releasing protoplasts and swarmers which act as accessory means of reproduction to generate new individuals. The ecological importance of this phenomenon is evaluated here by studying spores and fecal pellets. Alga fragments egested alive inside fecal pellets have some ecological advantages over free propagules. In intertidal habitats, the sticky nature of the pellets permits attachment to the substratum while protecting the contained algae from desiccation. In subtidal habitats, the pellets sink 8–22 times faster than the fastest sinking algal propagule. Quantification of swarmers and protoplasts released from algal fragments in the fecal pellets indicates densities of about 217 germlings·cm−2 and values of 300–700 propagules per pellet. Extrapolation of these results to field conditions based on number of grazers and pellets suggest production figures varying from 0.5–3.5 × 105to 1.3–1.5 × 106 propagules·m−2·12 h−1. These values are within known density ranges of settling spores in the field. The ecological significance of the phenomenon may be especially important at seasons when the density of grazers increases and the normal reproductive activity of the algae being consumed decreases.

An experimental analysis of interactions among several intertidal organisms

Observations were made on organisms on a rocky shore in southeastern Australia. Algae growing over barnacles seemed to attract carnivorous whelks at low tide, yet these whelks were less likely to attack individual barnacles completely covered by algae than those partially covered. A field experiment tested whether these patterns arose directly as the effect of an interaction of algae and whelks on the survival of barnacles. The communities in small plots were manipulated to provide a choice of bare space, barnacles only or barnacles and algae as discrete patches within a matrix of algal-covered barnacles. Whelks were released in each of these patches at half of the sites, and their subsequent location and activity were recorded, together with the mortality of barnacles, percentage of cover of algae, and the numbers of mobile grazing gastropods (as alternative prey for the whelks) in each plot. Whelks were observed eating barnacles and gastropods during the experiment. Although whelks ate many barnacles, the direct effect of algae on mortality of barnacles was only occasionally important, whelks did not seem to affect the numbers of grazers. We conclude that the observed pattern is due to a number of different interactions among these organisms. The experimental design is discussed and patterns of complexity within the community are summarized.

Grazing impacts on Mars: A record of lost satellites

Grazing impacts can be identified on the basis of the elongate shape of the resulting crater and a distinctive pattern of ejecta deposits. Over 170 such impact craters larger than 3 km are recognized on Mars, and they represent more than 5% of the total crater population of the ridged plains. In contrast, the moon exhibits only one comparable example larger than 3 km on the maria, a frequency consistent with theoretical estimates for an isotropic influx of impactors. Many Mars grazers appear to occur along great circles. The most recent examples generally impacted in an east‐west direction, whereas older grazers impacted in more northerly directions. We interpret the excessive number of grazers and the common impact directions as the result of satellites whose orbits tidally decayed with time. If all orbits originally had small inclinations similar to the orbits of Phobos and Deimos as well as the most recent grazers, then the change in impact direction with time can be explained as the result of shifts in the crust due to changes in the martian moments of inertia. The locations of the projected orbital axes (orbit‐pole points) on the martian surface indicate that the geographic poles of Mars originally were situated at lower latitudes. More than 95% of the mass represented by these proposed satellites impacted prior to the emplacement of the volcanic plains of Lunae Planum. The estimated combined mass of grazing impactors would form a satellite at least 225 km in diameter. These results may provide new clues for the origin of Phobos and Deimos and perhaps the angular momentum of Mars.