δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N values in reef corals <Emphasis Type="Italic">Porites lutea</Emphasis> and <Emphasis Type="Italic">P. cylindrica</Emphasis> and in their epilithic and endolithic algae

In summer 1998, shallow water corals at Sesoko Island, Japan (26°38′N, 127°52′E) were damaged by bleaching. In August 2003, partially damaged colonies of the massive Porites lutea and the branching P. cylindrica were collected at depths of 1.0–2.5 m. The species composition of epilithic algal communities on dead skeletal surfaces of the colonies (‘red turfs’, ‘green turfs’, ‘red crusts’) and the endolithic algae (living in coral skeletons) growing close to and away from living coral polyps was determined. Carbon and nitrogen stable isotope values of organic matter (δ¹³C and δ¹⁵N) from all six of these biological entities were determined. There were no significant differences in the isotope composition of coral tissues of the two corals, with P. lutea having δ¹³C of −15.3 to −9.6‰ and δ¹⁵N of 4.7–6.1‰ and P. cylindrica having similar values. Polyps in both species living close to an interface with epilithic algae had similar isotope values to polyps distant from such an interface. Despite differences in the relative abundance of the algal species in red turfs and crusts, their δ¹³C and δ¹⁵N values were not significantly different from each other (−18.2 to −13.9, −20.6 to −16.2, 1.1–4.3, and 3.3 to 4.9‰, respectively). The green algal turf had significantly higher δ¹³C values (−14.9 to −9.3‰) than that of red turfs and crusts but similar δ¹⁵N (1.2–4.1‰) to the red algae. The data do not suggest that adjoining associations of epilithic algae and coral polyps exchange carbon- and nitrogen-containing metabolites to a significant extent. The endolithic algae in the coral skeletons had δ¹³C values of −14.8 to −12.3‰ and δ¹⁵N of 4.0–5.4‰. Thus they did not differ significantly from the coral polyps in their carbon and nitrogen isotope values. The similarity in carbon isotope values between the coral polyps and endolithic algae may be attributed to a common source of CO₂ for zooxanthellae and endolithic algae, namely, from respiration by the coral host. While it is difficult to fully interpret similarity in the nitrogen isotope composition of coral tissue and of green endolithic algae and the difference in δ¹⁵N between green epilithic and endolithic algae, the data are consistent with nitrogen-containing metabolites from the scleractinian coral serving as a significant source of nitrogen for the endolithic algae.     

Descriptive ecology of offshore,deep-water,benthic algae in the temperate western North Atlantic Ocean

The vertical distribution, floristic composition and habitat ecology of sublittoral, benthic algae are described for two deep water, offshore stations in the Gulf of Maine, based on in situ observations and collections to 47 m depth during SCUBA and submarine dives. Twenty species of macroscopic algae were collected between 29 and 45 m. These occurred in two benthic algal associations: a Ptilota serrata association of fleshy, almost exclusively red algae from 29 to 37 m; and a deeper Lithothamnium glaciale association dominated by encrusting corallines and extended from ca 38 m to the lowest limit of macroscopic vegetation at 44 to 45 m. Below 45 m only endozoic and epizoic diatoms associated with sponges were observed. Owing to the seasonal stability, homogeneous composition and clear extinction depths of the two associations, we propose that the extinction depth of benthic seaweeds may provide a useful bioassay for making temporal comparison of water conditions at a single geographic region and, where substrata are similar, spatially among different geographic regions. Data obtained during the Jeffreys Ledge study are discussed in relation to this hypothesis.     

Habitat modification affects recruitment of abalone in central New Zealand

The habitat experienced during early life-history stages can determine the number and quality of individuals that recruit to adult populations. In a field experiment, biogenic habitat complexity was manipulated (presence or absence of foliose macroalgae) at two depths (2–3 m and 5–6 m) and the habitat-dependent effects on recruitment of the black foot abalone (Haliotis iris) were examined at three field sites along the south coast of Wellington, New Zealand (41°20′S, 174°47′E), between July and November 2005. Recruit density (<5 weeks post-settlement) was measured on cobbles covered with crustose coralline algae. Habitats of low complexity (barrens treatments) had consistently greater densities of recruits than habitats of high complexity (algae treatments). However, recruits in algae habitats were larger, and for deep habitats, there was greater survival in algae habitats compared with barrens habitats. While depth had no significant effect on early recruit (<2 weeks post-settlement) density, late recruit (<5 weeks post-settlement) density was greater in shallow habitats, and so it seems recruit survival was greater in shallow habitats. In this experiment, algal habitat complexity had strong effects on early recruit abundance, but habitat-dependent variations in recruit growth and survival may modify initial patterns of abundance and determine recruitment to adult abalone populations.     

Ontogenetic changes in small-scale movement by recruits of an exploited mussel: implications for the fate of larvae settling on algae

Many South African populations of the brown mussel Perna perna have been depleted through over-exploitation by subsistence harvesters. This is problematic because recovery after disturbance is very slow, partly because recruits are largely associated with adult mussels. However, unlike large recruits of 3.5–10 mm that exhibit spatial structure related to that of adults, a very high proportion of settlers and small recruits (0–3.5 mm) occur on foliose algae. We tested the hypothesis that recruits on algae move to adult mussel beds after a period of growth, with the null hypothesis that they die at a certain size. We conducted an indirect field study comparing the ratios of large to small recruits in 100% cover mussel patches at locations with high and low algal cover. A second laboratory experiment analysed whether the size of recruits on algae affects their active movement behaviour in response to nearby mussel patches. Large/small recruit ratios were slightly, but not significantly greater in high than low algal cover locations. Both small (2–2.5 mm) and medium (4.5–5.5 mm) recruits remained on algae and moved very short distances throughout the laboratory experiment, while larger recruits (9–10 mm) moved significantly further distances and more often into mussel patches. The results suggest that very large recruits are able to migrate actively to nearby mussel patches, indicating ontogenetic shifts in this behaviour. However, the absence of a significant difference in ratios between field locations with high and low algal cover suggests many large recruits are accidentally dislodged from the algae and presumably die. Thus settlement of P. perna onto algae is likely to be wasted, with consequences for sustainable management of the mussel resource.     

Diet, movement, and covering behavior of the sea urchin Toxopneustes roseus in rhodolith beds in the Gulf of California, México

The density, diet, movement, and covering behavior of Toxopneustes roseus (Agassiz) were investigated in rhodolith beds in the Gulf of California. Densities varied from a mean of 0.4 to 1.8/20 m² with most urchins occurring in aggregations. Spatial patterns of urchins varied with depth, with greatest abundance at intermediate depths (7.5–9.4 m) in the middle of the rhodolith bed. Urchins ate rhodoliths and nongeniculate coralline algal crusts almost exclusively, despite the availability of other algae. The mean amounts ingested were 3.87 and 7.96 g carbonate/individual per day. Even when food was abundant, animals were highly mobile, moving an average of 6.6–11.7 cm/h depending on site and time of day. Diel movement may be a behavioral adaptation to avoid surge, which is greatest during the day. Covering behavior may also be related to surge, as the ratio of covering material:body weight and the percent cover of material held were highest at the site with the most surge. While an urchin consumed rhodoliths, its movement spread the grazing impact over large areas. Bioturbation resulting from urchin feeding, movement, and covering activity probably benefits the rhodoliths by turning them, which maintains rhodolith integrity, prevents fouling, and contributes to bed persistence. Received: 22 February 1999 / Accepted: 3 July 2000     

Inhibition of larval settlement to a soft bottom benthic community by drifting algal mats: An experimental test

The hypothesis that drifting red algal mats inhibit settlement of planktonic larvae was tested in a field experiment in 1986. Substratum free of algae (caged boxes) was compared with substratum covered with algae (natural substratum and open boxes). Whereas settling densities of 1500 to 5500 ind. m^-2 of the bivalves Macoma balthica, Cardium glaucum and Mya arenaria were observed in sediment without algal cover, no recruits of the same species were found beneath the algae during the period of peak settlement (June to July). The same difference was observed for the polychaete Nereis diversicolor, although in this case occasional individuals were found beneath the algae. The results demonstrate that algae mats may be efficient inhibitors of larval settlement to benthic soft-substratum communities. It is concluded that algae act as a larval filter.     

A comparative study of zooplankton dynamics in two subtropical temporarily open/closed estuaries, South Africa

This study aims at comparing the zooplankton abundance/biomass of two South African estuaries affected to a different degree by sewage pollution. In particular the hypothesis that microzooplankton may increase relative to mesozooplankton, as eutrophication increases, is tested for the first time in African estuaries. The survey was carried out during a whole-year cycle, including both open and closed phases. Results show that zooplankton abundance and biomass are higher in the Mdloti than in the Mhlanga, during both the closed (U=1, P<0.001, U=16, P<0.001) and the open phase (U=101, P<0.05, U=88, P<0.01). There were no significant differences in abundance/biomass between the different reaches of each estuary, during either their open or closed phase (1-way ANOVA, P>0.05). The different levels of dominance of the calanoid copepod Pseudodiaptomus hessei, which accounted for 53–64 and 86–97% of the total abundance in the Mdloti and the Mhlanga, respectively, suggests a shift in the zooplankton community structure. There was a lower microzooplankton abundance/biomass contribution to the total zooplankton in the Mhlanga (59.6–15.8%), compared to the Mdloti (99.1–96.5%). The highest microzooplankton contribution in the Mdloti was observed during the closed phase, while in the Mhlanga this occurred during the open phase. This suggests that eutrophication in the Mhlanga may impact negatively on the microzooplankton community. On the other hand the less affected Mdloti exhibits an opposite trend with an increased microzooplankton component.     

Macroalgal morphology mediates particle capture by the corallimorpharian <Emphasis Type="Italic">Corynactis californica</Emphasis>

The shallow kelp forest at Santa Catalina Island, California (33.45 N, −118.49 W) is distinguished by several canopy guilds ranging from a floating canopy (Macrocystis pyrifera), to a stipitate, erect understory canopy (Eisenia arborea), to a short prostrate canopy just above the substratum (Dictyopteris, Gelidium, Laminaria, Plocamium spp.), followed by algal turfs and encrusting coralline algae. The prostrate macroalgae found beneath E. arborea canopies are primarily branching red algae, while those in open habitats are foliose brown algae. Densities of Corynactis californica, are significantly greater under E. arborea canopies than outside (approximately 1,200 versus 300 polyps m⁻² respectively). Morphological differences in macroalgae between these habitats may affect the rate of C. californica particle capture and serve as a mechanism for determining polyp distribution and abundance. Laboratory experiments in a unidirectional flume under low (9.5 cm s⁻¹) and high (21 cm s⁻¹) flow speeds examined the effect of two morphologically distinct macroalgae on the capture rate of Artemia sp. cysts by C. californica polyps. These experiments (January–March 2006) tested the hypothesis that a foliose macroalga, D. undulata, would inhibit particle capture more than a branching alga, G. robustum. G. robustum, found predominantly under the E. arborea canopy did not affect particle capture. However, D. undulata, found predominantly outside of the canopy, inhibited particle capture rates by 40% by redirecting particles around C. californica polyps and causing contraction of the feeding tentacles. These results suggest that the morphology of flexible marine organisms may affect the distribution and abundance of adjacent passive suspension feeders.     

Selective feeding of littoral harpacticoids on diatom algae: hungry gourmands?

We studied benthic harpacticoid grazing on diatom algae from two sites on the White Sea intertidal sandflat. Diatoms from sediments and from harpacticoid gut contents were sampled in situ, identified and counted, and grazing rates were calculated by two ways: (1) using potential daily ration estimations and (2) from the gut content and gut-residence time data. Paraleptastacus kliei did not contain any diatoms in the guts and presumably fed on other objects (bacteria or flagellates). Two other dominating species studied, Heterolaophonte minuta and Huntemannia jadensis, contained an average of 604 and 222 diatom cells per specimen. Diet composition differed significantly from the natural algal community. Two diatoms of intermediate cell size (Nitzschia palea var. debilis and Navicula sp.) contributed 92–97% of gut content for H. minuta and 58–81% for Hn. jadensis, whereas these diatoms amounted to only about 10% of biomass in native community. Mean consumption rates were estimated as 50–200 μg of wet biomass/day/cm², so the harpacticoids grazed only between 3 and 11% of the total microalgae biomass per day. The grazing impact on the two preferred diatom populations, however, was much more intensive, 10–30% per day for Navicula sp. and 55–228% for N. palea. Therefore, native harpacticoid populations demonstrate highly selective feeding and could be strongly limited by their food in spite of seemingly plentiful total abundance of microphytobenthos. This disproportionally high grazing pressure upon some species apparently could affect the structure of microalgae communities resulting in low relative abundance of mid-sized forms. We hypothesize that a very dynamic spatio-temporal distribution of epibenthic harpacticoids (short-living micropatches) may be the possible adaptation to such local food limitation.     

Carbon acquisition strategies for marine macroalgae

A model system was developed to analyse differences in carbon acquisition strategies among macroalgae. During photosynthesis in a limited volume of seawater the capability of the algae to assimilate inorganic carbon as well as to change the alkalinity of the seawater was analysed. These properties were then related to the status of the carbonate equilibrium system of the seawater. The experimental system was assumed to simulate the conditions in the boundary layer during periods of low water exchange or high intensity irradiations. Fundamental differences were found between different algal classes, suggesting that capabilities to adapt to specific environmental conditions may be connected with dissimilarities in carbon acquisition strategies. In general, green algae were able to reach the highest pH (10.8 at 5°C), and thus to achieve the highest reduction in the level of inorganic carbon via a simple HCO₃ ^–/OH^– ion exchange process. For brown algae, pH increases due to carbon uptake never exceeded pH 9.7 (9.5 in a saltwater scale). In spite of this, members of the Fucaceae (littoral brown algae) were able to extract almost all of the dissolved inorganic carbon (DIC). This was achieved through a gradual decrease in the alkalinity of the enclosed water, so that the carbon assimilation could continue without any concomitant increase in pH. For red algae, the specific response was an increase in the level of inorganic carbon. Thus, for this algal class, no specific strategy for handling a shortage of inorganic carbon was documented. Within each algal class, differences in pH and DIC compensation points could be related to differences in the depths at which the algal species occurred. This paper also introduces a low cost and convenient method of analysing DIC in seawater.     

Phytoplankton production patterns in Massachusetts Bay and the absence of the 1998 winter–spring bloom

The seasonal productivity cycle and factors controlling annual variation in the timing and magnitude of the winter–spring bloom were examined for several locations (range: 42°20.35′–42°26.63′N; 70°44.19′–70°56.52′W) in Boston Harbor and Massachusetts Bay, USA, from 1995 to 1999, and compared with earlier published data (1992–1994). Primary productivity (mg C m⁻² day⁻¹) in Massachusetts Bay from 1995 to 1999 was generally characterized by a well-developed winter–spring bloom of several weeks duration, high but variable production during the summer, and a prominent fall bloom. The bulk of production (mg C m⁻³ day⁻¹) typically occurred in the upper 15 m of the water column. At a nearby Boston Harbor station a gradual pattern of increasing areal production from winter through summer was more typical, with the bulk of production restricted to the upper 5 m. Annual productivity in Massachusetts Bay and Boston Harbor ranged from a low of 160 g C m⁻² year⁻¹ to a high of 787 g C m⁻² year⁻¹ from 1992 to 1999. Mean annual productivity was higher (mean=525 g C m⁻² year⁻¹) and more variable near the harbor entrance than in western Massachusetts Bay. At the harbor station productivity varied more than 3.5-fold (CV=40%) over an 8 year sampling period. Average annual productivity (305–419 g C m⁻² year⁻¹) and variability around the means (CV=25–27%) were lower at both the outer nearfield and central nearfield regions of Massachusetts Bay. Annual productivity in 1998 was unusually low at all three sites (<220 g C m⁻² year⁻¹) due to the absence of a winter–spring phytoplankton bloom. Potential factors influencing the occurrence of a spring bloom were investigated. Incident irradiance during the winter–spring period was not significantly different (P > 0.05) among years (1995–1999). The mean photic depth during the bloom period was significantly deeper (P < 0.05) in 1998, signifying greater light availability with depth. Nutrients were also in abundance during the winter–spring of 1998 with stratified conditions not observed until May. In general, the magnitude of the winter–spring bloom in Massachusetts Bay from 1995 to 1999 was significantly correlated with winter water temperature (r ²=0.78) and zooplankton abundance (r ²=0.74) over the bloom period (typically February–April). The absence of the 1998 bloom was associated with higher than average water temperature and elevated levels of zooplankton abundance just prior to, and during, the peak winter–spring bloom period. Received: 3 July 2000 / Accepted: 6 December 2000     

Transient dynamics of pelagic producer-grazer systems in a gradient of nutrients and mixing depths

Phytoplankton-grazer dynamics are often characterized by long transients relative to the length of the growing season. Using a phytoplankton-grazer model parameterized for Daphnia pulex with either flexible or fixed algal carbon:nutrient stoichiometry, we explored how nutrient and light supply (the latter by varying depth of the mixed water column) affect the transient dynamics of the system starting from low densities. The system goes through an initial oscillation across nearly the entire light-nutrient supply space. With flexible (but not with fixed) algal stoichiometry, duration of the initial algal peak, timing and duration of the subsequent grazer peak, and timing of the algal minimum are consistently accelerated by nutrient enrichment but decelerated by light enrichment (decreasing mixing depth) over the range of intermediate to shallow mixing depths. These contrasting effects of nutrient vs. light enrichment are consequences of their opposing influences on food quality (algal nutrient content): algal productivity and food quality are positively related along a nutrient gradient but inversely related along a light gradient. Light enrichment therefore slows down grazer growth relative to algal growth, decelerating oscillatory dynamics; nutrient enrichment has opposite effects. We manipulated nutrient supply and mixing depth in a field enclosure experiment. The experimental results were qualitatively much more consistent with the flexible than with the fixed stoichiometry model. Nutrient enrichment increased Daphnia peak biomass, decreased algal minimum biomass, decreased the seston C:P ratio, and accelerated transient oscillatory dynamics. Light enrichment (decreasing mixing depth) produced the opposite patterns, except that Daphnia peak biomass increased monotonously with light enrichment, too. Thus, while the model predicts the possibility of the "paradox of energy enrichment" (a decrease in grazer biomass with light enrichment) at high light and low nutrient supply, this phenomenon did not occur in our experiment.     

Phytoplankton growth and microzooplankton grazing rates in a restricted Mediterranean lagoon (Bizerte Lagoon,Tunisia)

Phytoplankton growth and microzooplankton grazing were investigated in the restricted Bizerte Lagoon in 2002 and 2004. The 2002 study, carried out at one station from January to October, showed significant seasonal variations in phytoplankton dynamics. High growth rates (0.9–1.04 day⁻¹), chlorophyll a (Chl a) concentrations (6.6–6.8 μg l⁻¹) and carbon biomass (392–398 μg C l⁻¹) were recorded in summer (July), when several chain-forming diatoms had intensively proliferated and dominated the carbon biomass (74%). In 2004, four stations were studied during July, a period also characterized by the high proliferation of several diatoms that made up 70% of the algal carbon biomass. In 2004, growth rates (0.34–0.45 day⁻¹) and biomass of algae (2.9–5.4 μg Chl a l⁻¹ and 209–260 μg C l⁻¹) were low, which may be related to the lower nutrient concentrations recorded in 2004. Microzooplankton >5 μm were mainly composed of heterotrophic dinoflagellates and ciliates. Microzooplankton biomass peaked during summer (2002 320–329, 2004 246–361 μg C l⁻¹), in response to the enhanced phytoplankton biomass and production. The grazer biomass was dominated by ciliates (71–76%) in July 2002 and by heterotrophic dinoflagellates (52–67%) in July 2004. Throughout the year and at different stations, microzooplankton grazed actively on phytoplankton, removing 26–58% of the Chl a and 57-84% of the primary production. In 2002, the highest grazing impact was observed on the large algae (>10 μm) during the period of diatom dominance. These results have a significant implication for carbon export to depth. Indeed, the recycling of most of the diatom production by the microbial food web in the upper water column would reduce the flux of material to the seafloor. This should be considered when modeling the carbon cycling in coastal environments and under conditions of diatom dominance. During both studies, ciliates had higher growth rates (0.5–1.5 day⁻¹) and a higher carbon demand (165–470 μg C l⁻¹ day⁻¹) than dinoflagellates (0.1–0.5 day⁻¹, 33–290 μg C l⁻¹ day⁻¹). Moreover, when grazer biomass was dominated by ciliates (in July 2002), herbivory accounted for 71–80% of the C ingested by microzooplankton while it accounted only for 14–23% when dinoflagellates dominated the grazer biomass (in July 2004). These results suggest that, in contrast to findings from open coastal waters, ciliate species of the restricted Bizerte Lagoon were more vigorous grazers of the large algae (diatoms) than were dinoflagellates.     

Short- and long-term effects of eutrophication on the secondary production of an intertidal macrobenthic community

Secondary production of a macrobenthic community at an intertidal mudflat was estimated for 33 successive months. Sampling was carried out along a eutrophication gradient, including non-eutrophied Zostera meadows, an intermediate muddy area, and a strongly eutrophied sand-muddy flat, where macroalgal blooms of Enteromorpha spp. usually occur. The Zostera meadows were always the most productive habitat (145–225 g ash-free dry weight m^–2 year^–1). In the short term, the macroalgal bloom benefited the total estuarine production by enhancing the annual production in the eutrophied area. Nevertheless, our results show that this increase was short lived and in no way sufficient to match the production in the Zostera meadows. In the long term, the present study provides evidence that the disappearance of macrophyte beds, as a result of ongoing eutrophication, constitutes a major threat to the sustainability of the estuarine ecosystem.Communicated by S.A. Poulet, Roscoff     

Under-ice feeding and diel migration by the planktonic copepodsCalanus glacialis andPseudocalanus minutus in relation to the ice algal production cycle in southeastern Hudson Bay,Canada

The marine planktonic copepodsCalanus glacialis Jaschnov andPseudocalanus minutus (Kroyer) typically dominate the copepod biomass in spring under the ice in southeastern Hudson Bay, Canada. Females of both species exhibited significant diel feeding cycles, as measured by gut pigment content, throughout a bloom of ice algae at the ice-water interface in 1986. Periods of grazing correlated well with a nighttime vertical migration by females to within 0.2 m of the ice-water interface, suggesting that feeding took place at or just below the thermohaline boundary between seawater and the interfacial layer containing the ice algae. Seasonal melting of the ice bottom in mid-May resulted in freshening of the surface layer and release of the ice algae into the water column. FemaleC. glacialis andP. minutus responded by ceasing migration to the interface. Gut pigment content, and by reasonable assumption, feeding activity in the water column, increased substantially immediately after this event. In mid-May, the water column phytoplankton consisted of flagellates, sedimenting ice algal cells, and diatoms (Navicula pelagica andChaetoceros sp.) previously found at the interface and then growing in the water column. We conclude that algae growing at the ice-water interface, and sedimenting or actively growing algae derived from this interfacial layer, are a regular and principal source of nutrition for these pelagic copepods during and immediately after the ice algal bloom.     

Spring sea ice photosynthesis,primary productivity and biomass distribution in eastern Antarctica, 2002–2004

While it is known that Antarctic sea ice biomass and productivity are highly variable over small spatial and temporal scales, there have been very few measurements from eastern Antarctic. Here we attempt to quantify the biomass and productivity and relate patterns of variability to sea ice latitude ice thickness and vertical distribution. Sea ice algal biomass in spring in 2002, 2003 and 2004 was low, in the range 0.01–8.41 mg Chl a m⁻², with a mean and standard deviation of 2.08 ± 1.74 mg Chl a m⁻² (n = 199). An increased concentration of algae at the bottom of the ice was most pronounced in thicker ice. There was little evidence to suggest that there was a gradient of biomass distribution with latitude. Maximum in situ production in 2002 was approximately 2.6 mg C m⁻² h⁻¹ with assimilation numbers of 0.73 mg C (mg Chl a)⁻¹ h⁻¹. Assimilation numbers determined by the ¹⁴C incubations in 2002 varied between 0.031 and 0.457 mg C (mg Chl a)⁻¹ h⁻¹. Maximum fluorescence quantum yields of the incubated ice samples in 2002 were 0.470 ± 0.041 with E _k indices between 19 and 44 μmol photons m⁻² s⁻¹. These findings are consistent with the shade-adapted character of ice algal communities. In 2004 maximum in situ production was 5.9 mg C m⁻² h⁻¹ with an assimilation number of 5.4 mg C (mg Chl a)⁻¹ h⁻¹. Sea ice biomass increased with ice thickness but showed no correlation with latitude or the time the ice was collected. Forty-four percent of the biomass was located in bottom communities and these were more commonly found in thicker ice. Surface communities were uncommon.     

Bacterivory of a mudflat nematode community under different environmental conditions

The fate of the benthic bacterial biomass is a topic of major importance in understanding how soft-bottom environments function. Because of their high abundance, production and nutritional value, benthic bacteria may constitute an important food resource for benthic fauna. The trophic role of bacteria for a nematode community on the Brouage mudflat (Marennes-Oléron-France), dominated by three species: Chromadora macrolaima (64% of the abundance), Daptonema oxycerca (15%) and Ptycholaimellus jacobi (8%), was determined in grazing experiments using ¹⁵N pre-enriched bacteria. On intertidal flats, seasonal, tidal and circadian cycles induce strong variations in environmental conditions. Grazing experiments were performed in order to measure the effects of abiotic (temperature, salinity and luminosity) and biotic (bacterial and algal abundances) factors on assimilation rates of bacteria by nematodes. In order to assess simultaneously bacteria and algal assimilation rates, algal abundances were modified adding ¹³C pre-enriched Navicula phyllepta. Assimilation rate was significantly lower at 5°C; moreover, general trend shows a prominent temperature effect with an optimum around 30°C. Assimilation at salinity 18 was not significantly different from the assimilation at salinity 31. Assimilation was higher under light conditions than in the dark. Above 10⁹ bacteria ml⁻¹, assimilation of bacteria remained unaffected by bacterial abundance. However, assimilation of algae increased with the algal concentration. Nematode kept feeding under conditions of stress, which are typical of the surficial sediment habitat and they appeared to be principally dependent on the algal resource.     

Abundance,distribution and prey composition of scyphomedusae in the southern North Sea

The annual cycle of abundance and distribution of the scyphozoan medusae Aurelia aurita, Cyanea lamarckii, C. capillata and Chrysaora hysoscella were studied in the southern North Sea in 2004 and 2005. Three different patterns of seasonal occurrence of medusae were distinguished: (1) the early occurring C. lamarckii (February–August), (2) C. capillata and A. aurita (April–August) and (3) the late appearing C. hysoscella (July/August–September). Cyanea lamarckii was the most frequently encountered species in this study; its highest mean abundance was 1.8 ± 2.7 ind. 100 m⁻³. The prey spectra of C. lamarckii, C. capillata and C. hysoscella contained several copepod and other crustacean species and thus make them potential competitors with fish larvae. Medusae in this study also consumed fish eggs and larvae, including clupeids, in all months analysed. Although peak spawning of sprat (Sprattus sprattus) coincides with the maximum abundance of medusae (May–June) the relative low abundance of all medusae species in this study makes jellyfish predation unlikely to be a factor controlling sprat recruitment in the time frame investigated.     

Sewage nutrient enrichment and phytoplankton ecology along the central coast of Lebanon

The abundance and taxonomic diversity of phytoplankton has been studied in relation to sewage pollution (proximity to outfalls) south of Beirut, Lebanon. Surface-water samples were collected from a series of beach stations extending from the American University of Beirut to 20 km south from June, 1973 to July, 1974. Samples were preserved, concentrated by settling, and the concentration of each taxon of phytoplankton enumerated in an inverted microscope. Water samples from the vicinity of two major sewer outfalls (Carlton and Khalde sewers) showed very high concentrations of NH₄ ⁺, NO₂ ^-, NO₃ ^- and PO₄ ^-3, a greater total concentration of phytoplankton, and a lower taxonomic diversity than samples remote from outfalls. A considerable variation in the occurrence of species and dominance occurred along the pollution gradient. Blue-green algae and dinoflagellates were dominant in polluted waters, while diatoms dominated in cleaner water away from major sewage outflow. From the dominance and relative distribution of the taxa along the pollution gradient, certain taxa (Oscillatoria spp., Spirulina spp., Phormidium spp., Synochococcus custos and S. elongatus, Gymnodinium spp., and Prorocentrum spp.) emerge as indicator species of pollution. These changes correspond to a typical degradation of a complex community to a less mature state by the inflow of nutrient-rich sewage (eutrophication) along a coastal region about 10 km long.     

Reversed parasite-mediated selection in sticklebacks from eutrophied habitats

Sexual selection is often assumed to increase the viability of populations by increasing the quality of offspring produced. Presently, human-induced environmental changes are altering the process of sexual selection by influencing male–male interactions and female mate choice. Here, we show that increased density of filamentous algae due to eutrophication reverses parasite-mediated selection during reproduction in threespine sticklebacks (Gasterosteus aculeatus). When we manipulated the density of artificial algae in a breeding area in the Baltic Sea, more males nested in dense than in sparse vegetation, but the males in dense vegetation were more parasitized. Interestingly, heavily parasitized males acquired more eggs than less parasitized males in dense vegetation but not in sparse vegetation. The higher probability of reproduction for parasitized males in dense algae growth could be due to impaired visibility relaxing male–male competition or reducing female choosiness. This could favour males in poor condition that often invest more in attracting females than males in good condition. In sparse vegetation, parasitized males may have a lower reproductive success due to intense male–male competition, careful female choice and high predation rate selecting against parasitized males. The results suggest that eutrophication could alter the fraction of the population that reproduces, which could have long-term evolutionary consequences.