Gerard Nieuwland

Temporal and spatial variations in heterotrophic nanoflagellate abundance in North Sea sediments.

Summer-winter variations in marine nanoflagellate densities at 3 depths in North Sea sediment~ (0-3, 30-33 and 60-63 mm) were studied using epifluorescence microscopy. Benthic flagellate densities in summer ranged from 7 to 859 X lo3 and in winter from 9 to l100 X 103 cells ~ m ~ . T h e effect of season on flagellate densities was different among stations. At 10 out of 15 stations summer values were significantly higher than winter values. The effect of season on flagellate densities was the same at all 3 depths. Flagellate densities in the sediment surface layer (0-3 mm) were in general 2 to 4 times higher than in the 2 deeper sampled layers (30-33, 60-63 mm). The extraordinary high flagellate densities near Esbjerg (Denmark) were remarkable: 859 X 103 cells in the surface layer in summer and 1100 X 103 cells in the 2 deeper layers during winter. In both seasons, at all depths and all stations most cells (50 to 75%) occurred in the 2 to 5 pm size class. Few flagellates were larger than 10 pm or smaller than 2 pm. Pooled winter and summer data of flagellate densities in the sediment surface layer showed a positive correlation with bacterial production and bacterial specific growth rate, explaining 20 and 30% respectively of the variance. In summer a positive correlation existed between flagellate density and bacterial specific growth rate and grain size, together explaining 53% of the variance. In winter nanoflagellate densities were significantly correlated with bacterial biomass and abundance accounting for 59% and 33%, respectively, of the variance. The data suggest that bacterial biomass/abundance during winter sets llmits to flagellate densities. Increased bacterial production was probably responsible for generally higher summer flagellate densities although grain size could become a limiting factor for flagellate densities in silty sediments during summer

Coral cavity sponges depend on reef-derived food resources: stable isotope and fatty acid constraints

The diet of cavity sponges on the narrow fringing reefs of Curaçao, Caribbean was studied. The origin and resources of the bulk food of these sponges, i.e., dissolved organic matter (DOM), were identified using stable carbon and nitrogen isotopes and fatty acid biomarkers. We found that phytoplankton and its derived DOM from the adjacent open sea and from reef overlying water is not the main source of food for most of the sponges examined nor is bacterioplankton. Interestingly, dual stable isotope signatures (δ13Corg, δ15Norg) and fatty acid biomarkers appoint coral mucus and organic matter derived from crustose coralline algae (CCA) as probable food sources for encrusting sponges. Mucus-derived DOM may contribute up to 66% to the diet of examined sponges based on results of dual isotope mixing model analysis. The contribution of CCA (as purported representative for benthic algae) was smaller with values up to 31%. Together, mucus- and CCA-derived substrates contributed for 48–73% to the diet of sponges. The presence of the exogenous fatty acid 20:4ω6 in sponges, which is abundant in coral mucus of Madracis mirabilis and in CCA, highlights these reef-derived resources as sources of nutrition for DOM feeding cavity sponges. The relatively high concentrations of exogenous 20:4ω6 in all sponges examined supports our hypothesis that the bulk of the food of the cavity sponge community is reef-derived. Our results imply that cavity sponges play an important role in conserving food and energy produced within the reef.

Organic sedimentation and macrofauna as forcing factors in marine benthic nanofagellate communities.

We investigated how benthic nanoflagellate communities in marine sediments respond to sedimentation of organic material and to the presence of macrofaunal organisms in controlled boxcosms. An input of 24 g C m−2 resulted in a sharp increase in densities, from 93 to 477 × 103 flagellates cm−3 within 11 days. At the onset, this increase was paralleled by enhanced bacterial production and bacterial numbers. When bacterial production collapsed, flagellate ingestion rates, varying from 17 to 67 bact flag−1 h−1, were sufficient to control bacterial abundance. The presence of macrofauna accelerated the burst in flagellate densities. With macrofauna the same maximum densities were reached, but later densities dropped to relatively low levels. Macrofaunal bioturbation resulted in higher flagellate densities deeper in the sediment (up to 1200% at 3 cm and up to 460% at 6 cm deep).

Bacteria, auto- and heterotrophic nanoflagellates, and their relations in mixed, frontal and stratified waters of the North Sea☆

Abstract The horizontal and vertical distributions of bacteria and bacterial productivity were compared with nanoflagellate densities in the southern part of the central North Sea. Mixed, frontal and stratified waters were sampled in transects during summer 1988. High bacterial abundance, 2.7 to 4.5 ∗ 10 6 cells·cm −3 , distinguished coastal from offshore waters. Bacterial production and nanoflagellate densities were also high in the coastal zone but reached comparable or even higher values further offshore in frontal systems. We crossed two conspicuous fronts: the Frisian Frontal zone and a frontal zone along the northern slope of the Dogger Bank. These fronts were characterized by enhanced bacterial production and/or enhanced bacterial specific growth rates. In fronts as well as mixed waters, nanoflagellate densities covaried with bacterial specific growth rates and reached highest numbers in fronts, e.g. heterotrophic nanoflagellate densities peaked in the Frisian Front with 6000 to more than 10 000 cells·cm −3 . These high densities were accompanied by low bacterial abundances (0.45 ∗ 10 6 cells·cm −3 ) suggesting a regulation of bacterial numbers by heterotrophic nanoflagellates. A comparable pattern was found in the Dogger Bank front. Biomass of autotrophic nanoflagellates was significantly correlated with biomass of heterotrophic nanoflagellates. A maximum of nanoflagellates was present in the thermocline in stratified waters. The carbon distribution between bacteria and nanoflagellates over the water column was always dominated by flagellates, except in the coastal zone. Offshore, in mixed waters, bacterial biomass made up 30 to 51% of the bacterial plus auto/heterotrophic nanoflagellate biomass. In fronts and stratified waters the biomass of heterotrophic nanoflagellates alone exceeded bacterial biomass. Bacterial production amounted to a fraction of 3 to 31% of the primary production.

Impact of nanoflagellate bacterivory on benthic bacterial production in the North Sea

Abstract In situ grazing of benthic heterotrophic nanoflagellates on bacteria was studied in a wide range of sediment types in the North Sea during summer and winter. Grazing rates were measured using the fluorescently labelled bacteria (FLB) technique. Several factors may potentially influence flagellate grazing, viz. temperature, sediment grain size, bacterial abundance and production, flagellate abundance and biovolume. Flagellate grazing rates were related to these variables. Total grazing of the benthic flagellate population was estimated by multiplying flagellate abundance by maximum/minimum grazing rates. A comparison with the total amount of bacterial cells produced provided an estimate of the impact of flagellate bacterivory on benthic bacterial production. Individual grazing rates ranged from 0 to 112 bacteria per flagellate per hour. Grazing rates showed no significant differences between summer and winter and did not correlate significantly with any of the above-mentioned variables. Maximum average grazing rates ranged from 15 to 39 bacteria per flagellate per hour. Minimum average values varied between 1 and 5 bacteria per flagellate per hour. The percentages of benthic bacterial production accounted for by flagellate consumption differ greatly depending on the grazing rate used. Using maximum grazing rates, 2 to 23% of the bacterial production during summer was consumed, while in winter this ranged from 23 to 528%. Minimum estimates were 10 to 20 times lower: 0.2 to 3% in summer and 1 to 50% in winter, higher percentages of the bacterial production were consumed, which was probably due to the relatively lower bacterial production in winter compared to summer.

Mesocosm experiments: mimicking seasonal developments of microbial variables in North Sea sediments

This study investigated the suitability of mesocosms for studying the seasonal development of microbial variables in the benthic system of the North Sea. Undisturbed sediment cores were taken from two locations in the North Sea, one with sandy sediment (28 m depth) and the other with silty sediment (38 m depth) and installed in mesocosms in January-April 1989. Cores were kept as in situ temperature in the dark until December 1989. One set of sandy and silty sediments was starved and the other set received a supply of organic matter in May-June, simulating the settlement of the spring bloom of Phaeocystis pouchetii. Seasonal developments in bacterial production (methyl 3H-thymidine incorporation), abundance and biomass of bacteria and nanoflagellates and oxygen consumption were compared between the mesocosms and the field in surface sediments every 1.5 to 2.5 months. Effects of seasonal temperature variations (range 6–17.5 °C) on microbial variables in starved mesocosms were limited, which possibly indicates a subordinate role of temperature in microbial processes in North Sea sediments. Organic matter produced a direct response in bacterial production and oxygen consumption in mesocosms. Bacterial and protozoan abundance also increased. The effect of the organic input disappeared within 2 months and values of enhanced variables declined to initial levels. The organic matter enrichment in mesocosms apparently did not provide sufficient energy to keep the microbenthos active at field levels through summer.

Long-term Shifts in Coral Communities On Shallow to Deep Reef Slopes of Curaçao and Bonaire: Are There Any Winners?

Tropical coral reefs are among the most biologically diverse and economically important ecosystems on earth. Nevertheless, we found dramatic changes in coral communities on the reef slopes of Curacao and Bonaire since 1973. Cover and abundance declined for virtually all coral species. The data show a shift from communities dominated by framework building species (e.g. Orbicella spp.) to communities consisting of small opportunistic, phenotypically plastic, species, including few remaining structural colonies. Madracis mirabilis, Porites astreoides, Diploria strigosa and Agaricia lamarcki are at present modest winners in the coral assemblage, although overall cover declined also for these species. Increased frequency and intensity of events inducing coral mortality and ongoing reduction in suitable hard substratum, provided by the remnants of large colony building species, could reduce the chance of these species to remain winners in the longer run. The observed loss in coral cover and the shift from larger structural to smaller opportunistic species reduced reef carbonate production by 67% and therewith, in combination with a trend towards smaller coral colonies, reef complexity. Alarmingly, reefs at mesophotic depths (30 - 40 m) did not escape the general degradation of the coral community. The negative effects are larger around densely populated areas where local stressors are adding to degradation caused, for instance, by region wide mass bleaching. Without proper conservation and management this already dramatic degradation will continue and turn more and more coral species into losers.