Biophysical controls on seasonal changes in the structure, growth, and grazing of the size-fractionated phytoplankton community in the northern South China Sea

The size-fractionated phytoplankton growth and microzooplankton grazing are crucial for the temporal change of community size structure, regulating not only trophic transfer but also the carbon cycle of the ocean. However, the size-dependent growth and grazing dynamics on a monthly or an annual basis are less addressed in the coastal ocean. In this paper, the seasonal responses of the size-fractionated phytoplankton growth and grazing to environmental change were examined over 1 year at a coastal site of the northern South China Sea. We found a nanophytoplankton-dominated community with strong seasonal variations in all size classes. Phytoplankton community growth rate was positively correlated to nutrients, with community grazing rate correlating to the total chlorophyll a at the station, reflecting a combined bottom-up and topdown effect on phytoplankton population dynamics. Further analyses suggested that the specific growth rate of microphytoplankton was significantly influenced by phosphate, and that of nanophytoplankton was influenced by light, although picophytoplankton growth was controlled by both nitrate and temperature. In addition, the specific grazing rate of nanophytoplankton was well correlated to phytoplankton standing stock, while that of micro- and pico-compartments was negatively influenced by ciliate abundance and salinity. Finally, a lower grazing impact for micro-cells (38 %) than nano- and pico-cells (72 % and 60 %, respectively) may support size-selective grazing of microzooplankton on small cells at this eutrophic system.

Pumping Rate and Size of Demosponges—Towards an Understanding Using Modeling

Filter-feeding sponges pump large amounts of water and contribute significantly to grazing impact, matter transport and nutrient cycling in many marine benthic communities. For ecological studies it is therefore of interest to be able to estimate the pumping rate of different species from their volume size or osculum cross-sectional area by means of experimentally determined allometric correlations. To help understand allometric data correlations and observed large variations of volume-specific pumping rate among species we developed a model that determines the pumping rate as a function of the size (volume) of a tubular-type demosponge described by 4 geometric length scales. The model relies on a choanocyte-pump model and standard pressure loss relations for flow through the aquiferous system, and density and pumping rate per choanocyte is assumed to be constant. By selecting different possibilities for increase of the length scales, which may also simulate different growth forms, we demonstrate that the model can imitate the experimental allometric correlations. It is concluded that the observed dependence of pumping rate on size is primarily governed by the hydraulics of pump performance and pressure losses of the aquiferous system rather than, e.g., decreasing density of choanocytes with increasing sponge size.

Response of Growth and Grazing Rate of Nanoflagellates on Synechococcus spp. to Experimental Nutrient Enrichment

As important bacterivores in planktonic food webs, mixotrophic nanoflagellates cancause mortality in marine Synechococcus spp. Our previous study found that the pigmented nanoflagellate (PNF) has a significant grazing impact on Synechococcus spp. In the current study, we applied the dilution approach to test the growth and grazing rates of nanoflagellates on Synechococcus spp. We then compared the differences between experimental nutrient additions and in situ conditions in the coastal waters of the East China Sea during the summer season from July to September. The growth rates of Synechococcus spp. in the ambient environment were between 0.54 and 0.62 day−1, which were slightly higher than the 0.56 and 0.66 day−1 with nutrient enrichment in summer. In contrast, our nutrient enrichment experiments produced a marked decline approximately from 21% to 58%in the nanoflagellate grazing rate on Synechococcus spp. The reason was that the mixotrophic PNFs directly used the added nutrients and reduced their supply of nutrients from prey during the incubation experiments.

Biophysical controls on seasonal changes in the structure, growth, and grazing of the size-fractioned phytoplankton community in the northern South China Sea

The size-fractionated phytoplankton growth and microzooplankton grazing are crucial for the temporal change of community size-structure, regulating not only trophic transfer but also carbon cycle of the ocean. However, the size-dependent growth and grazing dynamics on monthly or an annual basis are less addressed in the coastal ocean. In this paper, the seasonal responses of the size-fractionated phytoplankton growth and grazing to environmental change were examined over a one-year period at a coastal site of the northern South China Sea. We found a nanophytoplankton dominated community with strong seasonal variations of all size classes. Phytoplankton community growth rate was positively correlated to nutrients with community grazing rate correlating to the total chlorophyll-a at the station, reflecting a combined bottom-up and top-down effect on phytoplankton population dynamics. Further analyses suggested that the specific growth rate of microphytoplankton was significantly influenced by phosphate with that of nanophytoplankton by light, although picophytoplankton growth was controlled by both nitrate and temperature. In addition, the specific grazing rate of nanophytoplankton was well correlated to phytoplankton standing stock, while those of micro- and pico-compartments were negatively influenced by ciliate abundance and salinity. Finally, a lower grazing impact for micro-cells (38 %) than nano- and pico-cells (72 % and 60 %, respectively) may support a size-selective grazing of microzooplankton on small cells at this eutrophic system.

Phosphite Reduces the Predation Impact of Poterioochromonas malhamensis on Cyanobacterial Culture

Contamination by the predatory zooplankton Poterioochromonas malhamensis is one of the major threats that causes catastrophic damage to commercial-scale microalgal cultivation. However, knowledge of how to manage predator contamination is limited. Previously, we established a phosphite (Pt)-based culture system by engineering Synechococcus elongatus, which exerted a competitive growth advantage against microbial contaminants that compete with phosphate source. Here, we examined whether Pt is effective in suppressing predator-type contamination. Co-culture experiment of Synechococcus with isolated P. malhamensis revealed that, although an addition of Pt at low concentrations up to 2.0 mM was not effective, increased dosage of Pt (~20 mM) resulted in the reduced grazing impact of P. malhamensis. By using unsterilized raw environmental water collected from rivers or ponds, we found that the suppression effect of Pt was dependent on the type of environmental water used. Eukaryotic microbial community analysis of the cultures using environmental water samples revealed that Paraphysomonas, a colorless Chrysophyceae, emerged and dominated under high-Pt conditions, suggesting that Paraphysomonas is insensitive to Pt compared to P. malhamensis. These findings may provide a clue for developing a strategy to reduce the impact of grazer contamination in commercial-scale microalgal cultivation.

Reconciling Behavioural, Bioenergetic, and Oceanographic Views of Bowhead Whale Predation on Overwintering Copepods at an Arctic Hotspot (Disko Bay, Greenland)

Bowhead whales (Balaena mysticetus) visit Disko Bay, West Greenland in winter and early spring to feed on Calanus spp., at a time of year when the copepods are still mostly in diapause and concentrated in near-bottom patches. Combining past observations of copepod abundance and distribution with detailed observations of bowhead whale foraging behaviour from telemetry suggests that if the whales target the highest-density patches, they likely consume 26–75% of the Calanus standing stock annually. A parallel bioenergetic calculation further suggests that the whales' patch selection must be close to optimally efficient at finding hotspots of high density copepods near the sea floor in order for foraging in Disko Bay to be a net energetic gain. Annual Calanus consumption by bowhead whales is similar to median estimates of consumption by each of three zooplankton taxa (jellies, chaetognaths, and predatory copepods), and much greater than the median estimate of consumption by fish larvae, as derived from seasonal abundance and specific ingestion rates from the literature. The copepods' self-concentration during diapause, far from providing a refuge from predation, is the behaviour that makes this strong trophic link possible. Because the grazing impact of the whales comes 6–10 months later than the annual peak in primary production, and because Disko Bay sits at the end of rapid advective pathways (here delineated by a simple numerical particle-tracking experiment), it is likely that these Calanus populations act in part as a long-distance energetic bridge between the whales and primary production hundreds or thousands of km away.

Sea-ice habitat minimizes grazing impact and predation risk for larval Antarctic krill

Survival of larval Antarctic krill (Euphausia superba) during winter is largely dependent upon the presence of sea ice as it provides an important source of food and shelter. We hypothesized that sea ice provides additional benefits because it hosts fewer competitors and provides reduced predation risk for krill larvae than the water column. To test our hypothesis, zooplankton were sampled in the Weddell-Scotia Confluence Zone at the ice-water interface (0–2 m) and in the water column (0–500 m) during August–October 2013. Grazing by mesozooplankton, expressed as a percentage of the phytoplankton standing stock, was higher in the water column (1.97 ± 1.84%) than at the ice-water interface (0.08 ± 0.09%), due to a high abundance of pelagic copepods. Predation risk by carnivorous macrozooplankton, expressed as a percentage of the mesozooplankton standing stock, was significantly lower at the ice-water interface (0.83 ± 0.57%; main predators amphipods, siphonophores and ctenophores) than in the water column (4.72 ± 5.85%; main predators chaetognaths and medusae). These results emphasize the important role of sea ice as a suitable winter habitat for larval krill with fewer competitors and lower predation risk. These benefits should be taken into account when considering the response of Antarctic krill to projected declines in sea ice. Whether reduced sea-ice algal production may be compensated for by increased water column production remains unclear, but the shelter provided by sea ice would be significantly reduced or disappear, thus increasing the predation risk on krill larvae.

Variation in herbivore grazing behavior across Caribbean reef sites

Herbivorous fish can increase coral growth and survival by grazing down algal competitors. With coral reefs in global decline, maintaining adequate herbivory has become a primary goal for many managers. However, herbivore biomass targets assume grazing behavior is consistent across different reef systems, even though relatively few have been studied. We document grazing behavior of two scarid species in Antigua, Barbuda, and Bonaire. Our analyses show significant differences in intraspecific feeding rates, time spent grazing, and intensity of grazing across sites, which may alter the ecological impact of a given scarid population. We suggest several hypothesized mechanisms for these behavioral variations that would benefit from explicit testing in future research. As managers set targets to enhance herbivory on reefs, it is critical that we understand potential differences in scarid grazing impact. Our findings demonstrate the variability of grazing behavior across different reef sites and call for further investigation of the drivers and ecological implications of these inconsistencies.

Diversity and Physiological Tolerance of Native and Invasive Jellyfish/Ctenophores along the Extreme Salinity Gradient of the Baltic Sea

Global change has led to manifold changes of marine ecosystems and biodiversity world-wide. While it has been shown that certain jellyfish and comb jelly species have increased regionally, it remains to be investigated if this is a general trend or localized phenomenon. Especially for the economically important Baltic Sea, which is characterized by an extreme physical environmental gradient, this question has not been addressed to date. Here we present a detailed account of the gelatinous macro-zooplankton community including their physiological tolerance towards abiotic conditions and resulting distribution ranges in the Baltic. We show that the arrival and establishment of non-indigenous species has led to a rising importance of jellyfish and comb jellies in the Baltic. This accounts for the comb jelly Mnemiopsis leidyi, which was first observed in Northern Europe in 2005, as well as for the hydromedusae Blackfordia virginica, first sighted in 2014. Both species have been shown to attain high population densities with pronounced grazing impact in other invasive regions. Given the current and anticipated changes of the physical environment of the Baltic Sea, especially ongoing warming, amplification of their impact can be expected.

Patterns of Dispersion, Movement and Feeding of the Sea Urchin Lytechinus variegatus, and the Potential Implications for Grazing Impact on Live Seagrass

The sea urchin Lytechinus variegatus is a known grazer of both living and dead tissue of turtlegrass, Thalassia testudinum, occasionally denuding large areas of seagrass. Field studies have attempted to assess effects of herbivory on seagrass by enclosing urchins at various densities. However, it is unclear how unrestricted urchins affect seagrass at lower densities more typically observed in the field. This study describes movement, feeding, and distribution of L. variegatus within beds of T. testudinum in St. Joseph Bay, Florida (USA) to quantify this urchin’s impact as a seagrass grazer. Urchins were absent from portions of seagrass beds closest to shore, present at low densities midway across the bed, and at highest densities (up to ~5 individuals/m2) at the offshore edge of the bed. Urchins tended not to aggregate, moved twice as rapidly where seagrass cover was reduced, and moved > 20X faster when placed in areas of open sand. Dead seagrass tissue occurred 4—30X more frequently on oral surfaces than living seagrass tissue. Fecal pellets with dead seagrass tissue were > 3X more common than pellets with live seagrass tissue. Injury to seagrass leaves was more common along dead leaf sections than live sections (> 2—10X). Overall, spatial distributions, movement, and diet indicate that L. variegatus at densities observed in this study would tend to have minimal effects on living seagrass. Episodic periods of denuding grassbeds reported in the literature suggest L. variegatus switches to live seagrass tissue as dead tissue becomes scarce during times of high urchin density.