Trophic sources and linkages to support mesozooplankton community in the Kuroshio of the East China Sea

Many migratory fishes reproduce and recruit around the Kuroshio, and their survival of early life stages is supported by mesozooplankton. Mesozooplankton standing stocks and productivity equivalent to those on the continental shelf have been found in the Kuroshio; however, there is limited information on trophic sources and linkages to support the mesozooplankton community. Here, we evaluate mesozooplankton feeding on protists and their trophodynamics importance by removal bottle experiments in the Kuroshio of the East China (ECS‐Kuroshio). Pico‐ and nano‐autotrophs dominated chlorophyll a concentrations throughout the study sites across the continental shelf, within the Kuroshio path and in adjacent waters. Calanoid and poecilostomatoid copepods comprised more than 85% of mesozooplankton biomass. Significant mesozooplankton ingestion rates were found for nano‐autotrophs based on size‐fractionated chlorophyll; for haptophytes, chrysophytes, chlorophytes, and diatoms from pigment‐based phytoplankton analysis; and for naked ciliates in the microzooplankton. Based on the estimates of individual carbon budgets, nano‐autotrophs and naked ciliates ingested by mesozooplankton composed 39% of their food requirements, suggesting other available prey like nano‐heterotrophs. These results imply that mesozooplankton ingestion in the ECS‐Kuroshio has great impacts on protozoan and phytoplankton communities and their major trophic pathways are from nano‐sized auto‐ and heterotrophs and ciliates to copepods.     

Mesoscale physical processes and the distribution and composition of ichthyoplankton on the southern Brazilian shelf break

The southern Brazilian shelf supports the largest fish stocks in the country, and studies on physical–biological processes in the ecology of ichthyoplankton have been recommended to provide a better understanding of the variability of the recruitment of fishing resources. This study is the first to examine the influence of mesoscale physical processes on the distribution of early life stages of fish in this shelf‐break region. Collections of fish eggs and larvae and measurements of temperature and salinity were made at 13 stations along cross‐shelf transects in December 1997. Myctophidae, Bregmacerotidae, Clupeidae, Synodontidae and Engraulidae were the most abundant larvae in the northern region, while Engraulidae and Bregmacerotidae prevailed further south. In situ hydrographic data, NOAA/AVHRR images and merged TOPEX/POSEIDON + ERS‐1/2 satellite altimetry taken during the cruise revealed an anticyclonic eddy dominating the shelf around 31°S. Larval fish abundance was lower at the centre of this feature, suggesting that the eddy advected poorer offshore waters of tropical origin towards the inner shelf‐concentrating the larvae around the eddy.     

Effects of littoral habitat complexity and sunfish composition on fish production

Carey MP, Maloney KO, Chipps SR, Wahl DH. Effects of littoral habitat complexity and sunfish composition on fish production. Ecology of Freshwater Fish 2010: 19: 466–476. © 2010 John Wiley & Sons A/S Abstract – Habitat complexity is a key driver of food web dynamics because physical structure dictates resource availability to a community. Changes in fish diversity can also alter trophic interactions and energy pathways in food webs. Few studies have examined the direct, indirect, and interactive effects of biodiversity and habitat complexity on fish production. We explored the effects of habitat complexity (simulated vegetation), sunfish diversity (intra‐ vs. inter‐specific sunfish), and their interaction using a mesocosm experiment. Total fish production was examined across two levels of habitat complexity (low: 161 strands m^?2 and high: 714 strands m^?2) and two sunfish diversity treatments: bluegill only (Lepomis macrochirus) and bluegill, redear sunfish (Lepomis microlophus), and green sunfish (Lepomis cyanellus) combination. We also measured changes in total phosphorus, phytoplankton, periphyton, and invertebrates to explain patterns in fish production. Bluegill and total fish production were unaffected by the sunfish treatments. Habitat complexity had a large influence on food web structure by shifting primary productivity from pelagic to a more littoral pathway in the high habitat treatments. Periphyton was higher with dense vegetation, leading to reductions in total phosphorus, phytoplankton, cladoceran abundance and fish biomass. In tanks with low vegetation, bluegill exhibited increased growth. Habitat complexity can alter energy flow through food webs ultimately influencing higher trophic levels. The lack of an effect of sunfish diversity on fish production does not imply that conserving biodiversity is unimportant; rather, we suggest that understanding the context in which biodiversity is important to food web dynamics is critical to conservation planning.     

Dynamic regulation of larval fish self‐recruitment in a marine protected area

The factors that regulate the self‐recruitment of fish larvae were explored in Cabrera National Park (CNP), an insular Marine Protected Area (MPA) located off southern Mallorca (Western Mediterranean). Our study attributes the regulation of larval arrival to the MPA to a combination of retention by topographically generated circulation patterns around the island and shelf break frontal dynamics. Specifically, within the shelf, interaction of the wind‐induced oscillatory flow with the island was shown to generate ephemeral recirculation patterns that, over time, favor larval retention in the proximity of the MPA. According to our measurements, oscillatory flows produced by wind‐forced island‐trapped waves (ITWs) dominate the flow around CNP. ITW‐forced dispersion simulations were in agreement with the observed distributions of several typical fish species that breed in the CNP. A second regulator of environmental variability is the influence of boundary currents and open ocean mesoscale structures at the shelf break. These structures generate frontal zones that are comparatively more long‐lived than inner‐shelf circulation patterns, and they were shown to act as barriers to the offshore dispersion of coastal fish larval assemblages. Finally, inferences from larval size distributions around the MPA together with particle‐tracking model simulations suggested the relevance of behavioral processes for larval recruitment to the MPA. Based on these observations, the waters around the CNP can be viewed as a relatively stable system that allows breeding fish populations to rely on self‐recruitment for their long‐term persistence, independent of other source populations along the coast of Mallorca.     

Interannual variation in Neocalanus biomass in the Oyashio waters of the western North Pacific

We examined the interannual variation in Neocalanus copepod biomass in the Oyashio waters in spring and summer from 1972 to 1999. In the mid‐1970s, mesozooplankton biomass in spring was high; however, it decreased significantly in the late 1970s. The timing of the decrease in mesozooplankton biomass corresponded to the 1976/77 climatic regime shift. The biomass of N. flemingeri, which dominated the Neocalanus community, was roughly constant from 1980 to 1999. Although species‐level estimates of Neocalanus biomass were not available for the 1970s, a previous study reported that Neocalanus copepods were the predominant mesozooplankton in the Oyashio waters in spring during the 1970s. Neocalanus copepods dominated the mesozooplankton community throughout the 1970s, and their biomass decreased in the late 1970s. Springtime net community production, an index of new production, also decreased in the late 1970s. We suggest that the reduction in new production negatively affected Neocalanus food availability, resulting decreased copepod biomass. New production may have been limited by a combination of subsurface iron supplies, increased vertical density gradient, and reduced vertical water mixing in winter, which resulted in diminished iron entrainment in winter. In summer, mesozooplankton biomass significantly decreased and increased synchronously with the 1976/77 and 1988/89 climatic regime shifts. The biomass of N. plumchrus, which dominated the Neocalanus community, was low in the 1980s and increased in the early 1990s. The biomass of the second‐most dominant copepod, N. cristatus, also increased in the early 1990s. Neocalanus copepods were reported to be a dominant component of the mesozooplankton community in the 1970s; Neocalanus biomass was high in the mid‐1970s and decreased in the late 1970s. Japanese sardine (Sardinops melanostictus), an important predator of Neocalanus copepods, exhibited interannual variation in standing stock that was inversely related to mesozooplankton biomass. At their peak in 1984, sardines consumed 32–138% of the daily Neocalanus production during summer. Therefore, predation pressure on Neocalanus by Japanese sardine is likely to affect interannual variation in mesozooplankton biomass during the summer.     

Setting the record straight on drivers of changing ecosystem states

This short communication is a response to the critique by Greene (2012), who puts forward the argument that the dynamics of northwest Atlantic continental shelf ecosystems are strongly influenced by changes in Arctic climate, as indexed by surface salinity. The argument, in its essence, discounts any indirect effects from over‐exploitation of top predators on lower trophic levels in northwest Atlantic ecosystems. Frank et al. (Science 308 , 2005, 1621; Nature 477 , 2011, 86) reported the existence of cascading trophic interactions with particular emphasis on the eastern Scotian Shelf. Greene argues that the events occurring in the Gulf of Maine/Georges Bank region are representative of all Northwest Atlantic shelf systems, despite previous research (Frank et al. Trends Ecol. Evol. 22 , 2007, 236; Petrie et al. Fish. Oceanogr. 18 , 2009, 83) that has shown a differential pattern of forcing ranging from top‐down in species‐poor, cold water systems to bottom‐up in warmer, more species‐rich systems, including Georges Bank.     

A model‐based meso‐zooplankton production index and its relation to the ocean survival of juvenile coho (Oncorhynchus kisutch)

The ocean survival of coho salmon (Oncorhynchus kisutch) off the Pacific Northwest coast has been related to oceanographic conditions regulating lower trophic level production during their first year at sea. Coastal upwelling is recognized as the primary driver of seasonal plankton production but as a single index upwelling intensity has been an inconsistent predictor of coho salmon survival. Our goal was to develop a model of upwelling‐driven meso‐zooplankton production for the Oregon shelf ecosystem that was more immediately linked to the feeding conditions experienced by juvenile salmon than a purely physical index. The model consisted of a medium‐complexity plankton model linked to a simple one‐dimensional, cross‐shelf upwelling model. The plankton model described the dynamics of nitrate, ammonium, small and large phytoplankton, meso‐zooplankton (copepods), and detritus. The model was run from 1996 to 2007 and evaluated on an interannual scale against time‐series observations of copepod biomass. The model’s ability to capture observed interannual variability improved substantially when the copepod community size distribution was taken into account each season. The meso‐zooplankton production index was significantly correlated with the ocean survival of hatchery coho salmon from the Oregon production area, although the coastal upwelling index that drove the model was not itself correlated with survival. Meso‐zooplankton production within the summer quarter (July–September) was more strongly correlated with coho survival than was meso‐zooplankton production in the spring quarter (April–June).     

Does upwelling intensity determine larval fish habitats in upwelling ecosystems? The case of Senegal and Mauritania

European sardine (Sardina pilchardus) and round sardinella (Sardinella aurita) comprise two‐thirds of total landings of small pelagic fishes in the Canary Current Eastern Boundary Ecosystem (CCEBE). Their spawning habitat is the continental shelf where upwelling is responsible for high productivity. While upwelling intensity is predicted to change through ocean warming, the effects of upwelling intensity on larval fish habitat expansion is not well understood. Larval habitat characteristics of both species were investigated during different upwelling intensity regimes. Three surveys were carried out to sample fish larvae during cold (permanent upwelling) and warm (low upwelling) seasons along the southern coastal upwelling area of the CCEBE (13°–22.5°N). Sardina pilchardus larvae were observed in areas of strong upwelling during both seasons. Larval habitat expansion was restricted from 22.5°N to 17.5°N during cold seasons and to 22.5°N during the warm season. Sardinella aurita larvae were observed from 13°N to 15°N during cold seasons and 16–21°N in the warm season under low upwelling conditions. Generalized additive models predicted upwelling intensity driven larval fish abundance patterns. Observations and modeling revealed species‐specific spawning times and locations, that resulted in a niche partitioning allowing species' co‐existence. Alterations in upwelling intensity may have drastic effects on the spawning behavior, larval survival, and probably recruitment success of a species. The results enable insights into the spawning behavior of major small pelagic fish species in the CCEBE. Understanding biological responses to physical variability are essential in managing marine resources under changing climate conditions.     

The importance of episodic weather events to the ecosystem of the Bering Sea shelf

Climate variability on decadal time scales is generally recognized to influence high‐latitude marine populations. Our recent work in studying air–sea interactions in the Bering Sea suggests that interannual to decadal climate variability is important through its modulation of the frequencies and magnitudes of weather events on intraseasonal time scales. We hypothesize that it is these weather events that directly impact the marine ecosystem of the Bering Sea shelf. The linkages between the event‐scale weather and the ecosystem are illustrated with three examples: walleye pollock (Theragra chalcogramma), Tanner crabs (Chionoecetes bairdi), and coccolithophorid phytoplankton (Emiliania huxleyi). We hypothesize that the strong recruitment of walleye pollock that occurred in 1978, 1982, and 1996 can be attributed in part due to the seasonably strong storms that occurred in the early summer of those years. These storms caused greater than normal mixing of nutrients into the euphotic zone which presumably led to sustained primary productivity after the spring bloom and, possibly, enhanced prey concentrations for pollock larvae and their competitors. Recruitment of Tanner crab was particularly strong for the 1981 and 1984 year‐classes. These years had periods of prominent east wind anomalies along the Alaska Peninsula during the previous winter. Such winds promote flow through Unimak Pass, and hence an enhanced flux of nutrient‐rich water onto the shelf. This mechanism may have ultimately resulted in favorable feeding conditions for Tanner crab larvae. Finally, an unprecedented coccolithophorid bloom occurred over the Bering Sea shelf in the summer of 1997. This summer featured lighter winds and greater insolation than usual after a spring that included a very strong May storm. This combination brought about a warm, nutrient‐poor upper mixed layer by mid‐summer. This provided a competitive advantage for coccolithophorid phytoplankton in 1997 and to a lesser extent in 1998. Unusually high concentrations of coccolithophores persisted for the following two years although physical environmental conditions did not remain favorable. While slow variations in the overall aspects of the physical environment may be important for setting the stage, we propose that the significant multi‐year adjustments in the marine ecosystem of the Bering Sea shelf are more directly caused by major air–sea interaction events on intraseasonal time scales.     

Effects of cage fish farming on phytoplankton community structure,biomass and primary production in an aquaculture area in the middle Adriatic Sea

In this study, we investigated the physical, chemical and biological parameters of the water column at two stations under different trophic conditions during a 1‐year period (from September 2005 to September 2006) in an aquaculture area. The investigation was performed monthly at a station inside the fish farm in Maslinova Bay (middle Adriatic Sea, Croatia) and at a control station located in the Hvar Channel in the vicinity of the farm. The results showed that the concentrations of dissolved nutrients were slightly enhanced at the farm station in comparison with the control station, although those differences were not statistically significant, nor were the differences in oxygen saturation recorded during the investigation period. As opposed to the chemical parameters, considerably higher deviations were recorded for biological parameters as demonstrated by a significant increase in phytoplankton biomass and primary production (measured in situ) at the farm station, particularly during the summer season. Phytoplankton community structure analysis showed that the phytoplankton composition at the farm was mostly determined by seasonal changes rather than by the influence of fish farming. In Maslinova Bay, no causal links were identified between increased biomass and the appearance of toxic species.