Variability Of Hydrographic Conditions Research Articles

Cold Regime interannual variability of primary and secondary producer community composition in the southeastern Bering Sea.

Variability of hydrographic conditions and primary and secondary productivity between cold and warm climatic regimes in the Bering Sea has been the subject of much study in recent years, while interannual variability within a single regime and across multiple trophic levels has been less well-documented. Measurements from an instrumented mooring on the southeastern shelf of the Bering Sea were analyzed for the spring-to-summer transitions within the cold regime years of 2009–2012 to investigate the interannual variability of hydrographic conditions, primary producer biomass, and acoustically-derived secondary producer and consumer abundance and community structure. Hydrographic conditions in 2012 were significantly different than in 2009, 2010, and 2011, driven largely by increased ice extent and thickness, later ice retreat, and earlier stratification of the water column. Primary producer biomass was more tightly coupled to hydrographic conditions in 2012 than in 2009 or 2011, and shallow and mid-column phytoplankton blooms tended to occur independent of one another. There was a high degree of variability in the relationships between different classes of secondary producers and hydrographic conditions, evidence of significant intra-consumer interactions, and trade-offs between different consumer size classes in each year. Phytoplankton blooms stimulated different populations of secondary producers in each year, and summer consumer populations appeared to determine dominant populations in the subsequent spring. Overall, primary producers and secondary producers were more tightly coupled to each other and to hydrographic conditions in the coldest year compared to the warmer years. The highly variable nature of the interactions between the atmospherically-driven hydrographic environment, primary and secondary producers, and within food webs underscores the need to revisit how climatic regimes within the Bering Sea are defined and predicted to function given changing climate scenarios.

Spatio-temporal dynamics of ocean conditions and forage taxa reveal regional structuring of seabird–prey relationships.

Studies of predator–prey demographic responses and the physical drivers of such relationships are rare, yet essential for predicting future changes in the structure and dynamics of marine ecosystems. Here, we hypothesize that predator–prey relationships vary spatially in association with underlying physical ocean conditions, leading to observable changes in demographic rates, such as reproduction. To test this hypothesis, we quantified spatio-temporal variability in hydrographic conditions, krill, and forage fish to model predator (seabird) demographic responses over 18 years (1990–2007). We used principal component analysis and spatial correlation maps to assess coherence among ocean conditions, krill, and forage fish, and generalized additive models to quantify interannual variability in seabird breeding success relative to prey abundance. The first principal component of four hydrographic measurements yielded an index that partitioned “warm/weak upwelling” and “cool/strong upwelling” years. Partitioning of krill and forage fish time series among shelf and oceanic regions yielded spatially explicit indicators of prey availability. Krill abundance within the oceanic region was remarkably consistent between years, whereas krill over the shelf showed marked interannual fluctuations in relation to ocean conditions. Anchovy abundance varied on the shelf, and was greater in years of strong stratification, weak upwelling and warmer temperatures. Spatio-temporal variability of juvenile forage fish co-varied strongly with each other and with krill, but was weakly correlated with hydrographic conditions. Demographic responses between seabirds and prey availability revealed spatially variable associations indicative of the dynamic nature of “predator–habitat” relationships. Quantification of spatially explicit demographic responses, and their variability through time, demonstrate the possibility of delineating specific critical areas where the implementation of protective measures could maintain functions and productivity of central place foraging predators.

Springtime size-fractionated primary production across hydrographic and PAR-light gradients in Chilean Patagonia (41–50°S)

We combined on-deck and in situ measurements and satellite-derived data to study the spatial variability of springtime size-fractionated primary production and chlorophyll-a biomass along gradients of hydrographic conditions and surface PAR in central and northern Chilean Patagonia (41–50°S). This extensive and fragmented region encompasses numerous fjords and channels, as well as the northern and southern icefields (46–47°S, 48–52°S). Primary production displayed a latitudinal pattern decreasing southwards (6-fold lower), particularly toward areas influenced by rivers with a nival regime. Micro-phytoplankton (>20μm) dominated the primary production (57–93%) and chlorophyll-a (43–91%) of northern sites, where warmer and more saline surface waters exhibit greater PAR irradiance. Small phytoplankton cells (<2μm; 2–20μm) contributed>50% of carbon fixation and chlorophyll-a in the southernmost sites, especially those located near glaciers and major rivers, where surface temperature, salinity, and PAR irradiance were lowest. The long-term (2002–2012) average field of springtime PAR derived from satellite imagery showed a southward increase in longitudinal gradients, which indicates that spatial changes in surface light attenuation along this region are largely driven by glacier-derived freshwater inputs. A principal component analysis of surface temperature, salinity, and PAR produced an ordination of sites that was consistent with spatial changes in the balance of oceanic versus riverine influence on surface conditions along this region. Total primary production was significantly correlated (r=0.61, p=0.007) with the first principal component, which explained 65% of joint variability in hydrographic conditions and PAR. The same principal component clearly separated sites in northern Patagonia where micro-phytoplankton dominated total primary production – along the Reloncavi fjord and Inner Sea of Chiloe – from those located further south where other size fractions were equally or more important. We stress the need to include spatial variability in nutrient concentrations, which together with the strong light attenuation induced by glacier-derived freshwater may further explain the spatial patterns in primary productivity, phytoplankton biomass, and carbon fluxes along Chilean Patagonia.

Temperature effects on vital rates of different life stages and implications for population growth of Baltic sprat

Baltic sprat (Sprattus sprattus balticus S.) is a key species in the pelagic ecosystem of the Baltic Sea. Most stocks of small pelagic species are characterized by natural, fishery-independent fluctuations, which make it difficult to predict stock development. Baltic sprat recruitment is highly variable, which can partly be related to climate-driven variability in hydrographic conditions. Results from experimental studies and field observations demonstrate that a number of important life history traits of sprat are affected by temperature, especially the survival and growth of early life stages. Projected climate-driven warming may impact important processes affecting various life stages of sprat, from survival and development during the egg and larval phases to the reproductive output of adults. This study presents a stage-based matrix model approach to simulate sprat population dynamics in relation to different climate change scenarios. Data obtained from experimental studies and field observations were used to estimate and incorporate stage-specific growth and survival rates into the model. Model-based estimates of population growth rate were affected most by changes in the transition probability of the feeding larval stage at all temperatures (+0, +2, +4, +6 °C). The maximum increase in population growth rate was expected when ambient temperature was elevated by 4 °C. Coupling our stage-based model and more complex, biophysical individual-based models may reveal the processes driving these expected climate-driven changes in Baltic Sea sprat population dynamics.

Regional differences in the response of mesozooplankton to oceanographic regime shifts in the northeast Asian marginal seas

To understand responses of marine ecosystems to climate changes in the northwestern Pacific, especially responses to the 1998 regime shift, we related month-specific variability in hydrographic conditions to long-term changes in mesozooplankton in four regions adjacent to the Korean peninsula: the eastern Yellow Sea (EYS), northern East China Sea (NECS), and southwestern and northwestern Japan/East Sea (SJES and NJES). Sea surface (10-m depth) temperature in February has increased since the early 1990s in all four regions. Sea surface temperature in April and June has increased since the late 1990s in the SJES and EYS. Surface salinity has decreased, especially since the late 1990s, except NJES. Biomass of mesozooplankton in Korean sea regions (the EYS, NECS and SJES) began to increase after the early 1990s, with sharp increases after the late 1990s, indicating a regime shift triggered by the increased seawater temperatures. Unusually higher biomass was also occasionally observed in April, June or October after the late 1990s in the EYS and NECS. Abundances of the four major zooplankton groups (copepods, amphipods, chaetognaths and euphausiids) have generally increased since the late 1990s. The pattern of change in zooplankton abundance varied depending on taxonomic group and region, but we concluded that the four seas responded to the 1989 or 1998 regime shifts with respect to water temperature, salinity and zooplankton. We detected an additional sudden shift in both the zooplankton community and fisheries catch in the Korean waters, which we speculated was associated with the strong 1982/1983 El Niño event. The 1982–1983 shift was characterized by increased dominance of copepods, and was pronounced in the EYS, which is strongly influenced by river discharge. The results highlight the need for further retrospective analyses of regional ecosystems.

Seasonal and interannual dynamics in diatom production in the Cariaco Basin, Venezuela

We examine the diatom flux collected between November 1996 and April 1998, and between January and October 1999 at the time-series study site in the Cariaco Basin, off Venezuela. The temporal dynamics of the total diatom flux mainly reflect seasonal, trade wind-driven changes in surface hydrographic conditions, including changes associated with the El Niño/Southern Oscillation (ENSO). Highest diatom fluxes (>1.8×10 7 valves m −2 d −1) coincided with the upwelling season in boreal winters 1997 and 1999. Changes in the composition of the diverse diatom community reflect variations in hydrographic and atmospheric conditions, as well as nutrient availability. Cyclotella litoralis, a neritic diatom typical of nutrient-rich waters, along with resting spores of several Chaetoceros spp., dominate during periods of high diatom flux, following trade wind-driven upwelling. During the boreal summers of 1997 and 1999, nutrient-depleted surface waters resulted in low diatom fluxes (<5.2×10 6 valves m −2 d −1). The seasonal pattern of high diatom production was altered from July 1997 through April 1998, when the ENSO affected the Caribbean Sea. The occurrence of ENSO during boreal winter 1997–1998 caused a major change in the qualitative composition of the diatom assemblage: the highly diverse diatom assemblage was composed of a mixture of pelagic ( Nitzschia bicapitata, Thalassionema nitzschioides var. inflata, T. nitzschioides var . parva, Azpeitia tabularis) and coastal species ( C. litoralis, resting spores of Chaetoceros, T. nitzschioides var. nitzschioides). The simultaneous occurrence of neritic and open-ocean diatoms during boreal summers reflects the fact that the Cariaco Basin is influenced by both offshore and coastal waters, with considerable short-term variability in hydrographic conditions and nutrient availability.

Variability in hydrographic conditions to the east and northwest of South Georgia, 1996–2001

Six years of high-resolution hydrographic data from the eastern and northwestern sides of South Georgia (southwest Atlantic) are used to study the changing circulation and water mass properties of the region. One year of data from these locations was used previously to describe the oceanographic conditions at those times; using the much greater volume of data now available, we identify which features appear temporally robust and which are transient, and begin addressing topics relating to the forcing of the inter-annual variability and the potential consequences for the local ecosystem. Waters on the shelf and those over the adjacent deep ocean invariably have different hydrographic properties, though the transition between them can be abrupt or gradual. The onshelf/offshelf differences vary greatly from year to year, due to the combined influences of local and remote processes. There are several instances of strong physical coupling between the eastern and northwestern sides of South Georgia; this offers potential for distinguishing physically-induced ecosystems changes separately from biologically-induced ecosystems changes. On the northeast side of the shelf, close to Cumberland Bay, there is evidence of an often intense, but variable, cyclonic circulation that is the result of interaction with the local bathymetry. This may act as a retention mechanism, and enhance local productivity. Two examples of extreme cold anomalies are present in the series of measurements. One of these (in 2000/2001) affected a limited area at the eastern side of the region surveyed and was due to an intrusion of the Southern Antarctic Circumpolar Current Front. The other (in early 1998) was due to the combined effects of the passage of a large-scale ocean anomaly that had its origins upstream in the Pacific Ocean, and strong air/sea interaction. Both of these were associated with the strong 1997/1998 El Niño event. Whilst previous studies have observed a link between El Niño forcing and ocean response around South Georgia with a temporal lag of around 3 years, we observe a much more rapid response to the extremely strong 1997/1998 El Niño event. This indicates that the ocean and ecosystem around South Georgia are more immediately susceptible to extreme instances of remote climatic forcing than had been supposed.

Timing of deep convection in the Greenland and Iceland Seas

This paper reviews published information on the timing of hydrographic and atmospheric variations in the Iceland and Greenland Seas. Large scale variations in hydrography, advection features, regional convection and their general impact or connection with climate and hydro-biological conditions will be considered briefly in the area of the Subarctic Gyre in the northern North Atlantic and the Nordic Seas. The intermediate convection in the Iceland Sea may have ceased during the so-called “ice years” in North Icelandic waters in the late sixties (1965-1970). Convection to the bottom in the Greenland Sea seems to have ceased in the seventies or after 1972. A clear linkage between the period of the “Great Salinity Anomaly”, that was first observed in the Iceland Sea around 1968 and was advected through the Subarctic Gyre and returned to East Greenland waters and North Icelandic waters in 1981, and the timing of reduction of convection in the Greenland Sea is thus not obvious. Further detailed analysis is needed to resolve the questions about processes involved in the variability of hydrographic conditions in the northern North Atlantic and the Nordic Seas. It should be noted that conditions for convection may not only be related to atmospheric conditions or large scale advection of water characteristics but also to regional or local hydrographic conditions.

Distribution and abundance of larval fishes collected in the western Bransfield Strait region, 1986–87

Larval fishes were obtained from 0–50 m Bongo net and stratified 0–200 m Nansen net samples collected in the western Bransfield Strait region during four RACER (Research on Antarctic Coastal Ecosystem Rates) program cruises, December 1986 to March 1987. Three nototheniid species, Nototheniops larseni, Notothenia gibberifrons, and a form tentatively identified as Tremtomus scotti numerically dominated the total catch. Two typically abundant species in this area, Pleuragramma antarcticum and Notothenia kempi, were relatively uncommon. Gear differences, the restricted geographical coverage, and between-year variability in hydrographic conditions may explain species dominance differences between the RACER samples and prior surveys in the Antarctic Peninsula area. Largest abundances occurred in the vicinity of Gerlache Strait and island shelf areas; larvae were infrequently collected in Drake Passage waters. Individual species demonstrated different distributional patterns, suggesting that water mass influence, larval depth distribution and behavioral differences may be involved. Estimated January to March growth rates for N. larseni (0.06 mm day −1) and T. scotti (0.05 mm day −1) are similar but low compared to that of N. gibberifrons (0.12 mm day −1).