Deep-water Nutrient Research Articles

Distinct patterns of sedimentary phosphorus fractionation and mobilization in the seafloor of the Black Sea, Marmara Sea and Mediterranean Sea

Phosphorus (P) is a key element to all life that is used for structural and functional component of all organisms. The cycling of sedimentary P may differ depending on the redox-conditions of the overlying waters affecting the dynamics, and distribution of P-fractions and the elements that are highly coupled to P cycle. Though biogeochemistry of water column in the three interconnected marine basins of Black, Marmara and Mediterranean Seas have been studied extensively, few studies were carried out to understand sedimentary P dynamics in these regions. In this study, therefore, the biogeochemical cycling of sedimentary P and related variables such as porewater nutrients, sedimentary organic carbon, nitrogen and reactive iron were studied in selected sites at the three interconnected marine basins: Black Sea, Marmara and Northeastern (NE) Mediterranean Sea. The pool of “potentially mobile P” was also determined for the studied sites. The study results showed that porewater and sediment biogeochemistry displayed great variability in the studied sites with the maximum concentrations of porewater phosphate, ammonium, reactive silicate, surface sediment organic carbon, nitrogen, phosphorus and total phosphorus (TP) measured in the hypoxic Marmara Sea and suboxic/anoxic Black Sea. The decline in the TP concentrations of all sediment core samples indicated P-mobilization to the overlying water. The pool of “potentially mobile P” varied between 0.023 and 0.148 mol/m2 in the studied sites with the maximum values recorded in suboxic and anoxic/sulfidic parts of the Black Sea. This study predicts that the deoxygenation and eutrophication would further lead to the preferential release of P in these three interconnected marine basins, hence changing the remineralization, N/P molar ratios and eventually transform the deep-water nutrient stocks with implications for internal N/P control on marine ecosystems.

Nutrient co‐limitation in the subtropical Northwest Pacific

AbstractNutrients limiting phytoplankton growth in the ocean are a critical control on ocean productivity and can underpin predicted responses to climate change. The extensive western subtropical North Pacific is assumed to be under strong nitrogen limitation, but this is not well supported by experimental evidence. Here, we report the results of 14 factorial nitrogen–phosphorus–iron addition experiments through the Philippine Sea, which demonstrate a gradient from nitrogen limitation in the north to nitrogen–iron co‐limitation in the south. While nitrogen limited sites responded weakly to nutrient supply, co‐limited sites bloomed with up to ~60‐fold increases in chlorophyll a biomass that was dominated by initially undetectable diatoms. The transition in limiting nutrients and phytoplankton growth capacity was driven by a gradient in deep water nutrient supply, which was undetectable in surface concentration fields. We hypothesize that this large‐scale phytoplankton response gradient is both climate sensitive and potentially important for regulating the distribution of predatory fish.

Non-Redfieldian C:N:P ratio in the inorganic and organic pools of the Bay of Bengal during the summer monsoon

Nitrogen (N) and phosphorus (P) determine the strength of the ocean’s biological carbon (C) pump, and variation in the N:P ratio is key to phytoplankton growth. A fixed C:N:P ratio (106:16:1) in organic matter and deep-water nutrients was observed by Alfred C. Redfield. However, recent studies have challenged the concept of the Redfield Ratio, and its veracity remains to be examined in oceanic basins like the Bay of Bengal. For this purpose, we sampled the water in the Bay of Bengal for C, N, and P content in the organic and inorganic pools from the surface to 2000 m. Overall, the C:N:P ratio deviated greatly from the Redfield Ratio. The C:N:P ratio in particulate organic matter varied from 232:25:1 in the top layer (surface to the depth of the chlorophyll maximum) to 966:72:1 in the deep water (300-2000 m). In dissolved organic matter, the ratio varied from 357:30:1 in the top layer to 245:66:1 in the deep water. The N:P ratio in nutrients varied from 3 in the top layer to 12 in the deep water. The nutrient-depleted top layer (average NO3- + NO2- ~ 0.7 µmol l-1) with a low N:P ratio coupled with reported low primary production rates in the Bay suggested that the production was N limited. Concurrent N2 fixation rates were not sufficient to alter the observed C:N:P ratio. Eddies showed a mixed effect on the C:N:P ratio. Our C:N:P ratios in particulate organic matter are comparable to other tropical basins and supports the nutrient supply hypothesis for low latitude ecosystems.

Development of Lake Vesij\xe4rvi through four decades of remediation efforts

The diversion of sewage inputs in the mid-1970s led to an order of magnitude reduction in nutrient loading to Lake Vesijärvi, southern Finland. After the diversion, nutrient concentrations declined, consistent with a simple dilution model, and by the mid-1990s the chlorophyll concentration was reduced by 80%. The favourable development was supported by a 5-year mass removal of planktivorous and benthivorous fish and the stocking of predatory pikeperch (Sander lucioperca (L.)), although the exact mechanisms behind their effects remain obscure. Starting in 2010, oxygen-rich water from the top of the water column was pumped to the deepest parts of the lake, resulting in high deepwater oxygen concentration in winter. In summer, hypoxic or even anoxic conditions could not be avoided, but the duration of the anoxic period was markedly shortened. Because nitrate was never depleted, leaching of total nitrogen from the sediment was reduced and the same was also true for total phosphorus, but only in winter. The oxygenation stabilized deepwater nutrient concentrations to a low level, but this was not reflected in the epilimnetic total nutrient concentration or in a further decrease in the chlorophyll concentration.

Zooplankton-based δ13C and δ15N isoscapes from the outer continental shelf and slope in the subtropical western South Atlantic

Characterizing the patterns of stable carbon (δ13C) and nitrogen (δ15N) isotopes at the base of the food webs (baseline) is essential in ecological studies for assessing the feeding habits and migration patterns of marine predators. We analysed δ13C and δ15N in zooplankton samples collected in spring and autumn (2012–2015), along the south (SCM) and southeast (NCSM) Brazilian oceanic waters. An increase in δ13C from the shelf break towards the offshore was associated with the influence of continental water input and upwellings along the shelf break, which introduce 13C-depleted inorganic carbon to the surface, where it will be incorporated into the biological system through phytoplankton growth. An opposite trend was observed in δ15N, with higher values along the shelf break related to the intrusion of upwelled deep-water nutrients, contrasting with the oligotrophic offshore waters where N2 fixation takes place, resulting in primary and secondary production that is relatively more depleted in 15N. A latitudinal (north-south) decrease in δ13C along the offshore area coincided with known isotopic patterns in inorganic carbon, which were more 13C-depleted towards higher latitudes. On the other hand, an increase in δ15N was observed towards the south, reflecting the contrast between the N2 fixation in the more oligotrophic waters in the NCSM and the higher nutrient availability in the SCM. Additionally, δ13C was significantly higher in autumn and δ15N was higher in spring. Although inter-annual differences in δ15N were non-significant, δ13C was significantly lower in 2012 than in the later years. This study provides novel information regarding the patterns of baseline δ13C and δ15N in the southwestern Atlantic Ocean, which will be useful for future investigation of the ecology of higher trophic-level organisms occurring in this area.

Ocean greening off Hawai'i

Marine Science From June to August 2018, the eruption of Kīlauea volcano triggered a diatom-dominated phytoplankton bloom. Wilson et al. set sail to sample the plume, deploying subsea gliders and using satellite monitoring to measure the dynamics of this rare event in the nutrient-poor Pacific (see the Perspective by Ducklow and Plank). They found subsurface chlorophyll maxima not visible by remote sensing, performed transcriptome and N isotope marker analysis, and measured nutrients, partitioning of biomass into different organisms, and primary production. Much of the data are corroborated by physical modeling of the ocean dynamics. The authors conclude that the plume was fed by the lava heating subsurface water and triggering upwelling of deepwater nutrients to the surface rather than by direct injection of micronutrients from lava. Science , this issue p. [1040][1]; see also p. [978][2] [1]: /lookup/doi/10.1126/science.aax4767 [2]: /lookup/doi/10.1126/science.aay8088

Deep Water Nutrient Supply for an Offshore Ulva sp. Cultivation Project in the Eastern Mediterranean Sea: Experimental Simulation and Modeling

Offshore cultivation of marine macroalgae is a potential sustainable resource for fuel, food, and chemicals. Offshore, the high productivity of macroalgae cultivation depends on external nitrogen supply. The current work examines the idea of supplying nitrogen for Ulva sp. cultivation in the oligotrophic Eastern Mediterranean Sea (EMS) by artificial upwelling of nutrient-rich deep seawater (DSW). Growth rates, protein, and starch contents of Ulva sp. were measured for time varying fertilizations with nitrate concentrations corresponding to nutrient concentrations of DSW at increasing depths of the EMS. A maximal relative growth rate of 7.4% was measured for fertilizing ten times per week with 5.8 μM, which corresponds to the artificial upwelling from the depth of 700 m at EMS. Protein and starch contents ranged between 1–6 and 8–15% of dry weight. Finally, yields and energetic costs of DSW pumping were modeled for an example case of 10-ha offshore farm. The model predicts a high productivity but low energetic efficiency, which can be improved by coupling the biomass production with offshore power sources such as ocean thermal energy conversion.

Pelagic Subsidies Underpin Fish Productivity on a Degraded Coral Reef

Coral reefs harbor high productivity in nutrient-poor tropical oceans. This exceptional productivity can be explained by high recycling rates [1, 2], deep-water nutrient enrichment [3], and assimilation of external production [4]. Fishes consume this productivity through multiple trophic pathways and, as a result, dominate consumer biomass. Their reliance on pelagic versus benthic productivity pathways has been quantified from the tissues of individual fish [5, 6], but the contribution of different energetic pathways to the total productivity of coral reef fishassemblages remains unquantified. Here, we combined high-resolution surveys and individual biomass production estimates to generate the first energetic map of a full coral reef fish assemblage, from the smallest to the largest fishes [7, 8]. We found that the windward section of a coral reef on the Great Barrier Reef delivered an average fish productivity of 4.7kg ha-1 day-1, of which 41% was derived from water column photosynthesis, 29% by the epibenthic reef surface, 14% from cryptobenthic microhabitats, and 11% from adjacent sandy areas. The critical energetic contribution of pelagic subsidies would remain undetected if considering fish standing biomass alone, because the high productivity of reef planktivores originated from a relatively small biomass. Importantly, this study took place on a reef with only ∼6% of coral cover following multiple coral mortality events. Thus, our study offers hope that reefs subject to coral loss can still maintain considerable fish productivity, with planktivorous fishes providing major pelagic subsidies.

Secchi depths in lakes of Cape Cod National Seashore from 1996–2016 and relationships with morphometry, water chemistry, and housing densities

ABSTRACTSmith SM, Fox SE, Lee KD, Medeiros K, Plaisted HC. 2017. Secchi depths in lakes of Cape Cod National Seashore from 1996 to 2016 and relationships with morphometry, water chemistry, and housing densities. Lake Reserv Manage. 34:88–103.The landscape of Cape Cod (Massachusetts, USA) is scattered with numerous freshwater lakes (known as kettle ponds) that offer abundant recreational opportunities and provide important habitat for a wide variety of flora and fauna. The rapid urbanization of Cape Cod has been cause for concern regarding potential impacts on these waterbodies. Septic system delivery of nutrients to the ponds and atmospheric nitrogen deposition from fossil fuel combustion has been high for many decades in this region. These systems have also undergone significant shifts in pH and temperature due to regional changes in air quality and climate warming. All factors have the potential to alter pond trophic state and, consequently, water clarity. In this study, we examined August trends in kettle pond Secchi depths within Cape Cod National Seashore from 1996 to 2016. We further analyzed relationships by regression analysis between recent (2013–2014) measurements of Secchi depths and water quality constituents, physical features, shoreline housing densities, and the occurrence of gulls, fish stocking, and herring spawning. This provided a “snapshot” portrait of the current state of these ponds and their functioning. The results suggest that water clarity has remained unchanged or has even increased over the last 2 decades. Water clarity was negatively related to deep-water nutrient concentrations (total nitrogen and phosphorus), which themselves were negatively correlated with pond depth. The occurrence of gulls and anadromous and stocked fish populations had no apparent effect on water clarity, nor did any of the variables that described housing densities around ponds.

Novel techniques and insights into the deployment of pop-up satellite archival tags on a small-bodied deep-water chondrichthyan

Acquiring movement data for small-bodied, deep-water chondrichthyans is challenged by extreme effects of capture and handling stress, and post-release predation, however, it is urgently required to examine important fisheries interactions and assess the ecological role of these species within deep-water food webs. Here we suggest a novel release-cage mechanism to deploy pop-up satellite archival tags, as well as present vertical habitat data for a data-deficient, small-bodied, deep-water bycatch species, the Cuban dogfish (Squalus cubensis). Data were gathered from seven of eight High Rate X-Tags deployed on mature Cuban dogfish in the Exuma Sound, The Bahamas. Recovery periods appeared variable between individuals and are likely driven by capture-and-handling stress and tag burden. Application of the cross-correlation function to time-series depth and temperature data indicated three of the seven individuals suffered mortality through predation, which occurred during daytime, and suggests Cuban dogfish may constitute a proportion of deep-water apex predator diet in the Exuma Sound. Two animals were successfully released via a novel release-cage mechanism and displayed either no, or rapid (<15mins) vertically stationary recovery periods and were not consumed by predators; data for these individuals were recorded for the entire deployment duration (14 days). Vertical habitat data suggests Cuban dogfish are diel-vertical migrators, similar to other deep-water taxa, and exhibit a relatively broad temperature and depth range, which may be driven by preference for specific bathymetric structures. These techniques provide an important first step into acquiring and presenting vertical habitat data for small-bodied, deep-water chondrichthyans, which can be directly applied to fisheries and ecosystem-based management approaches.

Solubility of aerosol trace elements: Sources and deposition fluxes in the Canary Region

African dust inputs have important effects on the climate and marine biogeochemistry of the subtropical North Atlantic Ocean. The impact of dust inputs on oceanic carbon uptake and climate is dependent on total dust deposition fluxes as well as the bioavailability of nutrients and metals in the dust. In this work, the solubility of trace metals (Fe, Al, Mn, Co and Cu) and ions (Ca, sulphate, nitrate and phosphate) has been estimated from the analysis of a long-time series of 109 samples collected over a 3-year period in the Canary Islands. Solubility is primarily a function of aerosol origin, with higher solubility values corresponding to aerosols with more anthropogenic influence. Using soluble fractions of trace elements measured in this work, atmospheric deposition fluxes of soluble metals and nutrients have been calculated. Inputs of dissolved nutrients (P, N and Fe) have been estimated for the mixed layer. Considering that P is the limiting factor when ratios of these elements are compared with phytoplankton requirements, an increase of 0.58 nM of P in the mixed layer (∼150 m depth) and in a year can be estimated, which can support an increase of 0.02 μg Chla L−1 y−1. These atmospheric inputs of trace metals and nutrients appear to be significant relative to the concentrations reported in this region, especially during the summer months when the water column is more stratified and deep-water nutrient inputs are reduced.

Carbon and nitrogen stable isotope ratios of pelagic zooplankton elucidate ecohydrographic features in the oligotrophic Red Sea

Although zooplankton occupy key roles in aquatic biogeochemical cycles, little is known about the pelagic food web and trophodynamics of zooplankton in the Red Sea. Natural abundance stable isotope analysis (SIA) of carbon (δ13C) and N (δ15N) is one approach to elucidating pelagic food web structures and diet assimilation. Integrating the combined effects of ecological processes and hydrography, ecohydrographic features often translate into geographic patterns in δ13C and δ15N values at the base of food webs. This is due, for example, to divergent 15N abundances in source end-members (deep water sources: high δ15N, diazotrophs: low δ15N). Such patterns in the spatial distributions of stable isotope values were coined isoscapes. Empirical data of atmospheric, oceanographic, and biological processes, which drive the ecohydrographic gradients of the oligotrophic Red Sea, are under-explored and some rather anticipated than proven. Specifically, five processes underpin Red Sea gradients: (a) monsoon-related intrusions of nutrient-rich Indian Ocean water; (b) basin scale thermohaline circulation; (c) mesoscale eddy activity that causes up-welling of deep water nutrients into the upper layer; (d) the biological fixation of atmospheric nitrogen (N2) by diazotrophs; and (e) the deposition of dust and aerosol-derived N. This study assessed relationships between environmental samples (nutrients, chlorophyll a), oceanographic data (temperature, salinity, current velocity [ADCP]), particulate organic matter (POM), and net-phytoplankton, with the δ13C and δ15N values of zooplankton collected in spring 2012 from 16°28′ to 26°57′N along the central axis of the Red Sea. The δ15N of bulk POM and most zooplankton taxa increased from North (Duba) to South (Farasan). The potential contribution of deep water nutrient-fueled phytoplankton, POM, and diazotrophs varied among sites. Estimates suggested higher diazotroph contributions in the North, a greater contribution of POM in the South, and of small phytoplankton in the central Red Sea. Consistent variation across taxonomic and trophic groups at latitudinal scale, corresponding with patterns of nutrient stoichiometry and phytoplankton composition, indicates that the zooplankton ecology in the Red Sea is largely influenced by hydrographic features. It suggests that the primary ecohydrography of the Red Sea is driven not only by the thermohaline circulation, but also by mesoscale activities that transports nutrients to the upper water layers and interact with the general circulation pattern. Ecohydrographic features of the Red Sea, therefore, aid in explaining the observed configuration of its isoscape at the macroecological scale.

C : N : P stoichiometry at the Bermuda Atlantic Time-series Study station in the North Atlantic Ocean

Abstract. Nitrogen (N) and phosphorus (P) availability, in addition to other macro- and micronutrients, determine the strength of the ocean's carbon (C) uptake, and variation in the N : P ratio of inorganic nutrient pools is key to phytoplankton growth. A similarity between C : N : P ratios in the plankton biomass and deep-water nutrients was observed by Alfred C. Redfield around 80 years ago and suggested that biological processes in the surface ocean controlled deep-ocean chemistry. Recent studies have emphasized the role of inorganic N : P ratios in governing biogeochemical processes, particularly the C : N : P ratio in suspended particulate organic matter (POM), with somewhat less attention given to exported POM and dissolved organic matter (DOM). Herein, we extend the discussion on ecosystem C : N : P stoichiometry but also examine temporal variation in stoichiometric relationships. We have analyzed elemental stoichiometry in the suspended POM and total (POM + DOM) organic-matter (TOM) pools in the upper 100 m and in the exported POM and subeuphotic zone (100–500 m) inorganic nutrient pools from the monthly data collected at the Bermuda Atlantic Time-series Study (BATS) site located in the western part of the North Atlantic Ocean. C : N and N : P ratios in TOM were at least twice those in the POM, while C : P ratios were up to 5 times higher in TOM compared to those in the POM. Observed C : N ratios in suspended POM were approximately equal to the canonical Redfield ratio (C : N : P = 106 : 16 : 1), while N : P and C : P ratios in the same pool were more than twice the Redfield ratio. Average N : P ratios in the subsurface inorganic nutrient pool were ~ 26 : 1, squarely between the suspended POM ratio and the Redfield ratio. We have further linked variation in elemental stoichiometry to that of phytoplankton cell abundance observed at the BATS site. Findings from this study suggest that elemental ratios vary with depth in the euphotic zone, mainly due to different growth rates of cyanobacterial cells. We have also examined the role of the Arctic Oscillation on temporal patterns in C : N : P stoichiometry. This study strengthens our understanding of the variability in elemental stoichiometry in different organic-matter pools and should improve biogeochemical models by constraining the range of non-Redfield stoichiometry and the net relative flow of elements between pools.

Simulating long-term dynamics of the coupled North Sea and Baltic Sea ecosystem with ECOSMO II: Model description and validation

The North Sea and the Baltic Sea ecosystems differ substantially in both hydrology and biogeochemical processes. Nonetheless, both systems are closely linked to each other and a coupled modeling approach is indispensable when aiming to simulate and understand long-term ecosystem dynamics in both seas. In this study, we present first an updated version of the fully coupled bio-physical model ECOSMO, a 3d hydrodynamic and a N(utrient)P(hytoplankton)Z(ooplankton)D(etritus) model, which is now adopted to the coupled system North Sea–Baltic Sea. To make the model applicable to both ecosystems, processes relevant for the Baltic Sea (e.g. sedimentation, cyanobacteria) were incorporated into the model formulation. Secondly we assess the validity of the model to describe seasonal, inter-annual and decadal variations in both seas. Our analyses show that the model sufficiently represents the spatial and temporal dynamics in both ecosystems but with some uncertainties in the coastal areas of the North Sea, likely related to the missing representation of tidal flats in the model, and in the deep-water nutrient pool of the Baltic Sea. Finally we present results from a 61-year (1948–2008) hindcast of the coupled North Sea and Baltic Sea ecosystem and identify long-term changes in primary and secondary production. The simulated long-term dynamics of primary and secondary production could be corroborated by observations from available literature and shows a general increase in the last three decades of the simulation when compared to the first 30years. Regime shifts could be identified for both ecosystems, but with differences in both, timing and magnitude of the related change.

Transcriptional responses of surface water marine microbial assemblages to deep‐sea water amendment

Deep-water nutrient injection in the North Pacific Subtropical Gyre provides an aperiodic yet significant source of inorganic nutrients to the nutrient-limiting surface waters, and has been implicated in phytoplankton bloom formation. Here we examined short-term transcriptional responses of surface water picoplankton assemblages in a deep-sea water (DSW) mixing experiment. Both flow cytometric and transcriptomic analysis indicated stimulation of an Alteromonas-like population in the DSW-amended treatment after 12 h, relative to the control. Among the highly expressed alteromonad transcripts in DSW-treated samples, those encoding genes associated with chemotaxis, cell motility and carbon metabolism were most highly represented, relative to the control. Similarly, Prochlorococcus showed significantly higher levels of transcripts associated with carbon fixation and photosynthesis, as well as slightly increased cell density, relative to the control. Although other microbial taxa did not exhibit enhanced growth, DSW-stimulated changes in their genome-wide transcriptional profiles were still readily detectable. Cell-associated cyanophage DNA and cDNA profiles suggested DSW stimulation of phage-mediated cell lysis, in previously infected cells. Comparison of the DSW-responsive Alteromonas populations, to previously reported dissolved organic matter (DOM)-responding alteromonads (McCarren et al., 2010), revealed differential transcript abundances, predominantly among genes encoding mobile elements and phage-related genes. Transcript representation in other metabolic pathways differed significantly between Alteromonas populations in the two different treatments, suggesting perturbation-specific metabolic responses to DSW and DOM. In total, the results provide new insight into short-term responses of picoplankton to DSW mixing, which occur prior to the more well-studied, longer-term growth responses of larger phytoplankton species.

Effects of surface forcing on interannual variability of the fall phytoplankton bloom in the Gulf of Maine revealed using a process-oriented model

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 427:29-49 (2011) - DOI: https://doi.org/10.3354/meps09043 Effects of surface forcing on interannual variability of the fall phytoplankton bloom in the Gulf of Maine revealed using a process-oriented model Song Hu1,*, Changsheng Chen1,2, Rubao Ji1,3, David W. Townsend4, Rucheng Tian2, Robert C. Beardsley5, Cabell S. Davis3 1Marine Ecosystem and Environmental Laboratory, College of Marine Sciences, Shanghai Ocean University, 999 Hucheng Huanlu, Lingang New City, Shanghai 201306, PR China 2School for Marine Science and Technology, University of Massachusetts Dartmouth, 706 South Rodney French Blvd., New Bedford, Massachusetts 02744, USA 3Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 4School of Marine Sciences, University of Maine, 5706 Aubert Hall, Orono, Maine 04469, USA 5Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA *Email: shu@shou.edu.cn ABSTRACT: SeaWiFS (Sea-viewing Wide Field-of-view Sensor) chlorophyll data revealed strong interannual variability in fall phytoplankton dynamics in the Gulf of Maine, with 3 general features in any one year: (1) rapid chlorophyll increases in response to storm events in fall; (2) gradual chlorophyll increases in response to seasonal wind- and cooling-induced mixing that gradually deepens the mixed layer; and (3) the absence of any observable fall bloom. We applied a mixed-layer box model and a 1-dimensional physical-biological numerical model to examine the influence of physical forcing (surface wind, heat flux, and freshening) on the mixed-layer dynamics and its impact on the entrainment of deep-water nutrients and thus on the appearance of fall bloom. The model results suggest that during early fall, the surface mixed-layer depth is controlled by both wind- and cooling-induced mixing. Strong interannual variability in mixed-layer depth has a direct impact on short- and long-term vertical nutrient fluxes and thus the fall bloom. Phytoplankton concentrations over time are sensitive to initial pre-bloom profiles of nutrients. The strength of the initial stratification can affect the modeled phytoplankton concentration, while the timing of intermittent freshening events is related to the significant interannual variability of fall blooms. KEY WORDS: Fall phytoplankton bloom · Surface forcing · Freshening · Interannual variability · Gulf of Maine · Modeling Full text in pdf format PreviousNextCite this article as: Hu S, Chen C, Ji R, Townsend DW, Tian R, Beardsley RC, Davis CS (2011) Effects of surface forcing on interannual variability of the fall phytoplankton bloom in the Gulf of Maine revealed using a process-oriented model. Mar Ecol Prog Ser 427:29-49. https://doi.org/10.3354/meps09043 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 427. Online publication date: April 12, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Opal accumulation rates in the equatorial Pacific and mechanisms of deglaciation

A possible imprint on equatorial Pacific sediments of a deglacial reinvigoration of the Southern Ocean overturning is increased opal accumulation rate. This would arise from the transmission of silica‐rich deep water to the equatorial thermocline via Subantarctic Mode Water and an associated increase in diatom productivity. In search of this imprint, sediment cores from the central (TT013‐PC72) and eastern (V19‐30) equatorial Pacific have been analyzed for 230Th‐normalized opal accumulation rates over the past five and three glacial terminations, respectively. Equatorial opal accumulation rates sustained relatively low values over much of the records and were punctuated by large increases centered on some terminations, but not all. Furthermore, two periods of increased opal flux were observed that do not coincide with terminations. Sources other than the Southern Ocean may need to be considered in the silica budget of the equatorial Pacific, but the δ13C of Neogloboquadrina dutertrei can be used to support the presence of a deepwater nutrient signal in each case. Although a common deglacial mechanism, or a common imprint thereof, for each of the late Pleistocene glaciations remains elusive, the combination of opal flux and δ13C of N. dutertrei provides a diagnostic for past injection of deepwater nutrients into the Equatorial Undercurrent.

Spatial variability of nitrogen isotope ratios of particulate material from Northwest Atlantic continental shelf waters

Human encroachment on the coastal zone has led to concern about the impact of anthropogenic nitrogen (N) on estuarine and continental shelf waters. Western North Atlantic watershed budgets suggest that the export of human-derived N from estuaries to shelf waters off the east coast of the US may be significant; however, models based on water inputs and estimates of upwelling of deepwater nutrients to surface waters of the mid-Atlantic bight indicate that estuarine N may be a relatively minor component of the overall shelf N budget. Stable N isotope ratios could provide a means to assess the relative input of anthropogenic N to shelf waters, particularly since dissolved N from human sources has elevated δ 15N values (range: 7–30‰). We collected particulate material from surface shelf waters off the US east coast from 2000 to 2005 at near-shore sample sites proximal to the mouth of six estuaries and corresponding sites farther offshore. Near-shore (mean 33.7 km from estuary mouth) δ 15N values ranged from 5.5 to 7.7‰ Offshore values (mean 92.4 km from estuary mouth) were consistently lower than near-shore sites (average 4.7 ± 1.0‰ versus 6.8 ± 1.1‰), suggesting different N sources to near and offshore stations. Near-shore regions are often more productive, as mean monthly chlorophyll-a concentrations from the sea-viewing wide field-of-view sensor (SeaWiFS) were significantly higher at near-shore sites near the mouth of three of the six estuaries. A mass balance using a concentration-dependent mixing model with chlorophyll-a concentrations as a surrogate for dissolved inorganic nitrogen can account for all of the nitrogen at near-shore sites south of Cape Cod with estuarine nitrogen estimated to contribute 45–85% of the nitrogen to the near-shore surface particulate material. Our results support the hypothesis that estuarine nitrogen is influencing continental shelf ecosystems, and also provide preliminary evidence of the spatial extent of its influence on shelf waters in the mid-Atlantic bight.

Fall diet and bathymetric distribution of deepwater sculpin ( Myoxocephalus thompsonii) in Lake Huron

Deepwater sculpin Myoxocephalus thompsonii are an important component of Great Lake's offshore benthic food webs. Recent declines in deepwater sculpin abundance and changes in bathymetric distribution may be associated with changes in the deepwater food web of Lake Huron, particularly, decreased abundance of benthic invertebrates such as Diporeia. To assess how deepwater sculpins have responded to recent changes, we examined a fifteen-year time series of spatial and temporal patterns in abundance as well as the diets of fish collected in bottom trawls during fall of 2003, 2004, and 2005. During 1992–2007, deepwater sculpin abundance declined on a lake-wide scale but the decline in abundance at shallower depths and in the southern portion of Lake Huron was more pronounced. Of the 534 fish examined for diet analysis, 97% had food in the stomach. Mysis, Diporeia, and Chironomidae were consumed frequently, while sphaerid clams, ostracods, fish eggs, and small fish were found in only low numbers. We found an inverse relationship between prevalence of Mysis and Diporeia in diets that reflected geographic and temporal trends in abundance of these invertebrates in Lake Huron. Because deepwater sculpins are an important trophic link in offshore benthic food webs, declines in population abundance and changes in distribution may cascade throughout the food web and impede fish community restoration goals.

Indian Ocean circulation and productivity during the last glacial cycle

The Indian Ocean is an important part of the global thermohaline circulation system, receiving deep waters sourced from the Southern Ocean and being the location of upwelling and surface-ocean current flow which returns warm and salty waters to the Atlantic. It is also an ideal location to reconstruct the link between thermohaline circulation and deep-water nutrient contents. No mixing occurs between major deep-water masses along flow paths within the Indian Ocean, so changes in water-mass provenance reflect changes in deep-ocean circulation while nutrient contents reflect addition and dissolution of organic matter. We present neodymium (Nd) and carbon (C) isotope records from an equatorial Indian Ocean core (SK129–CR2) spanning the last 150 kyr. The Nd isotope record shows that an increased proportion of North Atlantic Deep Water (NADW) reached the Indian Ocean during interglacials (Marine Isotope Stages; MIS 1 and 5), and a reduced proportion during glacials (MIS 2, 4, and 6), and also that changes occurred during MIS 3. The magnitude and timing of deglacial and some MIS 3 variability is very similar to those in the RC11–83/TNO57–21 South Atlantic deep Cape Basin Nd isotope record ( Piotrowski et al., 2005). Since the deep Cape Basin is oceanographically upstream of the Indian Ocean from a deep water circulation perspective, the Nd isotopes suggest that Atlantic meridional overturning circulation changes were effectively propagated from the southeastern Atlantic into the central Indian Ocean via the Southern Ocean. Comparison of the Nd and C isotope records shows that deep-ocean circulation was decoupled from nutrient-content changes on glacial–interglacial timescales, in particular suggesting higher productivity during MIS 5. Increased equatorial productivity during MIS 5 is supported by benthic infaunal–epifaunal foraminiferal δ 13C gradients, as well as benthic foraminiferal δ 13C gradients along deep-water flow. Concurrent warming, indicated by planktonic foraminiferal Mg/Ca during MIS 5 at the site is consistent with changing thermocline temperature and may indicate a link to surface-ocean hydrographic changes.