Impact of the azores front propagation on deep ocean particle flux

The Azores Current originating as a branch of the Gulf Stream is a highly dynamic system in the subtropical North Atlantic. The associated front forms the northeastern boundary of the North Atlantic Subtropical Gyre. In this study we analyzed 42 years of assimilated modeled temperature fields to localize the position of the Azores Front at 22°W and observed a fast north- and southward propagation between 30°N and 37°N on monthly to decadal time scales. The North Atlantic Oscillation with correlated changes of the wind direction was identified as one driving mechanism. As the front is acting as a guide for Rossby waves, the signal of the front’s propagation is transferred to the western Atlantic and, among other atmospheric forcing mechanisms, induces a shifting of the Northern Wall of the Gulf Stream with one year delay. Shallower mixed layer depths in the northern frontal region of the Azores Current caused by the rise of the isotherms lead to nutrient supply and primary production different from those found in the southern frontal region of the current system. A high interannual variability is manifested in deep ocean particle flux, derived from a sediment trap in 2000 m water depth at the mooring site KIEL276 (33°N, 22°W) from 1993 to 2008, which is directly related to the phytoplankton bloom in the euphotic zone. This variability is explained by the propagation of the front and strong variations in the catchment areas of the sediment trap due to the associated eddy activity in the frontal region

Chlorophyll a reconstruction from in situ measurements: 1. Method description

Understanding the development of primary production is essential for projections of the global carbon cycle in the context of climate change. A chlorophyll a hindcast that serves as a primary production indicator was obtained by fitting in situ measurements of nitrate, chlorophyll a, and temperature. The resulting fitting functions were adapted to a modeled temperature field. The method was applied to observations from the Madeira Basin, in the northeastern part of the oligotrophic North Atlantic Subtropical Gyre and yielded a chlorophyll a field from 1989 to 2008 with a monthly resolution validated with remotely measured surface chlorophyll a data by SeaWiFS. The chlorophyll a hindcast determined with our method resolved the seasonal and interannual variability in the phytoplankton biomass of the euphotic zone as well as the deep chlorophyll maximum. Moreover, it will allow estimation of carbon uptake over long time scales

Calcification depth of deep-dwelling planktonic foraminifera from the eastern North Atlantic constrained by stable oxygen isotope ratios of shells from stratified plankton tows

Stable oxygen isotopes (delta O-18) of planktonic foraminifera are one of the most used tools to reconstruct environmental conditions of the water column. Since different species live and calcify at different depths in the water column, the delta O-18 of sedimentary foraminifera reflects to a large degree the vertical habitat and interspecies delta O-18 differences and can thus potentially provide information on the vertical structure of the water column. However, to fully unlock the potential of foraminifera as recorders of past surface water properties, it is necessary to understand how and under what conditions the environmental signal is incorporated into the calcite shells of individual species. Deep-dwelling species play a particularly important role in this context since their calcification depth reaches below the surface mixed layer. Here we report delta O-18 measurements made on four deep-dwelling Globorotalia species collected with stratified plankton tows in the eastern North Atlantic. Size and crust effects on the delta O-18 signal were evaluated showing that a larger size increases the delta O-18 of G. inflata and G. hirsuta, and a crust effect is reflected in a higher delta O-18 signal in G. truncatulinoides. The great majority of the delta O-18 values can be explained without invoking disequilibrium calcification. When interpreted in this way the data imply depth-integrated calcification with progressive addition of calcite with depth to about 300m for G. inflata and to about 500m for G. hirsuta. In G. scitula, despite a strong subsurface maximum in abundance, the vertical delta O-18 profile is flat and appears dominated by a surface layer signal. In G. truncatulinoides, the delta O-18 profile follows equilibrium for each depth, implying a constant habitat during growth at each depth layer. The delta O-18 values are more consistent with the predictions of the Shackleton (1974) palaeotemperature equation, except in G. scitula which shows values more consistent with the Kim and O'Neil (1997) prediction. In all cases, we observe a difference between the level where most of the specimens were present and the depth where most of their shell appears to calcify.Agência financiadora Portuguese Foundation for Science and Technology (FCT): SFRH/BD/78016/2011; UID/Multi/04326/2019 European Union Seventh Framework Programme (FP7/2007-2013): 228344-EUROFLEETS German Research Foundation (DFG): WA2175/2-1; WA2175/4-1 German Climate Modelling consortium PalMod - German Federal Ministry of Education and Research (BMBF)info:eu-repo/semantics/publishedVersio

Oxygen and hydrogen isotope signatures of Northeast Atlantic water masses

Only a few studies have examined the variation of oxygen and hydrogen isotopes of seawater in NE Atlantic water masses, and data are especially sparse for intermediate and deep-water masses. The current study greatly expands this record with 527 δ18O values from 47 stations located throughout the mid- to low-latitude NE Atlantic. In addition, δD was analyzed in the 192 samples collected along the GEOTRACES North Atlantic Transect GA03 (GA03_e = KN199-4) and the 115 Iberia-Forams cruise samples from the western and southern Iberian margin. An intercomparison study between the two stable isotope measurement techniques (cavity ring-down laser spectroscopy and magnetic-sector isotope ratio mass spectrometry) used to analyze GA03_e samples reveals relatively good agreement for both hydrogen and oxygen isotope ratios. The surface (0-100 m) and central (100-500 m) water isotope data show the typical, evaporation related trend of increasing values equatorward with the exception for the zonal transect off Cape Blanc, NW Africa. Off Cape Blanc, surface water isotope signatures are modified by the upwelling of fresher Antarctic Intermediate Water (AAIW) that generally has isotopic values of 0.0 to 0.5‰ for δ18O and 0 to 2‰ for δD. Along the Iberian margin the Mediterranean Outflow Water (MOW) is clearly distinguished by its high δ18O (0.5-1.1‰) and δD (3-6‰) values that can be traced into the open Atlantic. Isotopic values in the NE Atlantic Deep Water (NEADW) are relatively low (δ18O: –0.1 to 0.5‰; δD: –1 to 4‰) and show a broader range than observed previously in the northern and southern convection areas. The NEADW is best observed at GA03_e Stations 5 and 7 in the central NE Atlantic basin. Antarctic Bottom Water isotope values are relatively high indicating modification of the original Antarctic source water along the flow path. The reconstructed δ18O-salinity relationship for the complete data set has a slope of 0.51, i.e. slightly steeper than the 0.46 described previously by Pierre et al. (1994) for the tropical to subtropical Northeast Atlantic. This slope decreases to 0.46 for the subtropical North Atlantic Central Water (NACW) and the MOW and to 0.32 for the surface waters of the upper 50 m. The δD-salinity mixing lines have estimated slopes of 3.01 for the complete data, 1.26 for the MOW, 3.47 for the NACW, and 2.63 for the surface waters. The slopes of the δ18O-δD relationship are significantly lower than the one for the Global Meteoric Water Line with 5.6 for the complete data set, 2.30 for the MOW, 4.79 for the NACW, and 3.99 for the surface waters. The lower slopes in all the relationships clearly reflect the impact of the evaporation surplus in the subtropics.EC Grant agreement n° 228344 - EUROFLEETS, C2007-FCT/319/2006info:eu-repo/semantics/publishedVersio

Integrating conventional microscopy and molecular analysis to analyse the abundance and distribution of four Calanus congeners in the North Atlantic

Analysis of the demographic structure of Calanus species in the North Atlantic presents particular difficulties due to the overlapping spatial distributions of four main congeneric species (Calanus finmarchicus, Calanus helgolandicus, Calanus glacialis and Calanus hyperboreus). These species have similar morphologies, making microscopic discrimination only possible between some of the species at late copepodite or adult stages. However, molecular techniques now offer the possibility of screening significant numbers of specimens and unambiguously identifying them to species, regardless of developmental stage. Unfortunately, the processing rate of specimens by molecular methods is still too low to offer a realistic alternative to microscopy for analysis of samples from large field surveys. Here, we outline and test an approach involving the use of molecular methodology in conjunction with conventional microscopy to assess the species assignment of developmental stage abundances of Calanus congeners. Our study has highlighted many important methodological issues. First, it cannot be assumed that the species composition is homogeneous across the development stages; applying proportional species composition of adults to morphologically undistinguishable earlier development stages can result in error. The second important conclusion is that prosome length may be a highly unreliable discriminator of C. finmarchicus and C. glaciali

Factors controlling the depth habitat of planktonic foraminifera in the subtropical eastern North Atlantic

Planktonic foraminifera preserved in marine sediments archive the physical and chemical conditions under which they built their shells. To interpret the paleoceano-graphic information contained in fossil foraminifera, the recorded proxy signals have to be attributed to the habitat and life cycle characteristics of individual species. Much of our knowledge on habitat depth is based on indirect methods, which reconstruct the depth at which the largest portion of the shell has been calcified. However, habitat depth can be best studied by direct observations in stratified plankton nets. Here we present a synthesis of living planktonic foraminifera abundance data in vertically resolved plankton net hauls taken in the eastern North Atlantic during 12 oceanographic campaigns between 1995 and 2012. Live (cytoplasm-bearing) specimens were counted for each depth interval and the vertical habitat at each station was expressed as average living depth (ALD). This allows us to differentiate species showing an ALD consistently in the upper 100m (e.g., Globigerinoides ruber white and pink), indicating a shallow habitat; species occurring from the surface to the subsurface (e.g., Globigerina bulloides, Globorotalia inflata, Globorotalia truncatulinoides); and species inhabiting the subsurface (e.g., Globorotalia scitula and Globorotalia hirsuta). For 17 species with variable ALD, we assessed whether their depth habitat at a given station could be predicted by mixed layer (ML) depth, temperature in the ML and chlorophyll a concentration in the ML. The influence of seasonal and lunar cycle on the depth habitat was also tested using periodic regression. In 11 out of the 17 tested species, ALD variation appears to have a predictable component. All of the tested parameters were significant in at least one case, with both seasonal and lunar cyclicity as well as the environmental parameters explaining up to >50% of the variance. Thus, G. truncatulinoides, G. hirsuta and G. scitula appear to descend in the water column towards the summer, whereas populations of Trilobatus sacculifer appear to descend in the water column towards the new moon. In all other species, properties of the mixed layer explained more of the observed variance than the periodic models. Chlorophyll a concentration seems least important for ALD, whilst shoaling of the habitat with deepening of the ML is observed most frequently. We observe both shoaling and deepening of species habitat with increasing temperature. Further, we observe that temperature and seawater density at the depth of the ALD were not equally variable among the studied species, and their variability showed no consistent relationship with depth habitat. According to our results, depth habitat of individual species changes in response to different environmental and ontogenetic factors and consequently planktonic foraminifera exhibit not only species-specific mean habitat depths but also species-specific changes in habitat depth.Portuguese Foundation for Science and Technology (FCT) [SFRH/BD/78016/2011]; MARUM - Center for Marine Environmental Sciences; European Union [228344-EUROFLEETS]; DFG (German Research Foundation) [WA2175/2-1, WA2175/4-1]; German Climate Modeling consortium PalMod - German Federal Ministry of Education and Research (BMBF); CANIGO project (EU) [MAS-CT96-0060]; DFGinfo:eu-repo/semantics/publishedVersio

In situ Determination of Nitrate and Hydrogen Sulfide in the Baltic Sea Using an Ultraviolet Spectrophotometer

Evaluating the health status of marine ecosystems becomes ever increasingly important especially against the backdrop of rising pressures from human activities. This is true especially for coastal seas such as the Baltic Sea that is surrounded by highly industrialized countries. Nutrients and pollutants such as nitrate and hydrogen sulfide, which have a major impact on ecosystem functioning, are two of several key environmental indicators for assessing the status of natural waters, and therefore of considerable interest. The frequency and the spatial coverage of the nitrate and hydrogen sulfide measurements are currently limited by the cost of the laboratory analysis and personnel. Optical in situ sensors can help to overcome this challenge by allowing reagentless and fast detection of dissolved chemical species. A chemical-free optical sensor has been used for direct and simultaneous measurements of both key parameters, and the results were compared with traditional methods. The data were collected during an observational program conducted in the Baltic Sea in February 2018. We used the OPUS UV spectral sensor, which was deployed for the first time in coastal waters, in combination with a deep-sea telemetry system to enable near-real time measurements during CTD profiling. Data processing was carried out using a multiple linear regression procedure. Measurements from both OPUS and on-board analysis were in good agreement. The results showed, that in situ UV-VIS spectrophotometry provides the capability to determine the concentration distributions of nitrate and hydrogen sulfide in the brackish waters of the Baltic Sea

Hydrographic-hydrochemical assessment of the Baltic Sea 2017

ENGLISH ABSTRACT: The article summarizes the hydrographic-hydrochemical conditions in the western and central Baltic Sea in 2017. Based on meteorological conditions, the horizontal and vertical distribution of temperature, salinity, oxygen/hydrogen sulphide and nutrients are described on a seasonal scale. For the southern Baltic Sea area, the “cold sum” of the air temperature of 31.7 Kd in Warnemünde amounted to a mild winter in 2014/15 and ranks as 15th warmest winter since the beginning of the record in 1948. The summer “heat sum” of 159.5 Kd ranks on 28th position of the warmest summers over the past 70 years and is slightly above the long-term average of 153.4 Kd. Based on satellite derived Sea Surface Temperature (SST) 2017 was the eleventh-warmest year since 1990 and with 0.24 K slightly above the long-term SST average. March, April and October - December contributed to the average by their positive anomalies. July and August were characterized by negative anomalies. The anomalies reached maximum values of +2 K and -3 K. The situation in the deep basins of the Baltic Sea was mainly coined by beginning stagnation at bottom-near water depths of the eastern Gotland Basin and ongoing ventilation of the upper part 5 of the deep-water above 150 m as a consequence of weak inflows. For the first time within this phase of intensified inflow activity, starting in 2014, the ventilation of the Farö Deep at the Northern Central Basin was registered at the beginning of the year. In the course of 2017 two weak inflows showing total volumes of 210 km^³ (February) and 188 km^³ (October) were registered. In conclusion, the impact of the observed phase of intensified water exchange processes with subsequent consequences for the biogeochemical cycles is weakening. GERMAN ABSTRACT: Die Arbeit beschreibt die hydrographisch-hydrochemischen Bedingungen in der westlichen und zentralen Ostsee für das Jahr 2017. Basierend auf den meteorologischen Verhältnissen werden die horizontalen und vertikalen Verteilungsmuster von Temperatur, Salzgehalt, Sauerstoff/ Schwefelwasserstoff und Nährstoffen mit saisonaler Auflösung dargestellt. Für den südlichen Ostseeraum ergab sich eine Kältesumme der Lufttemperatur an der Station Warnemünde von 31,7 Kd. Im Vergleich belegt der Winter 2016/17 den 15. Platz der wärmsten Winter seit Beginn der Aufzeichnungen im Jahr 1948 und wird als mild klassifiziert. Mit einer Wärmesumme von 159,5 Kd rangiert der Sommer im Mittelfeld der 70jährigen Datenreihe und reiht sich auf Platz 28 der wärmsten Sommer ein. Das Langzeitmittel liegt bei 153,4 Kd. Auf der Grundlage von satellitengestützten Meeresoberflächentemperaturen (SST) war 2017 das elft- wärmste Jahr seit 1990 und mit 0,24 K etwas über dem langfristigen SST-Mittel. März, April und Oktober - Dezember trugen durch ihre positiven Anomalien zum Durchschnitt bei. Juli und August waren durch negative Anomalien gekennzeichnet. Die Anomalien erreichten Höchstwerte von +2 K und -3 K. Die Situation in den Tiefenbecken der Ostsee war im Wesentlichen geprägt durch bodennah einsetzende Stagnation im östlichen Gotland Becken und Belüftung der mittleren Wassersäule oberhalb 150 m im Zuge kleinerer Einströme. Zu Jahresbeginn wurde das im nördlichen Zentralbecken gelegene Farö Tief erstmals innerhalb der aktuellen Einstromphase belüftet. Im Jahresverlauf 2017 wurden zwei weitere schwache Einströme mit Volumina zwischen 210 km³ und 188 km³ im Februar sowie Oktober registriert. Zusammenfassend kann gesagt werden, dass die Auswirkungen der seit 2014 beobachten Phase von verstärkten Wasseraustauschprozessen mit entsprechenden Konsequenzen für die biogeochemischen Kreisläufe abklingen

The Baltic Sea Tracer Release Experiment. Part I: Mixing rates

In this study, results from the Baltic Sea Tracer Release Experiment (BATRE) are described, in which deep water mixing rates and mixing processes in the central Baltic Sea were investigated. In September 2007, an inert tracer gas (CF3SF5) was injected at approximately 200 m depth in the Gotland Basin, and the subsequent spreading of the tracer was observed during six surveys until February 2009. These data describe the diapycnal and lateral mixing during a stagnation period without any significant deep water renewal due to inflow events. As one of the main results, vertical mixing rates were found to dramatically increase after the tracer had reached the lateral boundaries of the basin, suggesting boundary mixing as the key process for basin-scale vertical mixing. Basin-scale vertical diffusivities were of the order of 10−5 m2 s−1 (about 1 order of magnitude larger than interior diffusivities) with evidence for a seasonal and vertical variability. In contrast to tracer experiments in the open ocean, the basin geometry (hypsography) was found to have a crucial impact on the vertical tracer spreading. The e-folding time scale for deep water renewal due to mixing was slightly less than 2 years, the time scale for the lateral homogenization of the tracer patch was of the order of a few months. Key Points: Mixing rates in the Gotland Basin are dominated by boundary mixing processes; The time scale for Gotland Basin deep water renewal is approximately 2 years; Mixing rates determined from the tracer CF3SF