A winter upwelling event in the Northern Galician Rias: Frequency and oceanographic implications

A winter shelf-water upwelling evidence (February 2008) is described by first time in the Northern Galician Rias (NW Iberian Peninsula). On February 20, after 9 consecutive days of upwelling favorable conditions, inside the O Barqueiro Ria was observed the presence of seawater below 10 m depth which replaced the less saline water previously observed in January. This situation was in agreement with the analyzed Ekman transport close to the northern Galician coast. Salinity and temperature distribution revealed that the upwelled water inside the rias corresponds to shelf bottom seawater which is not associated with Eastern North Atlantic Central Water or the Iberian Poleward Current. In addition, TS diagram indicated a higher influence of upwelling eastward (Viveiro–Barqueiro–Ortigueira). Nutrient salts' concentrations also suggested the presence of seawater from subsurface origin with a small variation with regard to the winter mixing in this marine area. Plankton showed the existence of spring conditions related to solar radiation increase associated to upwelling favorable winds. Some species also corroborated the intrusion of shelf-water inside the ria. Results from the analysis of Ekman transport data from 1967 to 2007 revealed that this event cannot be considered an isolated episode. In fact, the number of days per month under upwelling favorable conditions in winter (January–March) was not negligible (8–10 days) showing that upwelling events along the northern Galician coast are also possible during winter.     

夏季北黄海水体浊度分布特征的初步研究

应用2007月7月国家908专项北黄海区块水体调查获取的浊度资料,与同步获得的悬浮物质量浓度进行拟合分析,结果表明,中底层水的相关系数在0.94以上,表层相关度较差.根据调查海域浊度的水平大面分布及选取的6个典型断面的垂直分布,初步阐述了夏季北黄海水体浊度的分布特征——近岸高和底层高,山东半岛北部沿岸、成山角海域和老铁山水道以及庄河河口附近海域为高浊度区.夏季北黄海冷水团对水体浊度分布具有控制性影响:调查海域水体垂向层结稳定,北黄海冷水团海域中下层表现为高盐、低温、高密特征,水体浊度小,且浊度锋面的分布与温度较为相近.同时,夏季北黄海冷水团对悬浮物的分布起到了屏障作用——在山东半岛北部沿岸和辽东半岛南部沿岸阻止了近岸悬浮物的经向输送,而在山东半岛东部沿岸则阻止了近岸悬浮物的纬向输送.     

Hydrography of the Artabro Gulf in summer: western coastal limit of Cantabrian seawater and wind-induced upwelling at prior cape

The Artabro Gulf (1500 km² north-west of the Iberian Peninsula) has been studied based on data obtained in August 1993, under typical summer meteorological conditions (COPLA 893 cruise). This is the first study on the summer hydrography of the Artabro Gulf. During this season, the hydrography of this area is quite complex and must be understood in terms of three factors: 1) upwelling, 2) two different eastern North Atlantic Central Water (ENACW) water masses and 3) rias water exchange. A local wind-induced upwelling of ENACW_p (subpolar) occurs off Prior Cape. The plume is located off the gulf coastline, but inside the continental shelf. The frontal zone, rich in nutrients and chlorophyll, is parallel to the gulf line, starting in the Prior region and ending at the convergence of ENACW_p and ENACW_t (subtropical), giving it an arched shape. The Artabro Gulf is the place off the Galicia coast where ENACW_t and ENACW_p are observed and the gulf is the western limit of coastal Cantabrian Sea influence. The upwelling is strongly influenced by the topographic presence of the gulf which generates an ‘upwelling shadow’ where the rias water exchange is confined near the coast and the ENACW_p does not penetrate into the rias. The opposite occurs at the southernmost Rias Bajas, where ENACW_t upwells inside the rias.     

Comparison of Phytoplankton Species Composition and Abundance in the Rias Arosa and Muros (NW Spain)

Abstract. Seasonal changes in phytoplankton species composition and abundance were studied in the Rias de Arosa and Muros (NW Spain) in November 1975, April and August 1976. Data on density and percentage contribution of species were used to calculate spatial and temporal relationships of phytoplankton communities by affinity matrices. The phytoplankton communities of both rias differed in species composition and abundance during the three sampling seasons. Diatoms dominated and total abundance was always higher in Arosa. Both rias have upwelling of nutrient-rich oceanic water but the Arosa has an intense raft aquaculture of the edible mussel Mytilus edulis.     

Abyssal current along the northern periphery of Shikoku Basin

Abyssal currents along the northern periphery of the Shikoku Basin south of Japan were measured by current meters moored off Cape Daio-zaki, Cape Shiono-misaki and Cape Ashizuri-misaki and on the eastern foot of the northernmost part of the Kyushu-Palau Ridge. Total length of observation off Cape Shiono-misaki was about five years including the periods of the Kuroshio large meander and no meander. Analyses of current data show:

1. Mean currents with a magnitude of 5–10 cm sec^?1 were observed during the whole observation period at all of current meters which were set 400 m above the sea bottom that was deeper than 4,500 m. The mean current for each current meter was parallel to the local bottom contour arond each station and was toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left.
2. At each station located above the shelf toe off Cape Daio-zaki and off Cape Shiono-misaki and on the foot of the Kyushu-Palau Ridge, the mean current increases with depth (a bottomward intensification of the mean current), and the vertical extent of the mean current is estimated to be about 2,000 m above the sea bottom.
3. At a station located at 2,600 m depth on the continental slope off Cape Shiono-misaki, no bottom-ward intensification of currents was observed.

These results strongly suggest that a steady abyssal flow exists in the depths deeper than about 3,000 m along the northern and northwestern peripheries of the Shikoku Basin. Existence of the abyssal circulation system is also suggested, at least, in the north of the Shikoku Basin.     

Meiofauna and macrofauna community structure in relation to sediment composition at the Iberian margin compared to the Goban Spur (NE Atlantic)

Meiofauna and macrofauna communities and several sediment characteristics were compared between a slope situated far from the coast (Goban Spur) and two transects across the Iberian Margin with steep slopes and close to the shore. The northern Galician transect (off La Coruña) was situated in an area subjected to wind-induced upwelling events. The western Galician transect was also subjected to upwelling, was additionally influenced by outflows of water rich in organic matter from the Rías Bajas. This transect also included the Galicia Bank. Macrofauna density decreased exponentially from the shelf edge (154 m) to the abyssal plain (4951 m) and different communities occurred on the shelf, the upper- and lower slope and on the abyssal plain. Apart from two extremely low-density stations on the Iberian Margin, there were no significant differences in the meiofauna between the Goban Spur and the Iberian Margin. Along the La Coruña-transect a station where meiofaunal densities were low occurred at a depth of 1522 m, where the sediment was characterised by having a high median-grain size, ripple structures, a low C_org and total N content. There were relatively high numbers of macrofaunal filter-feeders but low numbers of crustaceans, indicating a high current velocity regime. On top of the Galicia Bank (˜770 m) the sediment consisted mainly of shells of pelagic foraminifers, and had low contents of C_org and N. The macrofauna was dominated by filter-feeding and carnivorous taxa. At both these stations meiofauna densities were low. Meiofauna densities and community structure differed between the Goban Spur and the Iberian Margin. Meiofauna densities on the Galician shelf were more than double those on the Goban Spur shelf. The two deep stations on the La Coruña transect and the deepest station on the Galicia Bank transect all contained meiofaunal densities that were higher than found at similar depths off the Goban Spur. The meiofaunal densities were inversely correlated with %CaCO₃ content and, excluding the shelf stations, were positively correlated with both %C_org and total N at the Iberian Margin. Neither upwelling nor the enriched outflows from the rias affected the macrofauna, but meiofaunal densities were greatly enhanced.     

Observations and mechanisms of upwelling in the northern KwaZulu-Natal Bight,South Africa

New in situ time-series data were acquired by two ADCP moorings placed on the shelf off Richards Bay on the east coast of South Africa at depths of 25 m and 582 m between October 2009 and August 2010. The 11-month inshore bottom-temperature record revealed five substantial upwelling events lasting 5–10 days each where temperatures decreased by about 7 °C to 17–18 °C. Satellite sea surface temperature data showed these events to coincide with cold-water plumes occupying the northern wedge of the KwaZulu-Natal (KZN) Bight. Numerous shorter duration (1–2 days) upwelling events with less vivid surface expressions were also observed throughout the entire record where bottom temperature dropped by 2–3 °C. The last four months of the record were characterised by a protracted cool period lacking a seasonal trend but punctuated with oscillations of warm and cooler bottom water. In contrast to earlier studies that suggested upwelling was topographically and dynamically driven by the juxtaposition of the Cape St Lucia offset and the Agulhas Current (a solitary mechanism), our analysis showed almost all major and minor cold-water intrusions to coincide with upwelling-favourable north-easterly winds that simultaneously force a south-westerly coastal current. Ekman veering in the bottom boundary layer of the Agulhas Current, and the concomitant movement of cold water up the slope, was found to coexist at times with coastal upwelling, but its absence did not impede inshore cold-water intrusions, calling into question its role as a primary driver of upwelling. Both major and minor upwelling events were observed to promote phytoplankton blooms in the northern KZN Bight which commonly extended to the Thukela River. Wind-driven upwelling was also observed in the inner bight between Richards Bay and Port Durnford, explaining the ribbon of coastal chlorophyll continuously observed on ocean colour images between Cape St Lucia and the Thukela River. Similarities in upwelling character and mechanisms are observed between the northern KZN Bight and the Florida Current shelf systems.     

Shifts from native to non-indigenous mussels: Enhanced habitat complexity and its effects on faunal assemblages

Ecosystem engineers such as mussels may affect strongly both the structure of benthic assemblages and the ecosystem functioning. The black-pygmy mussel Limnoperna securis is an invasive species that is spreading along the Galician coast (NW Spain). Its current distribution overlaps with the distribution of the commercial native mussel species Mytilus galloprovincialis, but only in the inner part of two southern Galician rias. Here, we analysed the assemblages associated with clumps of the two mussel species and evaluated if the invasive species increased complexity of habitat. To measure complexity of clumps we used a new method modified from the “chain and tape” method. Results showed that the identity of the mussel influenced macrofaunal assemblages, but not meiofauna. L. securis increased the complexity of clumps, and such complexity explained a high percentage of variability of macrofauna. The shift in dominance from M. galloprovincialis to L. securis may alter habitat structure and complexity, affecting the macrofaunal assemblages with unpredictable consequences on trophic web relations.     

Metal distributions and their fluxes at the coastal boundary of a semi-enclosed ria

Water column samples have been collected in the outer channel of the Ferrol Ria (NW Spain) during four occasions over a tidal cycle. The objective was to study the exchange of dissolved and particulate Cd, Cu, Pb and Zn and particulate Al, Fe and Si between the ria and the adjacent coastal waters. This study provides the first extensive dataset on dissolved and particulate metal concentrations in the water column of a Galician ria. Typical concentrations of dissolved Cd (96 ± 31 pM), Cu (8 ± 4 nM), Pb (270 ± 170 pM) and Zn (21 ± 10 nM) were similar than in other European Atlantic shelf and coastal waters. The fraction of metals in the particulate phase followed the trend: Pb > Cu Zn > Cd. The outgoing water from the ria was enriched in dissolved and particulate Cu, Pb and Zn compared with incoming waters, whereas Cd concentrations were similar for both waters. The suspended particulate matter was composed of a mixture of marine and continental material. The latter end-member was found to arise from the metal-rich ria bed sediments, which is diluted by the dominant metal-poor marine end-member. The net output flux of Cu from the channel is balanced by the freshwater inputs to the ria, and the net Zn flux gave a positive output to coastal waters. For Pb, the net flux to the coastal waters is less than that input from the rivers, as a result of its particle reactivity and deposition in sediments. On the contrary, a net input flux of dissolved Cd from coastal waters was observed, highlighting the oceanic source of this metal in the Galician rias. Results from the budget calculations are in agreement with the differential geochemical behavior of these elements in coastal waters.     

A two-dimensional particle tracking model for pollution dispersion in A Coruña and Vigo Rias (NW Spain)

A two-dimensional hydrodynamic model coupled to a particle tracking model is applied to study the dispersion processes and residence time in two Galician rias (A Coruña and Vigo, NW Spain) under summer conditions. In A Coruña a long residence time was found in the harbour area due to the existence of a dock, and a short one in the river area. On the contrary, in Vigo, the residence time is smaller in the harbour area, due to the Rande Strait, beyond which the river effect is negligible.     

Recent changes in the Labrador Sea Water within the Deep Western Boundary Current southeast of Cape Cod

Water properties measured by the central mooring in the Line W mooring array southeast of Cape Cod document a large character shift during the period of November 2001 to April 2008. The observed temperature, salinity and planetary potential vorticity (PPV) anomalies manifest changes in the formation region of the water masses present at Station W, specifically upper Labrador Sea Water (uLSW), deep Labrador Sea Water (dLSW) and Overflow Water (OW). During the observation period, the minimum in the PPV anomaly field relative to the record mean PPV profile migrated from 1500 m, where it was originally found, to 700 m. Temporal changes in the vertical distribution of temperature and salinity were correlated with the PPV changes. This suggests a dLSW-dominated first half of the record versus an uLSW-dominated second half. The structure of these anomalies is consistent with observations within the Labrador Sea, and their transit time to Line W agrees well with tracer-derived times for signals spreading along the western boundary. In that context, the observed water properties at Line W in the early 2000s reflected the intense deep convection in the Labrador Sea in the mid-1990s, with less intense convection subsequently affecting lighter isopycnals. The observed velocity field is dominated by high-frequency (periods of days to months) fluctuations, however, a fraction of the velocity variability is correlated with changes in water mass properties, and indicate a gradual acceleration of the southwestward flow, with a corresponding increase in Deep Western Boundary Current transport.     

Hydrological conditions and circulation off the west coast of the North Island,New Zealand

Temperature and salinity surveys were carried out in the Tasman Sea in winter (August 1973) and summer (February‐March 1974). In both surveys the presence of the Westland Current was indicated by the distribution of surface water properties; in summer it was associated with a subsurface salinity maximum. The current extended further northwards in summer than in winter. In summer, an east‐going geostrophic flow at about latitude 35°S separated on approaching New Zealand; part of the flow passed north around the North Island and part moved slowly eastwards in the deeper off‐shore water to at least latitude 38°S. The West Auckland Current was apparent in the winter, but not in the summer. To the west of Cape Reinga, relatively low values of surface temperature and salinity are probably associated with upwelling between Cape Reinga and the Three Kings Islands. Upwelling was observed along the coast between Kaipara and Manukau Harbour.     

An Inshore Poleward Current in the NW of the Iberian Peninsula Detected from Satellite Images, and its Relation withG. catenatum and D. acuminata Blooms in the Galician Rias

Toxic Gymnodinium catenatum blooms usually occur in the Galician Rias at the end of the upwelling season, which necessitates a ban on harvesting shellfish extraction, with subsequent economic losses for this sector. One of the possible causes cited in the literature is the advection of populations from outside the area but no evidence was available to substantiate this.Oceanographic conditions at the end of the upwelling season in the NW coast of the Iberian Peninsula (39°–43°N) have been studied for the years 1986, 1990, 1995 and 1998. Sea surface temperature data from satellite images, wind data, drifter tracks and in situ oceanic data from the Galician Rias have been combined to clarify the oceanographic structures in the area at the commencement of the blooms. On the inner shelf, an inshore poleward current advecting warm water has been identified after the cessation of upwelling. On the middle and outer shelf, a tongue of cold water as a remnant of the previous upwelling continued to move southward. On the slope and offshore, the poleward counter current reported by several authors was detected carrying warm oceanic water northwards.It is suggested that the inshore poleward current, not previously reported in the literature, could advect initial populations of dinoflagellates to the Rias from northern Portuguese waters. This would explain why blooms such as G. catenatum have been found usually in Portuguese waters several weeks before the Galician Rias, showing an apparent northward movement, but cells of this toxic dinoflagellate species have not been found in waters of the offshore poleward counter current.     

Direct hydroacoustic observations of chokka squid Loligo vulgaris reynaudii spawning activity in deep water

Unusual and distinct hydroacoustic targets were observed in November 1996, May 1998 and November 1998 during routine pelagic biomass surveys off the south coast of South Africa. During the November 1996 survey, seven such targets were observed near the bottom at depths of 115–125 m, directly south of the traditional inshore spawning grounds of chokka squid Loligo vulgaris reynaudii at Cape St Francis. The targets were close to prominent seabed ridges and extended 30–40 m off the bottom. In May 1998, three similar targets were observed at depths 55–80 m off Plettenberg Bay, another well-known squid spawning site. The shallowest target was identified, by means of a midwater trawl, as a mixture of mature male and female chokka. During the November 1998 survey, nine similar targets were again observed on the squid spawning grounds at Cape St Francis, also adjacent to seabed ridges. Drawing on fisheries hydroacoustic experience and knowledge of chokka squid spawning behaviour, the targets are believed to be aggregations of spawning squid.     

夏冬季北黄海水体浊度分布特征研究

应用2007年1月和7月国家908专项北黄海区块水体调查获取的浊度等资料,分析了夏季和冬季北黄海海域水体浊度的水平和垂向分布特征,初步阐述了夏、冬季北黄海水体浊度分布具有南北高,中间低的特征。无论是夏季还是冬季,山东半岛东北沿岸和辽东半岛东南沿岸为高浊度区,中部海域受北黄海冷水团的影响而维持低浊度。夏季,北黄海冷水团导致的水体层结效应产生了"水障"作用——悬浮物只能沿岸分布和输送;冬季,山东半岛东端外海的强海流切变锋阻碍了悬浮物的纬向输送。此外,研究还发现由于黄海冷水团和黄海暖流的作用,导致夏、冬季黄海中部的沉积动力环境有所差异。     

Macro- and megafauna communities in three deep basins of the South-East Atlantic

During ‘Meteor’ expedition ‘DIVA 2’ in 2005 the abyssal macro- and megafauna communities were studied in the northern Cape Basin, in the northern Angola Basin and in the eastern and western Guinea Basin. Water depths varied between 5040 and 5670 m.Surface deposit feeding or predatory ophiuroids dominated the megafaunal community in the northern Cape Basin, sponges, sipunculids and fish in the northern Angola Basin, and asteroids, crustaceans and fish in the eastern Guinea Basin, while in the western Guinea Basin sipunculids dominated.In the northern Cape Basin, peracarid crustaceans were the dominant macrofaunal group, followed by polychaetes and bivalves. In the Guinea Basin, polychaetes, peracarid crustaceans and bivalves dominated, although omnivorous or predatory free-living nematodes of macrofaunal size (>0.5 mm) made up 40–60% of the total abundance, with maxima in the western basin.The chlorophyll a content of sediments was lower in the northern Cape and Angola Basins than in the Guinea Basin, which was consistent with the differences in water masses, primary production and flux rate of organic matter in the three basins of the South-East Atlantic. The differences in structure and function of the macro- and megafauna communities in the three basins correlated with the differences in the amount of food reaching the seafloor in tropical and subtropical settings.     

Intrusion events of the intermediate Oyashio Water into Sagami Bay, Japan

Intermediate intrusion of low salinity water (LSW) into Sagami Bay was investigated on the basis of CTD data taken in Sagami Bay and off the Boso Peninsula in 1993–1994. In October 1993, water of low temperature (<7.0°C), low salinity (<34.20 psu) and high dissolved oxygen concentration (>3.5 ml I⁻¹) intruded along the isopycnal surface of {ie29-1} at depths of 320–500 m from the Oshima East Channel to the center of the bay. On the other hand, the LSW was absent in Sagami Bay in the period of September–November 1994, though it was always found to the south off the Boso Peninsula. Salinity and dissolved oxygen distributions on relevant isopycnal surfaces and water characteristics of LSW cores revealed that the LSW intruded from the south off the Boso Peninsula to Sagami Bay through the Oshima East Channel. The LSW cores were distributed on the continental slope along 500–1000 m isobaths and its onshore-offshore scales were two to three times the internal deformation radius. Initial phosphate concentrations in the LSW revealed its origin in the northern seas. These facts suggest that the observed LSW is the submerged Oyashio Water and it flows southwestward along the continental slope as a density current in the rotating fluid. The variation of the LSW near the center of Sagami Bay is closely related to the Kuroshio flow path. The duration of LSW in Sagami Bay is 0.5 to 1.5 months.     

Recent sedimentation and sedimentary budgets on the western Iberian shelf

Thus study is based on 31 box-core samples collected mainly from the mud-patches of the Galician and North-Portuguese shelf. Each of these cores, some decimetres in length, have been described, radiographied and sampled each centimetre for grain-size analysis and measurement of the particulate organic carbon (C_org) content. ²¹⁰Pb excess profiles have been measured using gamma spectrometry, which, combined with the analysis of the vertical variations of the sedimentary facies, have allowed estimates of sedimentation rates. These rates were found to vary between 0.05 and 0.40 cm yr⁻¹. Considering the latitudinal variation of sedimentation rates around the 100m contour, we note that the maximum rates occurred off the Douro estuary in the southern sector and also near the Galician rias to the north. The organic content (C_org) of sediments varied between 0.4 and 2%. Latitudinally C_org showed a decrease from the South to the North. These results coupled with the sedimentary map (see Dias, Gonzalez, Garcia & Diaz del Rio, 2001) have permitted us to establish the detrital mass budget and an inventory of C_org burial.     

Upwelling and ocean structures off Algoa Bay and the south-east coast of South Africa

As the Agulhas Current flows along the south-east coast of South Africa, a number of processes operate that bring cold, deep water up onto the narrow shelf. As a consequence, upwelling along the coastline is enhanced farther southward and downstream. This situation is investigated off Algoa Bay and along the south-east coast to Port Alfred, where measurements demonstrate that marked temperature variability occurs at the coastline, particularly in summer when temperature structures are more intense and easterly-component winds more common. There is no indication that upwelling is more prevalent at Port Alfred; increasing variability farther south is evident at Woody Cape/Cape Padrone, where the coastline veers westwards, forming the eastern boundary of Algoa Bay. Here it is found that, after a wind change to north-easterly, cold water is upwelled along the shoreline between 19 hours and 2.5 days later. Such upwelling progresses north-eastwards with the movement of the wind and weather systems, although colder water also moves south-westwards into Algoa Bay. Winds, currents, sea level and sea temperatures are highly correlated, with fluctuations in sea level measuring >50 cm being associated with coastal trapped waves (CTWs). Such barotropic wind-driven CTWs are frequently active during upwelling, although it is unclear whether there is any interaction between the two phenomena.