Thermocline Regulated Seasonal Evolution of Surface Chlorophyll in the Gulf of Aden

The Gulf of Aden, although subject to seasonally reversing monsoonal winds, has been previously reported as an oligotrophic basin during summer, with elevated chlorophyll concentrations only occurring during winter due to convective mixing. However, the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) ocean color data reveal that the Gulf of Aden also exhibits a prominent summer chlorophyll bloom and sustains elevated chlorophyll concentrations throughout the fall, and is a biophysical province distinct from the adjacent Arabian Sea. Climatological hydrographic data suggest that the thermocline, hence the nutricline, in the entire gulf is markedly shoaled by the southwest monsoon during summer and fall. Under this condition, cyclonic eddies in the gulf can effectively pump deep nutrients to the surface layer and lead to the chlorophyll bloom in late summer, and, after the transition to the northeast monsoon in fall, coastal upwelling driven by the northeasterly winds produces a pronounced increase in surface chlorophyll concentrations along the Somali coast.     

上升流和水团对浙江中部近海浮游动物生态类群分布的影响

依据2010年4月、7月和11月对浙江中部近海上升流海域进行的海洋调查资料,运用定量、定性方法,探讨了上升流对该海域浮游动物生态类群分布的影响.结果表明:3个季节共鉴定浮游动物64种,桡足类占主要优势,包括5个生态类群,分别是暖温带近海种、暖温带外海种、亚热带近海种、亚热带外海种和热带大洋种.在种类数组成上,春季以暖温带近海种为主,夏季则是亚热带近海种和亚热带外海种居多,秋季也是亚热带种居多,其中夏季暖温带种种类数要高于春季和秋季,这一现象与同时期东黄海沿海有所不同,主要是由于上升流将一些在海洋底部度夏的暖温种带至海洋表面造成的.此外,3个季节生态类群都是以近海种为主,表明沿岸流是影响这一海域的最主要的水团.在丰度组成上,4月暖温带近海种占总丰度的98.79％,7月暖温带近海种也是组成丰度的重要部分,10月则是亚热带近海种丰度最高.丰度组成所反映的规律与种类数组成规律一致.上升流的存在导致夏季近海暖温带种大量出现,是影响这一海域浮游动物生态类群组成的重要因素;受长江径流和椒江径流的影响,近海种成为主要生态类群,是这一海域浮游动物的一个重要的生态特征.     

Summer dynamics of the deep chlorophyll maximum in Lake Tahoe

Vertical profiles of chlorophyll and phytoplankton biomass weremeasured in Lake Tahoe from July 1976 through April 1977. Adeep chlorophyll maximum (DCM) persisted during summer and earlyautumn (July—October) near 100 m, well below the mixedlayer and at the upper surface of the nitracline. The DCM coincidedwith the phytoplankton biomass maximum as determined from cellcounts. In addition, the composition of the phytoplankton assemblagewas highly differentiated with respect to depth. Cyclotellastelligera was the predominant species in the mixed layer whilethe major species in the DCM layer included C. ocellata andseveral green ultraplanktonic species. In situ cell growth playsa substantial role in maintaining the DCM, but sinking of cellsfrom shallower depths and zooplankton grazing above the DCMmay contribute to the maintenance of the DCM. Calculations supportthe interpretation that the summer DCM persists at the boundarybetween an upper, nutrient-limited phytoplankton assemblageand a deeper, light-limited assemblage.     

Seasonality of peak metabolic rate in non‐migrant tropical birds

Birds that are year‐round residents of temperate and tropical regions have divergent life histories. Tropical birds have a slower ‘pace of life’, one characteristic of which includes lower peak metabolic rate and daily activity levels. Temperate resident birds are faced with seasonal variation in thermogenic demand. This challenge is met with seasonally increased peak metabolic rate during winter. These thermogenic demands are much lower in birds that are year‐round tropical residents. By measuring peak (summit) metabolic rate in tropical and temperate resident bird species during summer and winter, we asked whether tropical birds exhibit seasonality in peak metabolic rate, and if the direction of seasonality differs between tropical and temperate species. We measured summit metabolism in seven tropical and one temperate species during the winter and during the summer breeding season to test the hypothesis that summit metabolism of tropical residents would change seasonally. We consider whether metabolic seasonality is associated with breeding season for tropical species. We found that summit metabolism was significantly greater during the summer for most tropical residents, while the temperate resident matched several previous reports with higher summit metabolism in winter. We conclude that metabolic seasonality occurs in tropical residents and differs from temperate residents, suggesting that breeding during the summer may be driving relatively higher metabolism as compared to winter thermogenesis in temperate birds.     

Coccolith evidence for Quaternary nutricline variations in the southern South China Sea

Nutricline variations during the last 1560 ka in the southern South China Sea are reconstructed using the relative abundance of the coccolithophore Florisphaera profunda in ODP Site 1143. Nutricline depth shows both long-term large magnitude variation and high frequency glacial–interglacial variation. On the long-term scale, the nutricline experienced four significant changes, which occurred at 900, 480, 250, and 50 ka respectively. According to these changes, five stages could be recognized during the last 1560 ka. From 1560 to 900 ka, the nutricline depth increased gradually. Around 900 ka, the nutricline abruptly shallowed and then remained stable until 480 ka. The nutricline was shallow during the time interval between 480 and 250 ka. At 250 ka, it deepened again and increased gradually until 50 ka. After 50 ka, the nutricline depth decreased gradually to modern values. On the glacial–interglacial scale, the variations in nutricline depth show different patterns before and after 900 ka. Before 900 ka, the nutricline was deep during glacial periods and shallow during interglacials. However, after 900 ka, the nutricline was deep during interglacials and shallow during glacials. Spectral analysis of the relative abundance of F. profunda shows a similar trend. In addition to the eccentricity (113, 76 ka), obliquity (55, 39 ka), and precession (24, 19 ka), we also find a 431 ka cycle. The former three periods reflect glacial–interglacial variations in nutricline, and the period of 431 ka reflects long-term variations in nutricline. We suggested that the variations in nutricline in the southern South China Sea were caused by global and regional climate changes. Glacial–interglacial variations in nutricline are mainly controlled by the East Asian monsoon, and the long-term variations might be related to the global climatic events, such as the mid-Pleistocene Revolution and the mid-Brunhes event.     

Modelling the contribution of deep chlorophyll maxima to annual primary production in the North Sea

Seasonally stratified areas in temperate shelf seas are usually characterized by a strong spring bloom, followed by limited production within the surface mixed layer as nutrients are depleted in the post-bloom period. The bottom mixed layer remains nutrient-rich, due to regeneration processes. When the thermocline is within the euphotic zone, primary production at thermocline depth is possible as organisms can access nutrients from the bottom layer. Ship-based observations indicate that production within these deep chlorophyll maxima (DCM) can form a substantial part of the annual production, but this process is not captured by satellite observations. The spatial and temporal behaviour of DCM-related production is therefore less well known. To study the extent of DCM occurrence we applied a fully three dimensional, hydro-biogeochemical model to the North Sea. The simulation was used to quantify sub-surface production in space and time to determine the importance of production within the DCM layer with respect to annual and spring bloom primary production in the North Sea. The results showed that locally, in thermally stratified areas in the North Sea, up to 30% of annual net primary production occurred below 15 m depth. In a 21-year average, sub-surface production contributed 10% to the total annual net primary production in the entire North Sea. These values should be interpreted as lower estimates, as the model was unable to represent observed hot-spots of very high chlorophyll a concentrations at depth. The importance of DCM-related production may increase under future climate scenarios when the length of the stratified period may increase.     

Reconstructing nutricline dynamics of mid-Cretaceous oceans: evidence from calcareous nannofossils from the Niveau Paquier black shale (SE France)

A high-resolution calcareous nannofossil record is presented from the Lower Albian Niveau Paquier black shale from the Vocontian Basin (SE France). The Niveau Paquier black shale represents the regional equivalent of the supraregionally distributed Oceanic Anoxic Event 1b (OAE 1b). To reconstruct surface water fertility, a nutrient index based on Zeugrhabdotus erectus, Discorhabdus rotatorius (high fertility indicators), and Watznaueria barnesae (low fertility indicator) was established using principal component analysis. In addition, the distribution of Nannoconus spp. and absolute abundances of coccoliths (coccoliths per gram) were used for reconstructing nutricline dynamics of the surface waters. High surface water fertility coincides with low percentages of nannoconids and vice versa. Moreover, high percentages of nannoconids correlate with low absolute abundances of all other coccoliths. Based on the observed nannoplankton distribution pattern and a suggested similarity in ecological requirements between nannoconids and the modern taxon Florisphaera profunda, a model is proposed that couples nannoconid abundances with dynamics of the nutricline. Time series analyses of the nutrient index show fluctuations within the precessional band. The precession-controlled fluctuations of the surface water fertility may represent a monsoonal signal, with the nutrient supply in the surface waters depending on the strength of monsoonal activity. During periods of enhanced monsoonal activity, which were characterized by humid conditions and stronger winds, mixing of the upper water column was enhanced. During that time, the abundance of nannoconids decreased as a consequence of enhanced wind stress that improved vertical mixing and led to an entrainment of nutrients into the surface waters. This resulted in an increase of primary production. During periods of reduced monsoonal activity, marked by drier conditions and reduced wind stress, the surface waters were depleted in nutrients. As a result of a deep nutricline and a reduced nutrient supply to the upper photic zone in a stratified water column, percentages of nannoconids increased. According to the mechanisms outlined above, fluctuations of the nutrient index and nannoconid percentages have been used as a proxy for reconstructing the physical structure of mid-Cretaceous oceans steered by precession-forced monsoonal activity. The results of the study show that the formation of the Lower Albian OAE 1b from the Vocontian Basin occurred under strongly fluctuating surface waters fertility.     

Growing or reproducing in a temperate sea: optimization of resource allocation in a colonial ascidian

Relatively little is known about the life cycles of ascidians in temperate seas. Here, we investigated the biological cycle of the colonial ascidian Didemnum fulgens, a dominant species in some shallow localities of the NW Mediterranean Sea. Growth rates and frequencies of fission/fusion events were calculated over a period of 13 months, and the reproductive cycle determined after 32 months of observation. For analyses of reproduction, zooids were dissected in the laboratory and classified into five reproductive categories; these data were used to calculate a maturity index. For growth analyses, underwater photographs of marked colonies were used to estimate the surface area of D. fulgens colonies, calculate monthly growth rates, and document fusion and fission events. Clear seasonal patterns in reproduction and growth were observed, with distinct periods of investment into each function. Gonad maturation started in winter and larval release occurred in early summer, just before maximal sea temperatures were reached. After reproducing, colonies shrank and aestivated during the warmer summer months. Growth occurred during the cooler months, with maximal and minimal growth rates observed in winter and summer, respectively. Fusions and fissions occurred year‐round, although fissions were more frequent in fall (coincident with high growth rates) and fusions in spring (coincident with reproduction). These results add to the mounting evidence that ascidian life cycles in temperate seas are characterized by a trade‐off between investment in reproduction and growth, triggered by seasonal temperature shifts and constrained by resource availability during summer.     

Seasonality in coastal benthic ecosystems

For historical reasons, knowledge about seasonality in the dynamics of marine benthic suspension feeders from temperate areas comes mainly from studies of cold temperate seas. Recent surveys of Mediterranean taxa show different patterns from those observed in cold temperate seas, which are characterized by winter dormancy. In the Mediterranean, summer dormancy predominates among taxa and appears to be related to energetic constraints. Temperature and food availability are crucial to the dynamics of benthic suspension feeders. However, because these factors tend to be positively correlated in cold temperate seas, it is difficult to distinguish between their effects. Such correlation does not occur in Mediterranean ecosystems. The contrast between recent studies in the Mediterranean and in other areas can help to disentangle confounded environmental controls.     

Depth profiles of plankton,particulate organic matter and microbial activity in the eastern Canadian Arctic during summer

Summary Deep profiles of particulate organic matter, microplankton (phytoplankton and bacteria), zooplankton and their metabolic activities were investigated during two summer voyages to the eastern Canadian Arctic. Magnitudes and depth distributions were similar in many respects to observations from temperate and tropical waters. Strong gradients in most properties were observed in the upper 50–100 m and subsurface maxima were generally associated with the upper mixed-layer (>50 m). In addition to the general vertical decreases in plankton biomass and metabolic activity there was evidence for both rapid transport (sinking) of organic matter and for enhanced (above background) levels of microbial metabolic activity in deep waters (>500 m). Zooplankton depth distributions differed from the pattern generally observed at lower latitudes; in the Arctic, zooplankton abundance decreased to a lesser degree with depth than particulate organics and microplankton. The overwintering behavior of high-latitude zooplankton appeared to be the best explanation for their relatively high abundance at depth. Despite this, however, zooplankton apparently contributed little to the total column community metabolism.     

Ethylene and methane in the upper water column of the subtropical Atlantic

The vertical distributions of ethylene and methane in the upper water column of the subtropical Atlantic were measured along a transect from Madeira to the Caribbean and compared with temperature, salinity, oxygen, nutrients, chlorophyll-a, and dissolved organic carbon (DOC).Methane concentrations between 41.6 and 60.7 nL L-1 were found in the upper 20 m of the water column giving a calculated average flux of methane into the atmosphere of 0.82 g m-2 h-1. Methane profiles reveal several distinct maxima in the upper 500 m of the water column and short-time variations which are presumably partly related to the vertical migration of zooplankton.Ethylene concentrations in near surface waters varied in the range of 1.8 to 8.2 nL L-1. Calculated flux rates for ethylene into the atmosphere were in the range of 0.41 to 1.35 g m-2 h-1 with a mean of 0.83 g m-2 h-1. Maximum concentrations of up to 39.2 nL L-1 were detected directly below the pycnocline in the western Atlantic. The vertical distributions of ethylene generally showed one maximum at the pycnocline (about 100 m depth) where elevated concentrations of chlorophyll-a, dissolved oxygen, and nutrients were also found; no ethylene was detected below 270 m depth. This suggests that ethylene release is mainly related to one, probably phytoplankton associated, source, while for methane, enhanced net production occurs at various depth horizons. For surface waters, a simple correlation between ethylene and chlorophyll-a or DOC concentrations could not be observed. No considerable diurnal variation was observed for the distribution and concentration of ethylene in the upper water column.     

Reproduction and growth of the Arctic hyperiid amphipod Themisto libellula Mandt

Summary The hyperiid amphipod, Themisto libellula, is abundant in Arctic seas and is an important prey for fish, birds and mammals. Characterization of its life cycle has been hampered by the lack of winter collections. In this study, that portion of the hyperiid population inhabiting the upper 30 m of the water column in Frobisher Bay was sampled periodically in winter, spring and summer during five consecutive years. In one year the winter and spring collections included large numbers of ovigerous females. These samples provide new information about timing of reproduction, size at maturity, fecundity, embryonic development, hatching, release and rate of growth of T. libellula in the Arctic. The average length of the ovigerous females was 23.3 mm. This and the summer growth rate of about 6 mm/month indicates that they can reproduce when one year old. Eggs are laid prior to early February, and although some young may be released from the marsupium as early as mid-March, most are liberated during April and May. The fecundity is significantly correlated with female length and an average 23 mm long individual had 275 embryos. The size-specific fecundity is similar to that of hyperiids from temperate and subtropical waters.     

A one-dimensional model of vertical stratification of Lake Shira focussed on winter conditions and ice cover

In meromictic lakes such as Lake Shira, horizontal inhomogeneity is small in comparison with vertical gradients. To determine the vertical distribution of temperature, salinity, and density of water in a deep zone of a Lake Shira, or other saline lakes, a one-dimensional (in vertical direction) mathematical model is presented. A special feature of this model is that it takes into account the process of ice formation. The model of ice formation is based on the one-phase Stefan problem with the linear temperature distribution in the solid phase. A convective mixed layer is formed under an ice cover due to salt extraction in the ice formation process. To obtain analytical solutions for the vertical distribution of temperature, salinity, and density of water, we use a scheme of vertical structure in the form of several layers. In spring, the ice melts as top and bottom. These processes are taken into account in the model. The calculated profiles of salinity and temperature of Shira Lake are in good agreement with field measurement data for each season. Additionally, we focussed on the redox zone, which is the zone in which the aerobic layers of a water column meet the anaerobic ones. Hyperactivity of plankton communities is observed in this zone in lakes with hydrogen sulphide monimolimnion, and Lake Shira is among them. The location of the redox zone in the lake, which is estimated from field measurements, coincides with a sharp increase in density (the pycnocline) during autumn and winter. During spring and summer, the redox zone is deeper than the pycnocline. The location of pycnocline calculated with the hydro physical model is in good agreement with field measurement data.     

Interaction effects of zooplankton and CO2 on phytoplankton communities and the deep chlorophyll maximum

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Interannual variability in the distribution of the phytoplankton standing stock across the seasonal sea-ice zone west of the Antarctic Peninsula

The spatial distribution of phytoplankton cell abundance, carbon(C) biomass and chlorophyll a (Chl a) concentration was analysedduring three summers (1996, 1997 and 1999) in a seasonal sea-icearea, west of the Antarctic Peninsula. The objective of thestudy was to assess interannual variability in phytoplanktonspatial distribution and the mechanisms that regulate phytoplanktonaccumulation in the water column. Phytoplankton C biomass andChl a distributions were consistent from year to year, exhibitinga negative on/offshore gradient. The variations in C concentrationhad a close and non-linear relationship with the upper mixedlayer depth, suggesting that the vertical mixing of the watercolumn is the main factor regulating phytoplankton stock. Themagnitude of C gradients was 5-fold higher during 1996 thanduring 1997 and 1999. This was ascribed to interannual variationsin the concentration of diatom blooms in the region influencedby sea-ice melting. Vertical distribution of the phytoplankton,as estimated from Chl a profiles, also varied along an on/offshoregradient: Chl a was distributed homogeneously in the upper mixedlayer in coastal and mid-shelf stations and concentrated inthe deep layer (40–100 m) occupied by the winter waters(WW, remnants of the Antarctic surface waters during summer)in more offshore stations. The region with a deep Chl a maximumlayer (DCM layer) was dominated by a phytoplankton assemblagecharacterized by a relatively high concentration of diatoms.The extent of this region varied from year to year: it was restrictedto pelagic waters during 1996, extended to the shelf slope during1997 and occupied a major portion of the area during 1999. Itis hypothesized that iron depletion in near surface waters dueto phytoplankton consumption, and a higher concentration inWW, regulated this vertical phytoplankton distribution pattern.Furthermore, we postulate that year-to-year variations in thespatial distribution of the DCM layer were related to interannualvariations in the timing of the sea-ice retreat. The similaritybetween our results and those reported in literature for otherareas of the Southern Ocean allows us to suggest that the mechanismsproposed here as regulating phytoplankton stock in our areamay be applicable elsewhere.     

Chaotic behaviour of an ocean ecosystem model under seasonal external forcing

A four-component ecosystem model of the oceanic upper mixedlayer (UML) forced by the annual cycle of UML depth, solar irradiationand dissolved inorganic nitrogen (DIN) entrainment from theseasonal pycnocline is presented. The model solution demonstratesthe following types of temporal variability: a periodical regimewith the frequency of the external forcing, a regime with aperiod of more than 1 year, quasi-periodic, and chaotic motion.The model results suggest that the last three types describingthe interannual variability can occur only at low latitudesin regions of strong upwelling where the DIN concentration inthe seasonal pycnocline is high. However, the range of externalforcing parameters in which such behaviour takes place is sonarrow that it is unlikely to be a common phenomenon in theocean. The quasi-periodic or chaotic variability of the modelecosystem is very sensitive to the initial conditions, and thereforeany exact prediction of model behaviour is impossible. Nevertheless,a prediction of model ecosystem behaviour can be obtained interms of a probability density. The annual cycle of the modelcomponents calculated in this way shows that the dispersionof the trajectories during the winter period is markedly smallerthan during the summer. It implies that the dynamics of themodel ecosystem during the summer period is less predictable.     

Temperate and tropical algal-sea anemone symbioses

Abstract. In this review, we seek to develop new insights about the nature of algal‐sea anemone symbioses by comparing such associations in temperate and tropical seas. Temperate seas undergo pronounced seasonal cycles in irradiance, temperature, and nutrients, while high irradiance, high temperature, and low nutrients are seasonally far less variable in tropical seas. We compare the nature of symbiosis between sea anemones (= actinians) and zooxanthellae (Symbiodinium spp.) in both regions to test tropical paradigms against temperate examples and to identify directions for future research. Although fewer anemone species are symbiotic in temperate regions, they are locally dominant and ecologically important members of the benthic community compared to the tropics. Zooxanthella densities tend to be lower in temperate anemones, but data are limited to a few species in both temperate and tropical seas. Zooxanthella densities are far more stable over time in temperate anemones than in tropical anemones, suggesting that temperate symbioses are more resistant to fluctuations in environmental parameters such as irradiance and temperature. Light‐saturated photosynthetic rates of temperate and tropical zooxanthellae are similar, but temperate anemone hosts receive severely reduced carbon supplies from zooxanthellae during winter months when light is reduced. Symbiont transmission modes and specificity do not show any trends among anemones in tropical vs. temperate seas. Our review indicates the need for the following: (1) Investigations of other temperate and tropical symbiotic anemone species to assess the generality of trends seen in a few “model’ anemones. (2) Attention to the field ecology of temperate and tropical algal‐anemone symbioses, for example, how symbioses function under seasonally variable environmental factors and how zooxanthellae persist at high densities in darkness and winter. The greater stability of zooxanthella populations in temperate hosts may be useful to understanding tropical symbioses in which bleaching (loss of zooxanthellae) is of major concern. (3) Study of the evolutionary history of symbiosis in both temperate and tropical seas. Continued exploration of the phylogenetic relationships between host anemones and zooxanthella strains may show how and why zooxanthellae differ in anemone hosts in both environments.     

Monthly variations in calix growth, polyp tissue, and density banding of the Mediterranean scleractinian Cladocora caespitosa (L.)

Monthly skeletal growth of the scleractinian, temperate coral Cladocora caespitosa (L.) from the Ligurian Sea (NW Mediterranean) was analysed for a period of 1 year and compared with seawater parameters. Measurements on corallite sections and on X-ray images showed that the formation of the high-density (HD) band and two dissepiments are favoured by fall–winter conditions, characterised by high quantities of rain, rough seas, and cold seawater. In summer, when the low-density (LD) band is formed, the corallites stretch upward and form one new dissepiment and one deep calix, where the polyps recede almost completely in August. These findings confirmed the adaptation of the temperate coral to winter environmental conditions, characterised by low irradiance and high availability of nutrients and food particles resuspended from bottom sediments. On the contrary, the high seawater temperature, irradiance, and ammonia contents stressed the coral in August and, when they persist in September, may cause the onset of mortality events.     

Pigment gut contents of copepods and deep phytoplankton maximum in the Western Mediterranean

The feeding activity of Centropages typicus in the Western Mediterraneanwas evaluated during the summer stratification period, whena deep phytoplankton and chlorophyll maximum (DCM) develops.The lack of significant day-night differences in gut contentssuggests continuous phytoplankton ingestion, despite nictemeralmigration of this species. Copepods captured in the DCM hadgreater phytoplankton gut contents, both in nocturnal and diurnalsamples. The observed pattern of phytoplankton ingestion andvertical migration implies copepod-mediated upward transportof nutrients, which might contribute to the maintenance of primaryproduction in the photic layers.     

中国海毛颚类物种多样性的研究

中国海已记录毛颚类有37种,可分为3个生态类群,即暖水性、暖温性和深水类群。种数是由北往南、由近岸向外海递增。中国海毛颚类可划分暖温带区系、热带- 暖温带混合区系和热带大洋区系。大多数种类广泛分布于印度洋、大西洋和太平洋等水域。