Ontogenetic Vertical Migration and Life cycle of Neocalanus Plumchrus (Crustacea: Copepoda) in the Oyashio Region, with Notes on Regional Variations in Body Sizes

The ontogenetic vertical migration and life cycle of Neocalanusplumchrus were investigated by anal-yzing monthly populationstructure at Site H in the Oyashio region from September 1996through October 1997. Additional sampling was also done at severalstations covering the entire subarctic Pacific, Okhotsk Seaand Japan Sea as a basis for regional comparison of life cyclesand body sizes. At Site H, N. plumchrus spawned October to Aprilbelow 250 m depth. Young copepodite stages (C1–C5) occurredduring June late in the phytoplankton bloom. The C5 migratedto the deeper layers in July–August where they moltedto adults. Development time of C5 to C6 was highly variable.The ontogenetic vertical migration of N. plumchrus ranged fromthe surface to 1000–2000 m depth, and the life cycle wasannual. Temporal data on population structure and vertical distributionsuggestedthe annual life cycle was generally synchronized throughoutthe subarctic Pacific and its marginal seas. Geographical comparisonof C5 prosome length indicated the occurrence of significantlylarge specimens in the Oyashio region and Okhotsk Sea but smallspecimens in the Japan Sea. Possible causes for regional variabilityin body sizes are discussed.     

The family Xystonellidae (Ciliophora, Tintinnina) in the Adriatic Sea

This paper reports quantitative, horizontal and vertical distributiondata of the 11 species of the family Xystonellidae found inthe Adriatic Sea and in the Strait of Otranto. The samples werecollected in coastal and open waters, with a plankton net equippedwith a closing system, in the period from 1973 to 1986. At thedeeper stations of the South Adriatic Pit, hauls were takenin eight vertical layers, while at other stations sampling dependedon the respective depth of each station. Xystonellidae are oceanictintinnines, mainly present in the South Adriatic and the Straitof Otranto throughout the year. They are characteristic of SouthAdriatic and Eastern Mediterranean water masses. Horizontaldistribution of lower mesopelagic species is limited by the200 m isobath. Only sparse individuals of the upper mesopelagicand epipelagic species were found in the Jabuka Pit, probablydue to the late autumn and winter current. These species werenot registered to the north of this region. They were registeredbelow the 200 m depth as dominant tintinnines, while in summerthey represented an average of 81% of the total tintinnine population.     

Seasonal variation of macrobenthic infauna in the Johor Strait,Singapore

Sediment samples were collected every three months from December 2001 to December 2002 to study the seasonal variation of soft-bottom macrobenthic community at a site (1°25′30′′N, 103°52′00′′E; depth: 6.5 m) in the Johor Strait, Singapore. Seasonal changes in species number, abundance and diversity of macrobenthic infauna significantly differed, the lowest values for the three parameters were recorded in December, the highest values for species number and species diversity in March and the highest abundance in September. Species composition and community structure also varied gradually over time. The differences in dominant species and community structure between December 2001 and December 2002 suggest an interannual variation of macrobenthic infauna.     

Seasonal and daily movements of the bluntnose sixgill shark (<Emphasis Type="Italic">Hexanchus griseus</Emphasis>) in the strait of Georgia from satellite tag data

The bluntnose sixgill shark (Hexanchus griseus Bonnaterre 1788) is a large, deepwater apex predator that, in a few unique locations such as the Strait of Georgia, Canada, occupies shallow, inshore waters as juveniles. The occurrence of pregnant females in the Strait of Georgia suggests that this is an important area for parturition and juvenile rearing. Bluntnose sixgill sharks are listed under the Canadian Species at Risk Act, and an understanding of their behaviour in habitat selection during important life stages is an important component for conservation measures. We employed satellite tags to identify the daily and seasonal depth and thermal habitats of juvenile bluntnose sixgill sharks in the Strait of Georgia. Tag release and depth information suggests that juvenile bluntnose sixgill sharks within the Strait of Georgia remain there until they mature and migrate to offshore deepwaters. The dominant depth habitat utilized by the juvenile sharks in our study was deepwater (>200 m), with only occasional occupation of depths <100 m, indicating that even within coastal areas, juveniles still prefer deepwater. Overall the sharks occupied shallower depths at night than during the day with increased vertical activity in both the day and night compared to crepuscular periods. Seasonally, the sharks occupied shallower depths in the summer than in winter, but inconsistency in the temperatures with which those depths were associated suggests that their vertical behaviour is influenced by local foraging opportunities and not by thermoregulation.     

Highly Ordered Vertical Structure of Synechococcus Populations within the One-Millimeter-Thick Photic Zone of a Hot Spring Cyanobacterial Mat

A variety of contemporary techniques were used to investigate the vertical distribution of thermophilic unicellular cyanobacteria, Synechococcus spp., and their activity within the upper 1-mm-thick photic zone of the mat community found in an alkaline siliceous hot spring in Yellowstone National Park in Wyoming. Detailed measurements were made over a diel cycle at a 61°C site. Net oxygenic photosynthesis measured with oxygen microelectrodes was highest within the uppermost 100- to 200-μm-thick layer until midmorning, but as the day progressed, the peak of net activity shifted to deeper layers, stabilizing at a depth of 300 μm from midday throughout the afternoon. Examination of vertical thin sections by bright-field and autofluorescence microscopy revealed the existence of different populations of Synechococcus which form discrete bands at different vertical positions. Denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA gene segments from horizontal cryosections obtained at 100-μm-thick vertical intervals also suggested vertical stratification of cyanobacterial, green sulfur bacterium-like, and green nonsulfur bacterium-like populations. There was no evidence of diel migration. However, image analysis of vertical thin sections revealed the presence of a narrow band of rod-shaped Synechococcus cells in which the cells assumed an upright position. These upright cells, located 400 to 800 μm below the surface, were observed only in mat samples obtained around noon. In mat samples obtained at other time points, the cells were randomly oriented throughout the mat. These combined observations reveal the existence of a highly ordered structure within the very thin photic zone of this hot spring microbial mat, consisting of morphologically similar Synechococcus populations that are likely to be differentially adapted, some co-occurring with green sulfur bacterium-like populations, and all overlying green nonsulfur bacterium-like populations.     

Natural variability or anthropogenically-induced variation? Insights from 15 years of multidisciplinary observations at the arctic marine LTER site HAUSGARTEN

Time-series studies of arctic marine ecosystems are rare. This is not surprising since polar regions are largely only accessible by means of expensive modern infrastructure and instrumentation. In 1999, the Alfred Wegener Institute, Helmholtz-Centre for Polar and Marine Research (AWI) established the LTER (Long-Term Ecological Research) observatory HAUSGARTEN crossing the Fram Strait at about 79° N. Multidisciplinary investigations covering all parts of the open-ocean ecosystem are carried out at a total of 21 permanent sampling sites in water depths ranging between 250 and 5500 m. From the outset, repeated sampling in the water column and at the deep seafloor during regular expeditions in summer months was complemented by continuous year-round sampling and sensing using autonomous instruments in anchored devices (i.e., moorings and free-falling systems). The central HAUSGARTEN station at 2500 m water depth in the eastern Fram Strait serves as an experimental area for unique biological in situ experiments at the seafloor, simulating various scenarios in changing environmental settings. Long-term ecological research at the HAUSGARTEN observatory revealed a number of interesting temporal trends in numerous biological variables from the pelagic system to the deep seafloor. Contrary to common intuition, the entire ecosystem responded exceptionally fast to environmental changes in the upper water column. Major variations were associated with a Warm-Water-Anomaly evident in surface waters in eastern parts of the Fram Strait between 2005 and 2008. However, even after 15 years of intense time-series work at HAUSGARTEN, we cannot yet predict with complete certainty whether these trends indicate lasting alterations due to anthropologically-induced global environmental changes of the system, or whether they reflect natural variability on multiyear time-scales, for example, in relation to decadal oscillatory atmospheric processes.     

Larval fish distribution and abundance in the Antarctic Peninsula region and adjacent waters

The spatial distribution, species composition and abundance of ichthyoplankton in the Bransfield Strait and adjacent waters were studied during two cruises in the Antarctic spring 1991/92 and summer 1992/93 seasons. A multiple plankton net (Bioness) and a Bongo net were used to collect samples at 35 stations in 1991/92 and 75 stations in 1992/93. Early larval stages (14 species) and juveniles (13 species) representing the known Bransfield Strait ichthyofauna were present in the water column. The nototheniids predominated in the entire study area. The greatest species diversity was found in the uppermost 200 m of the water column in the Bransfield Strait. Notothenia gibberifrons and Nototheniops larseni dominated in spring, whilst in summer Pleuragramma antarcticum dominated in association with N. larseni. The dominant species in the Gerlache Strait were P. antarcticum and Notothenia kempi, while P. antarcticum and Trematomus scotti were predominant in the Bellingshausen Sea area.     

Swimming ability and behaviour of post-larvae of a temperate marine fish re-entrained in the pelagic environment

The degree to which behaviour, vertical movement and horizontal transport, in relation to local hydrodynamics, may facilitate secondary dispersal in the water column was studied in post-larval Sillaginodes punctata in Port Phillip Bay, Australia. S. punctata were captured in shallow seagrass beds and released at the surface in three depth zones (1.5, 3 and 7 m) off-shore at each of two sites to mimic the re-entrainment of fish. The behaviour, depth and position of S. punctata were recorded through time. The direction and speed of local currents were described using an S4 current meter and the movement of drogues. Regardless of site, fish immediately oriented toward the bottom, and into the current after release. In shallow water (1.5 m), 86% of fish swam to the bottom within 2 min of release. At one site, the net horizontal displacement of fish was largely unrelated to the speed and direction of local currents; at a second site, fish could not maintain their position against the current, and the net horizontal displacement was related to the speed and direction of currents. In the intermediate depth zone, wide variability in depths of individual fish through time led to an average depth reached by fish that was between the shallow and deep zones. Based on daily increments in the otoliths, however, this variability was not related significantly to the time since entry of fish into Port Phillip Bay. In the deepest depth zone, 81% of fish remained within 1 m of the surface and their horizontal displacement was significantly related to the direction and speed of currents. Secondary dispersal of post-larval fish in the water column may be facilitated by the behaviour and vertical movements of fish, but only if fish reach deeper water, where their displacement (direction and distance) closely resembles local hydrodynamic regimes. In shallow water, fish behaviour and vertical migration actually reduce the potential for secondary dispersal.     

Zooplankton distribution and diversity in a frontal area of the Aegean Sea

This study was carried out in the northern Aegean Sea during late summer and spring. The aim was to examine the spatial and temporal distribution of the zooplankton community across a dynamic frontal area and to investigate how the oceanographic heterogeneity structures the composition of the zooplankton assemblages. The low-salinity and cold Black Sea water coming from the Dardanelles Strait is modified by mixing with the underlying warm and saline Aegean water. These hydrological features result in a pronounced thermohaline front in the northern Aegean Sea throughout the year. In both seasons, zooplankton was collected using both 45 and 200 µm mesh plankton nets. A high abundance of zooplankton was observed in the surface layer at the stations closest to the Dardanelles Strait on the stratified side of the front. The zooplankton distribution and community structure in the northern Aegean Sea were strongly influenced by the hydrological features. The frontal structure acts as a boundary for the zooplankton community. The surface layer at the stratified stations had the lowest copepod diversity, from where it increased with depth and horizontally as the stratification weakened outside of the front. The total abundance of zooplankton collected with the 200 µm net was between two and 20 times lower than samples taken with the 45 µm net. The most pronounced differences were observed for the adults and copepodids of the small genera Oithona, Oncaea and Microsetella. Thus, to manage and understand the transfer of primary production up the food chain in the Aegean Sea, the smaller fraction of copepods should be taken into account in future investigations.     

Highly ordered vertical structure of Synechococcus populations within the one-millimeter-thick photic zone of a hot spring cyanobacterial mat

A variety of contemporary techniques were used to investigate the vertical distribution of thermophilic unicellular cyanobacteria, Synechococcus spp., and their activity within the upper 1-mm-thick photic zone of the mat community found in an alkaline siliceous hot spring in Yellowstone National Park in Wyoming. Detailed measurements were made over a diel cycle at a 61 degrees C site. Net oxygenic photosynthesis measured with oxygen microelectrodes was highest within the uppermost 100- to 200-microm-thick layer until midmorning, but as the day progressed, the peak of net activity shifted to deeper layers, stabilizing at a depth of 300 microm from midday throughout the afternoon. Examination of vertical thin sections by bright-field and autofluorescence microscopy revealed the existence of different populations of Synechococcus which form discrete bands at different vertical positions. Denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA gene segments from horizontal cryosections obtained at 100-microm-thick vertical intervals also suggested vertical stratification of cyanobacterial, green sulfur bacterium-like, and green nonsulfur bacterium-like populations. There was no evidence of diel migration. However, image analysis of vertical thin sections revealed the presence of a narrow band of rod-shaped Synechococcus cells in which the cells assumed an upright position. These upright cells, located 400 to 800 microm below the surface, were observed only in mat samples obtained around noon. In mat samples obtained at other time points, the cells were randomly oriented throughout the mat. These combined observations reveal the existence of a highly ordered structure within the very thin photic zone of this hot spring microbial mat, consisting of morphologically similar Synechococcus populations that are likely to be differentially adapted, some co-occurring with green sulfur bacterium-like populations, and all overlying green nonsulfur bacterium-like populations.     

Comparing Bongo net and N70 mesozooplankton catches: using a reconstruction of an original net to quantify historical plankton catch data

If Southern Ocean plankton communities are changing in response to climate, biases in various nets need to be evaluated to help understand regional and temporal differences between historical and contemporary sample collections. A comparison of the catching ability of a Bongo net (mesh aperture 200?μm) and a reconstructed version of an N70 net (upper mesh aperture 445?μm, lower mesh aperture 195?μm) as used by the Discovery Investigations was therefore undertaken. Forty Bongo and forty N70 samples were obtained from 10 stations in the Southern Ocean during December 2009. Bongo net catch abundance was ~3 times greater than the N70 and ~4 times greater when only copepod instars <0.5?mm body length were considered. The Bongo net captured more plankton at all chlorophyll a (Chl a) concentrations encountered, although the difference was less at high Chl a when the N70 appeared to filter all size classes of copepod more efficiently, as well as retaining a relatively greater proportion of non-copepod plankton. Application of these findings was made to a previous study in which N70 samples from 1926/1927 were compared to a series of Bongo net hauls made post-1995. By ‘correcting’ abundances for net bias among a common set of 45 taxa, N70 data were within the range of variability seen in the Bongo net samples. Making such inter-net comparisons allows us to use historical collections and better judge the nature and magnitude of change in these plankton communities.     

Response of Acala cotton to nitrogen rates in the San Joaquin Valley of California

The responses of Acala cotton (Gossypium hirsutum L.) in California to a range of applied nitrogen (N) treatments were investigated in a 5-year, multisite experiment. The experiment's goals were to identify crop growth and yield responses to applied N and provide information to better assess the utility of soil residual N estimates in improving fertilizer management. Baseline fertilizer application rates for the lowest applied N treatments were based on residual soil nitrate-N (NO3-N) levels determined on soil samples from the upper 0.6 m of the soil collected prior to spring N fertilization and within 1 week postplanting each year. Results have shown positive cotton lint yield responses to increases in applied N across the 56 to 224 kg N/ha range in only 41% (16 out of 39) of test sites. Soil NO3-N monitoring to a depth of 2.4 m in the spring (after planting) and fall (postharvest) indicate most changes in soil NO3- occur within the upper 1.2 m of soil. However, some sites (those most prone to leaching losses of soluble nutrients) also exhibited net increases in soil NO3-N in the 1.2- to 2.4-m depth zone when comparing planting time vs. postharvest data. The lack of yield responses and soil NO3-N accumulations at some sites indicate that more efforts should be put into identifying the amount of plant N requirements that can be met from residual soil N, rather than solely from fertilizer N applications.     

Some distributional characteristics of small zooplankton at two stations in the Otranto Strait (Eastern Mediterranean)

The qualitative composition, numerical abundance and vertical distribution of protozoan and microcopepod communities were studied in the Otranto Strait at two stations during five cruises from August 1986 to May 1990. The samples were collected with a plankton net of 53-m mesh size equipped with a closing system in six or seven vertical layers. The principal component analysis (PCA) was used to analyse the spatial and temporal variability of small zooplanktonic assemblages. Data are presented for 36 species of radiolarians, 46 tintinnines and for 22 oncaeid copepods. Differences between the eastern shore and the western shore were not significant, according to the PC analysis of the total density values for all assemblages. However, on the basis of the distribution of PC loadings, it was possible to distinguish three vertical layers whose variability contributes in different amounts to the total variability in the small zooplankton community at both stations. The water masses have crucial significance for the spatial and seasonal distribution of all assemblages. Each type of water mass is connected by the presence of characteristic species.     

Optimizing methods to estimate zooplankton concentration based on generalized patterns of patchiness inside ballast tanks and ballast water discharges

Zooplankton populations are spatially heterogeneous in nature and inside ship ballast tanks. Sampling methods should take heterogeneity into account, particularly when estimating quantitative variables such as abundance or concentration. It is particularly important to generate unbiased estimates of zooplankton concentration in ballast water when assessing compliance with new international ballast water discharge standards. We measured spatial heterogeneity of zooplankton within ballast water using three sampling methodologies. In‐tank pump samples were collected at fixed depths within the vertical part of the ballast tank (side tank). Vertical net‐haul samples were collected from the upper portion of the tank as a depth‐integrated and historically relevant method. In‐line, time‐integrated samples were collected during ballast discharge by an isokinetic sample probe, likely representing the double bottom part of the ballast tank. The bias and precision associated with each sampling method were evaluated in reference to the estimated average abundance of the entire ballast tank, which was modeled from the data collected by all methods. In‐tank pump samples provided robust evidence for vertical stratification of zooplankton concentration in the side tank. A consistent trend was also observed for in‐line discharge samples, with zooplankton concentration decreasing through time as the ballast tank is being discharged. Sample representativeness, as compared to the tank average, varied depending on the depth or tank volume discharged. In‐line discharge samples provided the least biased and most precise estimate of average tank abundance (having lowest mean squared error) when collected during the time frame of 20%–60% of the tank volume being discharged. Results were consistent across five trips despite differences in ballast water source, season, and age.     

Field comparison of an LHPR net sampling system and an Optical Plankton Counter (OPC) in the Southern Ocean

A field comparison of an optical plankton counter (OPC) anda Longhurst-Hardy Plankton Recorder (LHPR) was undertaken aroundthe sub-Antarctic island of South Georgia during austral spring,1997, to investigate the capacity of the OPC to accurately describefine-scale plankton distributions. Initially, formalin-preserveddevelopmental stages of representative zooplankton were passedthrough a benchtop OPC and a relationship established betweenmaximum cross sectional area (CSA) of the plankton and the digitaloutput value (DOV) of the OPC. The LHPR with an OPC attachedto the net frame was then deployed at two stations representingcontrasting plankton abundances. Outputs from two oblique haulstaken at a shelf station, and one at an oceanic station, werecompared. Following corrections for volume swept by both instruments,and standardization of OPC particle and net plankton countson the basis of sampler mouth area, the OPC was found to ‘overestimate'relative to the net by up to several orders of magnitude. However,following the correction for rate of flow though the OPC, andthe application of a size threshold to the OPC data excludingthe proportion of particles <0.25 mm² CSA, based on the expectedretention of plankton by the net there was a greatly improvedand significant correlation. Overall there was no consistentover- or undersample by the OPC relative to the net, with meanOPC:net ratios being 1–1.8 for net plankton densitiesof up to 6000 ind. m^–3. There was also a generally goodagreement between the abundances of the rarer large biomassdominant calanoid copepods estimated by both samplers. An OPCoverestimate of 2–3x relative to the net was seen on theascent portion of the haul at the oceanic station, attributableto the presence of high abundances (estimated density     

Seasonal distribution of sympagic amphipods near Chesterfield Inlet, N.W.T., Canada

The seasonal distribution of sympagic amphipods was investigated in the Chesterfield Inlet area of northwestern Hudson Bay (63°30′N). Amphipod abundance was measured by photographic samples and species composition was determined by sweep net samples. Twelve species of amphipods were collected, the most common being Ischyrocerus anguipes, Pontogeneia inermis, Apherusa megalops and Weyprechtia pinguis. The major environmental variable affecting amphipod distribution was water depth. Amphipod abundance was highest near 20 m and near zero past 50 m. The maximum recorded abundance was 1367 m⁻². A minor factor affecting the distribution of amphipods was snow depth, through its modifying effect on light and thereby the growth of ice algae. Amphipods began to inhabit the sea ice shortly after its formation. From the beginning of March, the number of amphipods on the ice increased steadily to about the 3rd week of April, after which numbers declined. This pattern coincided with the seasonal ice algae abundance. Amphipods reduced ice algal biomass over 20-m depth by 63%. No evidence of diurnal changes in abundance was observed. Received: 15 May 1996 / Accepted: 4 November 1996     

Adenosine 5′-Triphosphate Flux Through the North Inlet Marsh System

The distribution, fluctuation, and short-term transport of total microbial biomass (measured as adenosine 5′-triphosphate [ATP]) was investigated in a large salt marsh creek. Hourly samples were collected synoptically for 25 h from 10 boats positioned across the 320-m width of the creek. Samples were collected from three depths ranging from 0.2 to 8.0 m. Hourly data obtained from each station were graphed, plotting depth against ATP. Subsequently, interpolated ATP values were generated for every one-tenth depth from the surface to the bottom with the use of an 11-point proportional divider. A total of 2,750 values were generated, and a mean value of 0.865 mg of ATP per m³ was determined. Maximum levels of ATP were found at high tide and minimal values were found at low tide. The distribution of ATP concentrations was found to be complex, with no suggestion of vertical stratification; however, horizontal divisions were apparent. ATP values corrected for direction of flow or velocity indicated two ebb-directed channels; however, when considered in total, there was a net import of ATP through the interface. The total import of ATP for this 25-h sampling period was calculated to be 3.58 kg, corresponding to a net transport of 39.8 mg of ATP per s through the cross section. Results suggest that detailed characterization of a creek transect in terms of ATP or any similar parameter requires the simultaneous measurements of both the concentration of the parameter in question and the velocity at the time and point from which the sample was taken.     

Biodiversity and structure of the suprabenthic assemblages from South Shetland Islands and Bransfield Strait,Southern Ocean

During the austral summer 1995, suprabenthic samplings were carried out at 24 stations (depth range 45–649 m) located around Livingston Island, within the caldera of Deception Island and in the Bransfield Strait. At each station, the near-bottom motile fauna was simultaneously collected with a multinet Macer-GIROQ sled in three water layers above the bottom. This study presents original data on the occurrence, diversity, vertical distribution and abundance of suprabenthic taxa in this near-bottom environment. The most speciose taxa were amphipods (at least 140 spp.), followed by isopods (66 spp.), pycnogonids (31 spp.) and mysids (19 spp.). Total abundances ranged between 31 ind./100 m² (Bransfield Strait, 361 m depth) and 6817 ind./100 m² (South Livingston Island, 163 m depth). According to stations, the groups numerically dominant and more frequent were amphipods (17 stations) or mysids (seven stations). Four suprabenthic assemblages were discriminated in the study area, apparently more structured by the degree of shelter-exposure and development of sessile epifauna than by water depth or sediment features.     

Buoyancy Regulation and the Potential for Vertical Migration in the Oceanic Cyanobacterium Trichodesmium

Diel protein and carbohydrate content in Trichodesmium thiebautii was measured to evaluate the relationship to buoyancy status. Carbohydrate:protein ratio was the best predictor of buoyancy and fit a cosine curve with increasing values during the day and decreasing values at night in cycles that paralleled observed diel buoyancy patterns. This ratio also increased in short-term experiments as a function of light and increased in parallel with decreasing positive buoyancy. We used changes in this ratio to estimate the potential for vertical migration. Whereas limited vertical excursions in the upper 70 m are possible, deeper migrations appear unlikely unless respiration rates decrease significantly. N:P ratios in sinking and floating colonies were used to test for the P acquisition at depth (vertical migration). We noted that pooled N:P ratios were not significantly different between sinking and ascending colonies (N:P = 65.6 and 66.3, respectively) collected along the northern Australian coast, much like published results from north of Hawaii. Highly significant differences (p <0.0001) were observed in the western Gulf of Mexico between sinking and ascending colonies (N:P = 87.0 and 43.5, respectively) and provide the best direct evidence to date of vertical migration for P acquisition. Our physiological data on compositional changes during buoyancy reversals suggest a complex relationship between light and nutrients. It appears likely that light and P metabolism interact to regulate the vertical extent of migrations, with deep vertical migration regulated by P metabolism superimposed on a mixed-layer light-driven migration. The variability in N:P ratios suggests that care should be taken in assuming buoyancy reversals always result in P acquisition in this oceanic cyanobacterium.     

Net nitrogen mineralization and leaching in response to warming and nitrogen deposition in a temperate old field: the importance of winter temperature

While climate warming can increase plant N availability over the growing season by increasing rates of N mineralization, increased N mineralization over winter at a time when plant roots are largely inactive, coupled with an increased frequency of soil freeze–thaw cycles, may increase soil N leaching losses. We examined changes in soil net N mineralization and N leaching in response to warming and N addition (6 g m^?2 year^?1) in a factorial experiment conducted in a temperate old field. We used two warming treatments, year-round and winter-only warming, to isolate the effects of winter warming on soil N dynamics from the year-round warming effects. We estimated net N mineralization using in situ soil cores with resin bags placed at the bottom to catch throughput, and we measured N leaching using lysimeters located below the plant rooting zone at a depth of 50 cm. There were minor effects of warming on changes in soil extractable N and resin N in the soil cores over winter. Nevertheless, the overall effects of both warming and N addition on net N mineralization (the sum of changes in soil extractable N and resin N) were not significant over this period. Likewise, there were no significant treatment effects on the concentration of N in leachate collected below the plant rooting zone. However, in response to winter warming, net N mineralization over summer was approximately double that of both the ambient and year-round warming treatments. This result demonstrates a potentially large and unexpected effect of winter warming on soil N availability in this old field system.