Winter Seawater Temperature Research Articles

Early arrival of spring-spawning Atlantic herring Clupea harengus at their spawning ground in the Kiel Fjord, western Baltic, relates to increasing winter seawater temperature.

The disturbance of marine organism phenology due to climate change and the subsequent effects on recruitment success are still poorly understood, especially in migratory fish species, such as the Atlantic herring (Clupea harengus; Clupeidae). Here we used the commercial catch data from a local fisher over a 50-year period (1971-2020) to estimate western Baltic spring-spawning (WBSS) herring mean arrival time Q50 (i.e., the week when 50% of the total fish catches had been made) at their spawning ground within the Kiel Fjord, southwest Baltic Sea, and the duration of the spawning season for each year. The relationship between the seawater temperature in the Kiel Bight and other environmental parameters (such as water salinity, North Atlantic and Atlantic multidecadal oscillations) and Q50 was evaluated using a general linear model to test the hypothesis that fish arrived earlier after warm than cold winters. We also estimated the accumulated thermal time to Q50 during gonadal development to estimate the effects of seawater temperature on the variations of Q50. The results of this study revealed a dramatic decrease in herring catches within the Kiel Fjord since the mid-1990s, as documented for the whole southwestern Baltic Sea. Warmer winter seawater temperature was the only factor related to an earlier arrival (1 week for one January seawater temperature degree increase) of herring at their spawning ground. The relationship was found for the first time on week 52 of the year prior to spawning and was the strongest (50% of the variability explained) from the fourth week of January (8 weeks before the mean Q50 among the studied years). A thermal constant to Q50 (~316°C day) was found when temperatures were integrated from the 49th week of the year prior to spawning. These results indicate that seawater temperature enhanced the speed of gonadal maturation during the latest phases of gametogenesis, leading to an early fish arrival under warm conditions. The duration of the spawning season was elongated during warmer years, therefore potentially mitigating the effects of trophic mismatch when fish spawn early. The results of this study highlight the altering effects of climate change on the spawning activity of a migratory fish species in the Baltic Sea where fast global changes presage that in other coastal areas worldwide.

Adaptation to seasonal reproduction and environment-associated factors drive temporal and spatial differentiation in northwest Atlantic herring despite gene flow.

Understanding how marine organisms adapt to local environments is crucial for predicting how populations will respond to global climate change. The genomic basis, environmental factors and evolutionary processes involved in local adaptation are however not well understood. Here we use Atlantic herring, an abundant, migratory and widely distributed marine fish with substantial genomic resources, as a model organism to evaluate local adaptation. We examined genomic variation and its correlation with environmental variables across a broad environmental gradient, for 15 spawning aggregations in Atlantic Canada and the United States. We then compared our results with available genomic data of northeast Atlantic populations. We confirmed that population structure lies in a fraction of the genome including likely adaptive genetic variants of functional importance. We discovered 10 highly differentiated genomic regions distributed across four chromosomes. Nine regions show strong association with seasonal reproduction. One region, corresponding to a known inversion on chromosome 12, underlies a latitudinal pattern discriminating populations north and south of a biogeographic transition zone on the Scotian Shelf. Genome-environment associations indicate that winter seawater temperature best correlates with the latitudinal pattern of this inversion. The variation at two so-called 'islands of divergence' related to seasonal reproduction appear to be private to the northwest Atlantic. Populations in the northwest and northeast Atlantic share variation at four of these divergent regions, simultaneously displaying significant diversity in haplotype composition at another four regions, which includes an undescribed structural variant approximately 7.7 Mb long on chromosome 8. Our results suggest that the timing and geographic location of spawning and early development may be under diverse selective pressures related to allelic fitness across environments. Our study highlights the role of genomic architecture, ancestral haplotypes and selection in maintaining adaptive divergence in species with large population sizes and presumably high gene flow.

Body mass index does not decline during winter for the sedentary marine gastropod Crepidula fornicata.

Seasonal extremes in environmental conditions can substantially limit the growth and reproduction of animals. Sedentary marine animals are particularly susceptible to winter food limitation since they cannot relocate to more favourable conditions. In several temperate-zone bivalve species, substantial winter tissue mass declines have been documented; however, no comparable studies have been conducted on intertidal gastropods. Here, we investigate whether the suspension-feeding intertidal gastropod Crepidula fornicata also loses substantial tissue mass during the winter. We calculated body mass index (BMI) for individuals collected in New England at different times of year for 7 years to determine whether BMI declines through winter or varies seasonally. Remarkably, C. fornicata body mass did not decline significantly during winter months; indeed, a relatively poorer body condition was associated with higher seawater temperature, higher air temperature and higher chlorophyll concentration. In a laboratory experiment, we found that C. fornicata adults that were not fed for three weeks at 6°C (local winter seawater temperature) showed no detectable declines in BMI compared to field-collected individuals. Future studies should document energy budgets of C. fornicata and other sedentary marine animals at low winter seawater temperatures, and the impact of short-term elevated temperatures on those energy budgets.

Warming enhances carbon dioxide and methane fluxes from Red Sea seagrass (<i>Halophila stipulacea</i>) sediments

Abstract. Seagrass meadows are autotrophic ecosystems acting as carbon sinks, but they have also been shown to be sources of carbon dioxide (CO2) and methane (CH4). Seagrasses can be negatively affected by increasing seawater temperatures, but the effects of warming on CO2 and CH4 fluxes in seagrass meadows have not yet been reported. Here, we examine the effect of two disturbances on air–seawater fluxes of CO2 and CH4 in Red Sea Halophila stipulacea communities compared to adjacent unvegetated sediments using cavity ring-down spectroscopy. We first characterized CO2 and CH4 fluxes in vegetated and adjacent unvegetated sediments, and then experimentally examined their response, along with that of the carbon (C) isotopic signature of CO2 and CH4, to gradual warming from 25 ∘C (winter seawater temperature) to 37 ∘C, 2 ∘C above current maximum temperature. In addition, we assessed the response to prolonged darkness, thereby providing insights into the possible role of suppressing plant photosynthesis in supporting CO2 and CH4 fluxes. We detected 6-fold-higher CO2 fluxes in vegetated compared to bare sediments, as well as 10- to 100-fold-higher CH4 fluxes. Warming led to an increase in net CO2 and CH4 fluxes, reaching average fluxes of 10 422.18 ± 2570.12 µmol CO2 m−2 d−1 and 88.11±15.19 µmol CH4 m−2 d−1, while CO2 and CH4 fluxes decreased over time in sediments maintained at 25 ∘C. Prolonged darkness led to an increase in CO2 fluxes but a decrease in CH4 fluxes in vegetated sediments. These results add to previous research identifying Red Sea seagrass meadows as a significant source of CH4, while also indicating that sublethal warming may lead to increased emissions of greenhouse gases from seagrass meadows, providing a feedback mechanism that may contribute to further enhancing global warming.

Densities of reef-associated fish and corals on offshore platforms in the Gulf of Mexico

The northern Gulf of Mexico possesses 1320 offshore oil/gas platforms, currently being removed at a rapid rate. Thirteen species of scleractinian corals inhabit these structures, and a positive association is known to exist between reef-associated fishes and coral. Here, we studied 22 platforms on the Mississippi River delta, an area devoid of natural reefs. We quantified the abundance of reef-associated fishes on platforms occurring in 19–334 m depths. We recorded 18 reef-associated fish species, most commonly Stegastes leucostictus (Muller and Troschel, 1848), Stegastes partitus (Poey, 1868), Abudefduf saxatilis (Linnaeus, 1758), Abudefduf taurus (Muller and Troschel, 1848), and Chromis multilineata (Guichenot, 1853). In water depths ≥25 m, average fish densities ranged from 1.0 to 15.3 m–2. No reef-associated fishes were observed on structures in 66 m depth, the fish densities dropped to zero. Average coral densities per platform in waters >30 m deep ranged from 0.4 to 173.0 colonies m–2. Individual sample densities ranged from 0 to 945.0 colonies m–2. There was a significant positive correlation between average reef-associated fish density and bottom water depth. There was also a positive correlation between average fish density and average coral density. The data indicated that coral and fish densities were lower in inshore waters. These waters are generally characterized by low salinity, low winter seawater temperatures, high turbidity, and high nutrient levels. Our data showed that the fish community is linked to the coral community, as is known to occur on natural reefs. We believe that these numerous organisms add environmental value to the platforms.

The life history characteristics of invasive lionfish (Pterois volitans and P. miles) in Bermuda

Since the first reported sighting of Indo-Pacific lionfish (Pterois volitans and P. miles) in the Atlantic Ocean over 30 years ago, growing evidence suggests they may have a marked negative impact upon ecosystems and demersal communities. While lionfish populations expanded rapidly in most locations following their initial establishment, the population in Bermuda, the first location outside of the United States to be invaded, appears to be growing at a slower pace. This study investigated the life history characteristics of the invasive lionfish population in Bermuda to help understand population dynamics and thus potential impacts on Bermuda’s coral reef ecosystem. Annual growth rings in lionfish otoliths were counted to describe population structure and establish size-at-age, which was then utilized to estimate growth parameters using von Bertalanffy models. Macroscopic and histological staging of ovaries, calculations of gonadosomatic and hepatosomatic indices, and enumerations of mature oocytes were used to describe reproductive seasonality and capacity. Our findings show that lionfish in Bermuda appear to grow faster and attain larger mean sizes than they do in their native range or elsewhere in the invaded range. Lionfish in Bermuda, however, appear to reach maturity at larger sizes and have a shorter spawning season, a likely result of the cool winter seawater temperatures in the region. It is possible that the combination of these life history characteristics could mitigate or delay the ecological impact that invasive lionfish may have on this marine ecosystem.

Seascape genetics of a flatfish reveals local selection under high levels of gene flow

Local adaptation is often found to be in a delicate balance with gene flow in marine species with high dispersal potential. Genotyping with mapped transcriptome-derived markers and advanced seascape statistical analyses are proven tools to uncover the genomic basis of biologically relevant traits under environmental selection. Using a panel of 426 gene-linked single nucleotide polymorphisms (SNPs), we scanned 17 samples (n¼539) of sole (Solea solea L.) from the Northeast Atlantic Ocean and applied a node-based seascape analysis. Neutral loci confirmed a clear distinction between the North Sea–Baltic Sea transition zone and the other Eastern Atlantic samples. At a more subtle level, the latter unit split in an English Channel and North Sea group, and a Bay of Biscay and Atlantic Iberian coast group. A fourth group, the Irish and Celtic Sea, was identified with 19 outlier loci. A pattern of isolation by distance (IBD) characterized the latitudinal distribution. Seascape analyses identified winter seawater temperature, food availability and coastal currents to explain a significant component of geographically distributed genetic variation, suggesting that these factors act as drivers of local adaptation. The evidence for local adaptation is in line with the current understanding on the impact of two key ecological factors, the life-history trait winter mortality and the behaviour of inshore/offshore spawning. We conclude that the subtle differentiation between two metapopulations (North Sea and Bay of Biscay) mirrors local adaptation. At least three genomic regions with strong population differentiation point to locally divergent selection. Further functional characterization of these genomic regions should help with formulating adaptive management policies.

Occurrence and reproduction of tropical fishes in ocean warming hotspots of Japanese temperate reefs

Reproduction of tropical species beyond their geographic range associated with ocean warming is regarded as the key indicator of a range shift. However, the lack of historical breeding records poses challenges for detecting distinct range shifts of tropical fishes. To obtain baseline data of the current status of the occurrence and breeding activity of tropical pomacentrid and apogonid fishes in ocean warming hotspots of temperate reefs (Kochi and Wakayama, 33°N) of Japan, we conducted a two-year underwater visual survey and synthesized those data with recently published information. By combining data from the present as well as past studies, the results confirmed the occurrence of 52 pomacentrid and 34 apogonid species, whereas breeding activity was confirmed for 19 and 16 species, respectively. Species richness and abundance of recruitment periphery and breeding active species were high at the warmer site adjacent to the Kuroshio Current. Most observed species were found beyond their known geographic range. Some species showing active breeding were widespread tropical fishes (e.g., Amphiprion clarkii, Pomacentrus coelestis and Apogon notatus) and probably have established breeding populations irrespective of recent global warming. The winter sea water temperature around the study sites will continue to rise, increasing by >2 °C by the end of the century; therefore, our results are highly relevant and represent the first step to elucidate the potential range extension of tropical fishes into temperate reefs with climate change.

Spreading patterns of the invasive Caulerpa cylindracea Sonder along the west Istrian Coast (northern Adriatic Sea, Croatia)

The northern Adriatic Sea represents the northernmost and thus the coldest biogeographic sector of the Mediterranean Sea. In 2004, the invasive green alga Caulerpa cylindracea was recorded for the first time in the northern Adriatic at a site of the west Istrian Coast. Until 2010, additional C. cylindracea mats have only formed up to 7 km northward from the first colonisation site. Subsequently, the alga was also recorded at sites widespread along the entire coast. Both the first 2004 colonisation event and the 2011–2014 colonisation of distant sites occurred during periods of winter seawater temperatures higher than 9 °C. In general, algal spreading was markedly slow. Approximately 10 years after the first record, C. cylindracea has affected less than 1% of the entire west Istrian coastline. The colonisation predominantly occurred in ports and urbanised bays (seaside resorts) suggesting that anthropogenic activities might enhance algal diffusion.

Long-term patterns in 50 years of scyphomedusae catches in the western Dutch Wadden Sea in relation to climate change and eutrophication

Using a unique 50-year high-resolution time series of daily kom-fyke catches, long-term patterns of scyphomedusae in the western Dutch Wadden Sea were analysed and related to changes in environmental conditions [eutrophication in the 1980s–1990s and recent climate change (increased water temperature)] in the area. Over the years, species composition and general pattern of appearance has remained the same: the first species that occurred in spring was Aurelia aurita, followed by Cyanea lamarckii/C. capillata. Chrysaora hysoscella and Rhizostoma octopus occurred from June to July onwards. All species appeared earlier in recent decades and first appearance and peak occurrence of A. aurita was in part inversely related to previous winter seawater temperature. Last occurrence of C. hysoscella was related to summer seawater temperature and the species is present longer in recent decades. Phenological relationships might have been decoupled since the seasonality of the phytoplankton bloom did not change. All species showed large inter-annual abundance fluctuations, with prolific years followed by sparse years. Peak catches of the coastal species A. aurita occurred in the late 1970s−early 1990s when eutrophication peaked, however, without a significant relationship with total nitrogen input into the area. Unlike for phenology, the patterns of mean abundance of any species did not show a relationship to climate change in the area. This might imply that population regulating mechanisms do not operate during the planktonic phase but during the sessile demersal polyp stages.

The Possibility of Overwintering byIdiosepius paradoxus(Cephalopoda: Idiosepiidae) at the Northern Limits of its Distribution

Abstract. Idiosepius paradoxus (Ortmann, 1881) has been previously collected only in the four months from September to December at Usujiri, in northern Japan, which is considered its northern limit. Previous reports indicate that I. paradoxus is not able to overwinter at this distribution boundary. In winter 2006–2007, seawater temperatures were higher than in previous years. To investigate whether I. paradoxus overwinters in Usujiri, we collected specimens monthly and compared their occurrence pattern with seawater temperature data. In 2007, samples were collected between April and August, months in which they had not been previously collected in Usujiri. Winter seawater temperatures in 2007 were higher than in any other year. These results suggested that I. paradoxus successfully overwintered in Usujiri. However, winter water temperatures in 2008 returned to their usual lows, and specimens were not found in April. Therefore, overwintering success appeared to be temporary, and range expansion toward high...

Denman Island disease (causative agent Mikrocytos mackini) in a new host, Kumamoto oysters Crassostrea sikamea

Mikrocytos mackini, causative agent of Denman Island disease in Pacific oysters Crassostrea gigas and other oyster species, was found in 2011 in a previously unreported host, the Kumamoto oyster C. sikamea, in Humboldt Bay, California, USA. The detection was also the first reported finding of M. mackini in California. Prevalence was estimated as high as approximately 27% from pooled samples analyzed by PCR. Higher prevalence appeared related to longer residence time in the bay and somewhat colder than typical winter seawater temperatures. No M. mackini was detected in Humboldt Bay juvenile Kumamoto oysters or Pacific oyster brood or seed stock in 2011 or 2012.

Glycerol uptake is by passive diffusion in the heart but by facilitated transport in RBCs at high glycerol levels in cold acclimated rainbow smelt (Osmerus mordax)

Rainbow smelt (Osmerus mordax) is a small fish that accumulates glycerol at low winter seawater temperatures. In laboratory-held fish, glycerol concentration typically reaches 225 mM in plasma and in all cells. Glycerol uptake by the heart and red blood cells (RBCs) was assessed by tracking [(14)C(U)]glycerol into the acid-soluble pool. In fish acclimated to 9-10°C a decrease in perfusion/incubation temperature from 8 to 1°C resulted in a decrease in glycerol uptake with a Q(10) of 3.2 in heart and 2.4 in RBCs. Acclimation to ∼1.5°C did not result in an adaptive enhancement of glycerol uptake as rates were unchanged in heart and RBCs. Glycerol uptake at 1°C was by passive diffusion in heart as evidenced by a linear relationship between glycerol uptake and extracellular glycerol concentration and a lack of inhibition by phloretin. In contrast, in RBCs, glycerol uptake with respect to glycerol concentration showed two linear relationships with a transition point around 50 mM extracellular glycerol. The slope of the second phase was much steeper and eliminated with the inclusion of phloretin. In RBCs from Atlantic salmon (Salmo salar), a related species that does not accumulate glycerol, glycerol uptake showed only a single linear curve and was not inhibited by phloretin. The data imply a strong facilitated component to glycerol uptake in rainbow smelt RBCs at high glycerol concentrations. We propose this is related to cyclic changes in RBC glycerol content involving a loss of glycerol at the gill and a reaccumulation during passage through the liver.

Parental size and environmental conditions affect egg capsule production by Nassarius reticulatus (Linnaeus 1758) (Gastropoda: Nassariidae)

In shallow coastal habitats scavenging netted whelks Nassarius reticulatus attached egg capsules to the stipes of red algae Chondrus crispus and occasionally on Furcellaria lumbricalis and Plumaria plumose. In the laboratory egg capsules were laid on aquaria sides and lids by individuals ≥ 21 mm shell length. Larger size classes produced more egg capsules and spawned over a longer period and in doing so partitioned less energy into shell growth. Large netted whelks (25–28.9 mm) produced larger capsules which contained significantly more and larger eggs than those produced by smaller individuals (21–24.9 mm). Egg capsule production continued throughout the year by regularly fed N. reticulatus held at ambient seawater temperatures. Egg production increased in the spring and summer with peak production during June (15 °C), decreased between August and October and resumed again during the winter (November to February at ∼ 7 °C). During the summer (15–16 °C) egg capsules were smaller and contained smaller eggs than those deposited during the winter (7–10 °C), although the number of eggs · capsule − 1 was similar. Enforced food limitation reduced the number and size of the egg capsules, the number and size of eggs produced · female − 1 and the duration of the breeding period. Hatching success of N. reticulatus egg capsules was high (95%) even at winter seawater temperatures (11–8.5 °C) and the duration of embryonic development was fastest between 15 and 17.5 °C.

Environmental anomalies, energetic reserves and fatty acid modifications in oysters coincide with an exceptional mortality event

Mortality of oysters Crassostrea gigas occurred along the coasts of France during sum- mer 2008, causing a serious crisis in French oyster production. During spring to summer 2008, 5 groups of 1-yr old oysters of different origins and ploidy levels deployed in the Thau lagoon (France) were sampled for biometrical and biochemical analyses; environmental parameters were monitored in the same area. Mortality occurred in 2 episodes: 13 May to 10 June and 24 June to 9 July. Wild-collected oysters showed mortality sooner than other groups but total overall mortality was 85% in all groups. Mortality occurred when seawater temperature was >19°C during the repro- ductive season when energetic reserves of oysters were at their lowest. Energy reserve levels and mortality in oysters seemed to be independent of reproductive effort, and most likely reflected an unusual deficit in phytoplankton. The level of bacterial fatty acids in oysters increased markedly before the second mortality event, probably indicating an enhancement of the microbial loop or decomposition processes. At the same time, unsaturation of oyster fatty acids decreased, suggesting a diet change, acclimatization to increasing temperature or, most likely, oxidative damage due to an enhanced production of reactive oxygen species, which are often associated with stressful situations. Wild-collected and hatchery-produced oysters, which exhibited different mortality patterns, showed persistent differences in fatty acid (20:5n-3 and 22:6n-3) content in their membrane, despite the fact that they were reared in the same environment. Mortality started when fatty acid 20:4n-6 content increased to ~3% in the polar lipids, suggesting that this ratio could make a useful indicator of mor- tality risk. Finally, the 2008 mortality event coincided with a nationwide increase of ~1.5°C in winter seawater temperature, which may impact the oyster and its pathogens.

A latitudinal gradient of seasonal temperature variation recorded in oyster shells from the coastal waters of France and The Netherlands

Cathodoluminescence (CL) microscopy of the foliated calcite shell hinge sections of live-collected oyster Crassostrea gigas collected at seven locations along a latitudinal gradient from the Netherlands in the North Sea to the Atlantic coast of France, revealed variations in luminescence that were attributable to seasonal variations in calcification of the hinge. Photomicrographs of hinge sections and luminescence profiles were analyzed to define a micro-sampling strategy that was adopted to drill the hinge samples to determine their isotopic composition. Reconstructed seasonal seawater temperatures determined from the stable oxygen isotope (δ18O) composition along growth profiles from 32 oyster shell hinges showed distinct seasonal isotopic cycles that were compared with in situ measured seawater temperatures and salinities at each location. Comparison of the amplitude of the (δ18O) signal and the annual maximum and minimum seawater temperatures demonstrated that C. gigas shells from several locations provided a reliable record of seasonal seawater temperature variation. The exception to this was oysters from the Netherlands and northern France where winter growth rates at low temperatures were slow so that insufficient shell was deposited to allow adequate spatial resolution of sampling and this resulted in time-averaging of the reconstructed seawater temperatures and an overestimation of winter seawater temperature. A potential variability in δ18Ow–salinity relationship at low salinities could also explain the high difference between measured and predicted seawater temperatures in Dutch areas. The finding that latitudinal differences in oyster hinge growth rates and/or changes in the δ18Ow–salinity relationship can result in bias of the seawater temperature deduced from the stable isotopic composition of the hinge should be taken into account when reconstructing latitudinal gradients in seawater temperature.

Overwinter mortality of young-of-the-year Ezo abalone in relation to seawater temperature on the North Pacific coast of Japan

The Ezo abalone Haliotis discus hannai is the only species of abalone inhabiting the coastal waters affected by the cold Oyashio current in the Pacific waters of northern Honshu, Japan. Overwinter mortality of young-of-the-year (YOY) Ezo abalone was examined for 2000, 2001, and 2003 to 2006 year classes in Kadonohama Bay, northern Japan, to test the hypothesis that winter sea- water temperature controls recruitment fluctuation of this northernmost abalone and regulates their distribution limit. Eggs of Ezo abalone hatch between late summer and mid-autumn, and grow to a size of 2 to 8 mm in shell length by winter. Overwinter mortality rates varied among years (range: 0 to 97.4%), and there was a strong negative relationship between the instantaneous mortality rate and the winter temperature. High mortalities of YOY abalone were observed in February 2001 and March 2006, respectively. In both cases, the Oyashio current extended south to the study area, and the mean temperature fell to below 5°C. High mortalities were not observed in the other years when the mini- mum temperatures were higher than 6°C, as the Oyashio current did not extend to the study area. The differences in shell length between early winter and post-winter suggest that the low tempera- ture tolerance of YOY abalone may be size-dependent. Although not conclusive, the winter seawater temperature is an important factor affecting the survival of YOY and is the principal factor for the variation in the recruitment of Ezo abalone and the limit of their northward distribution.

Seagrasses from the Nansei Islands, Southern Japanese Archipelago: species composition, distribution and biogeography

The Nansei Islands in the southern Japanese Archipelago have 15 taxa of seagrasses from seven genera within three families. Seagrasses in this region grow on coral sands or coral debris in shallow reefs and on sandy or muddy substrata in the shallow areas of bays and inlets. Certain Halophila species grow in deep water off some islands. Enhalus acoroides only reaches to Ishigaki I. with winter sea water temperature (WST) at 23 °C, while Okinawa I. (WST at 21.6 °C) is the northern biogeographic limit for Halophila decipiens, H. okinawensis, H. major and H. gaudichaudii. Amami-oshima I. (WST at 20.7 °C) is the northern border for Thalassia hemprichii, H. minor, H. ovalis, Cymodocea serrulata, Cymodocea rotundata, Syringodium isoetifolium, Halodule uninervis and Halodule pinifolia. Halophila mikii the sole seagrass collected from Yakushima I. (WST at 19.3 °C), is of volcanic origin. The distribution of tropical seagrasses in the Nansei Islands is clearly associated with the warm Kuroshio Current, WST and habitat availability. Zostera japonica is the only temperate species occurring in the region. Meadows of Z. japonica, H. ovalis and Halodule pinifolia have disappeared from certain localities in the Archipelago, due probably to human activities and natural siltation.

Vegetation patterns and spontaneous regression of Caulerpa taxifolia (Vahl) C. Agardh in Malinska (Northern Adriatic, Croatia)

The green alga Caulerpa taxifolia was recorded at Malinska in 1994 and this actually represents the highest northern latitude (45°7′30″N) at which this invasive alga has been found in the world. The alga was widespread at four sites from which it was eradicated by suction pumps during 1996 and 1997. However, it immediately and intensively recolonized all but one site. Throughout 1998 and 1999 the renewed vegetation showed consistent seasonal patterns. The alga nearly disappeared in April and May while regenerating from over-wintering parts of the thalli in summer. The maximum development occurred in autumn and winter with values of biomass (around 200 g dry weight m −2) and frond number (around 2000 m −2) generally lower than those reported for the north-western Mediterranean. Values for the frond length (10–18 cm) were in the same range as those in the north-western basin. Throughout 1998 and 1999 the biomass was closely correlated to frond number and length (adjusted R 2 = 0.90). During the following years C. taxifolia entered a phase of regression. The total colonized area, which amounted to several thousands of square metres in 1998, spontaneously declined in 2000 and 2001 so that only several thalli were found in 2004 during a detailed survey of the settlement. No major changes in winter seawater temperatures, ranging from 9.5 to 10.5 °C, were observed in the area from 1994 to 2004. Thus, other unknown processes could likely play a role on specific vegetation patterns of C. taxifolia in Malinska. Accordingly, it is difficult to explain why the still surviving thalli did not proliferate during the favourable summer–autumn period.

Seasonal changes of the introduced Caulerpa racemosa var. cylindracea (Caulerpales, Chlorophyta) at the northwest limit of its Mediterranean range

A study of the meadows of the invasive Caulerpa racemosa var. cylindracea (Sonder) Verlaque, Huisman et Boudouresque was carried out over one year at Marseilles (Provence, France) where the alga is thriving, probably since 1994, in the cold waters of the north western Mediterranean Sea. At an early phase of colonisation, the C. racemosa meadow is characterized by a patchy distribution pattern. Several years are necessary to obtain a dense and continuous meadow. In one area colonized for more than 4 years, C. racemosa has developed a continuous meadow with wide seasonal variations. Maximum development was reached in autumn (biomass: 82 ± 3 g DW m −2; length of stolons: 1162 ± 86 m m −2; number of apices: 8360 ± 405 m −2; number of erect axes: 20955 ± 1499 m −2) and the minimum from winter to early spring (respectively, 0.3 ± 0.1 g DW m −2; 3 ± 1 m m −2; 220 ± 55 apices m −2; 35 ± 15 erect axes m −2). Seasonal variations in the growth rate were highly significant. The season of high growth lasted from June to October. The apical growth rate of a stolon reached a maximum of 7.5 ± 0.3 mm day −1 in early October, then began to decrease significantly from the end of October to December, before becoming nearly nil from January to early May. Annual net production rate expressed in terms of stolon length and biomass was estimated as 5801 m m −2 a −1 and 612 g DW m −2 a −1, respectively. During the growth period, the turnover rate of the C. racemosa stolons was estimated at from 25 to 46 days. The growth rate was closely correlated to the seawater temperature ( R 2 = 0.83), whereas no significant correlation was found between growth and irradiance. During the growth period, a decrease in temperature rapidly affects the growth rate, which soon recovers its earlier level when the temperature rises again. In winter, the growth rate decreased rapidly with the seasonal drop in the seawater temperature. Grazing by fish ( Sarpa salpa and Boops boops) can also affect the growth rate from September to December by consumption of the erect axes and stolon apices, enhancing the ramification of stolons. Seasonal changes at Marseilles are much sharper than those reported for warmer Mediterranean localities (French Riviera, Italy, Croatia): in winter and early spring C. racemosa meadows decreased and locally disappeared, leaving a barren substrate. C. racemosa survives the lower winter seawater temperatures of the north-western Mediterranean Sea probably in the form of zygotes and/or small fragments (rhizoids, stolons, propagules).