Can salinity-induced mortality explain larval vertical distribution with respect to a halocline?

For the larvae of two echinoderm species that coexist in Atlantic Canada (bipinnaria of the sea star Asterias rubens and 4- and 6-arm echinoplutei of the sea urchin Strongylocentrotus droebachiensis), we examined the effect of short- and long-term exposure to salinity (ranging from 18 to 35) on the probability of larval survival in laboratory experiments. We also related larval vertical distributions in response to sharp haloclines generated in the laboratory to survival probability in the salinity of different layers in the water column. For both species and developmental stages, survival probability decreased with decreasing salinity, and a salinity range of 24-27 emerged as the critical threshold for larval tolerance. The relationship between the proportion of larvae that crossed a halocline into the top water layer and the survival probability of larvae in the salinity of that layer was significant for both species. Interestingly, the shape of this response was species-specific but not stage-specific for S. droebachiensis. Our findings suggest that larval avoidance of low-salinity water layers may be an adaptive behavior that increases survival and indirectly influences larval distribution.     

The effects of haloclines on the vertical distribution and migration of zooplankton

While the influence of horizontal salinity gradients on the distribution and abundance of planktonic organisms in estuaries is relatively well known, the effects of vertical salinity gradients (haloclines) are less well understood. Because biological, chemical, and physical conditions can vary between different salinity strata, an understanding of the behavioral response of zooplankton to haloclines is crucial to understanding the population biology and ecology of these organisms. We studied four San Francisco Bay copepods, Acartia (Acartiura) spp., Acartia (Acanthacartia) spp., Oithona davisae, and Tortanus dextrilobatus, and one species of larval fish (Clupea pallasi), in an attempt to understand how and why zooplankton respond to haloclines. Controlled laboratory experiments involved placing several individuals of each species in two 2-m-high tanks, one containing a halocline (magnitude varied between 1.4 and 10.0 psu) and the other without a halocline, and recording the location of each organism once every hour for 2-4 days using an automated video microscopy system. Results indicated that most zooplankton changed their vertical distribution and/or migration in response to haloclines. For the smaller taxa (Acartiura spp., Acanthacartia spp., and O. davisae), this behavior took the form of accumulating in or below the halocline, while the effects on the larger species (C. pallasi and T. dextrilobatus) were more subtle. C. pallasi yolk sac and 3- to 6-day-old larvae seemed to pause or remain in the halocline during their diel migration, while 14- to 17-day-old larvae appeared to avoid the halocline by remaining in deeper, more saline water. There were very few statistically significant effects of haloclines on the vertical distribution of T. dextrilobatus. Subsequent mortality experiments with Acartiura spp., Acanthacartia spp. and T. dextrilobatus indicated that the behavioral changes seen in the halocline studies were not associated with any salinity-induced mortality per se, although more subtle affects of physiological stress could not be ruled out. These results point to a high degree of flexibility in vertical migration behavior within a given species as well as large variation between species. Such behavioral flexibility is likely to be very important in allowing planktonic organisms generally, and estuarine organisms in particular, to maintain or alter position relative to currents, food, and predators.     

The effect of salinity on larval survival and development in the sea urchin Echinometra lucunter

Summary

Reductions in salinity can have adverse effects on larval development and larval survival in some invertebrate taxa but not others. Salinity tolerance of larvae may be particularly important in echinoderms because they are both poor ion regulators and stenohaline. I examined the effect of six levels of salinity (15, 18, 21, 24, 27 and 33 PSU) on survival and rate of development of larvae in the subtropical sea urchin Echinometra lucunter. In the short-term, mortality rate was significantly lower in 33 PSU than in all other salinities except 27 PSU, and it was significantly greater in 15 and 18 PSU than in all higher salinities. In the long-term, daily and cumulative mortality were significantly greater in 15 PSU than in most other salinities over 11 days of development (except for cumulative mortality in 18 PSU). They were significantly greater in 18 PSU than in 21 PSU or 33 PSU over a period of 13 days. Furthermore, daily mortality was significantly greater in 18 PSU than in 24 PSU or 27 PSU at 13 d after fertilization. Daily and cumulative mortality were significantly lower in 33 PSU than in 21, 24 or 27 PSU over a period of 17 days. Although in the control (33 PSU) 75% of larvae completed development to the 8-arm stage at 35 d, no larvae developed further than the 4-arm stage in 18, 21, 24 or 27 PSU; in 15 PSU, ~60% of larvae did not develop further than swimming blastulae. Since prolonged exposure to salinities as high as 27 PSU (frequently recorded in the adult habitat) can result in great larval losses, adaptive behaviours that prevent larvae from entering water layers of low salinity will enhance their chance for survival.     

Can bivalve veligers escape feeding currents of adult bivalves?

While the stock of introduced Pacific oysters (Crassostrea gigas) increased in the Oosterschelde estuary (SW Netherlands), so did the filtration pressure of all bivalve species together. In the same period, stocks of native bivalves declined slightly. The expansion of Pacific oysters in Dutch estuaries might be partially due to better abilities of their larvae to avoid or escape filtration, compared to larvae of native bivalves. In this context, escape and swimming abilities of Pacific oyster larvae and the larvae of the native blue mussel (Mytilus edulis) were compared.Swimming behaviour of C. gigas larvae and larvae of M. edulis was recorded in still water and in a suction current mimicking a bivalve feeding current, in a horizontal and in a vertical plane. Larval swimming behaviour in a suction flow field was reconstructed by subtracting local water movement vectors from the total movement of larvae, yielding movement paths due to larval swimming alone.Swimming speeds and the rate of displacement in vertical direction of C. gigas and M. edulis larvae were related to larval shell length, and to the pitch of up- or downward swimming.Larvae of both species did not show escape reactions in a suction flow field. With increasing shell length, larval swimming speeds of both species increased significantly. Swimming speeds of C. gigas larvae were significantly higher than swimming speeds of M. edulis larvae, resulting in a faster vertical displacement. The ability to migrate to more favourable water layers faster may offer C. gigas an advantage over native bivalves with slower swimming larvae.     

Metazoans of redoxcline sediments in Mediterranean deep-sea hypersaline anoxic basins

Background

The deep-sea hypersaline anoxic basins (DHABs) of the Mediterranean (water depth ~3500 m) are some of the most extreme oceanic habitats known. Brines of DHABs are nearly saturated with salt, leading many to suspect they are uninhabitable for eukaryotes. While diverse bacterial and protistan communities are reported from some DHAB haloclines and brines, loriciferans are the only metazoan reported to inhabit the anoxic DHAB brines. Our goal was to further investigate metazoan communities in DHAB haloclines and brines.

Results

We report observations from sediments of three DHAB (Urania, Discovery, L’Atalante) haloclines, comparing these to observations from sediments underlying normoxic waters of typical Mediterranean salinity. Due to technical difficulties, sampling of the brines was not possible. Morphotype analysis indicates nematodes are the most abundant taxon; crustaceans, loriciferans and bryozoans were also noted. Among nematodes, Daptonema was the most abundant genus; three morphotypes were noted with a degree of endemicity. The majority of rRNA sequences were from planktonic taxa, suggesting that at least some individual metazoans were preserved and inactive. Nematode abundance data, in some cases determined from direct counts of sediments incubated in situ with CellTracker^TM Green, was patchy but generally indicates the highest abundances in either normoxic control samples or in upper halocline samples; nematodes were absent or very rare in lower halocline samples. Ultrastructural analysis indicates the nematodes in L’Atalante normoxic control sediments were fit, while specimens from L’Atalante upper halocline were healthy or had only recently died and those from the lower halocline had no identifiable organelles. Loriciferans, which were only rarely encountered, were found in both normoxic control samples as well as in Discovery and L’Atalante haloclines. It is not clear how a metazoan taxon could remain viable under this wide range of conditions.

Conclusions

We document a community of living nematodes in normoxic, normal saline deep-sea Mediterranean sediments and in the upper halocline portions of the DHABs. Occurrences of nematodes in mid-halocline and lower halocline samples did not provide compelling evidence of a living community in those zones. The possibility of a viable metazoan community in brines of DHABs is not supported by our data at this time.

     

On barriers to hybridization between Strongylocentrotus droebachiensis (O.F. Müller) and S. Pallidus (G.O. Sars)

Strongylocentrotus droebachiensis (O. F. Müller) and Strongylocentrotus pallidus (G. O. Sars) occur together over much of their ranges in both the Atlantic and Pacific Oceans and remain distinct species, but no strong barrier to genetic exchange has been found in a study in the San Juan Islands. Co-occurrence of embryos of both species in the plankton indicates similar times of spawning. Ova of S. droebachiensis are fertilized by sperm of S. pallidus but the reciprocal cross gives a percent fertilization near zero. Hybrids of both crosses are viable with pigmentation resembling S. pallidus in the larval stage and S. droebachiensis in the adult stage. Ova of first generation female hybrids are more readily fertilized by sperm of S. pallidus, so that gene flow through female hybrids is more likely to be from S. droebachiensis to S. pallidus than the reverse. Male hybrids were not tested. S. droebachiensis and S. pallidus from deeper waters have lower fecundity than shallow water S. droebachiensis, which suggests that in the deeper waters individuals are food limited. The hypothesis that S. pallidus uses the sterile male technique to reduce reproductive output of S. droebachiensis is considered and rejected.     

The effect of the quality of food patches on larval vertical distribution of the sea urchins Lytechinus variegatus (Lamarck) and Strongylocentrotus droebachiensis (Mueller)

Although most invertebrate larvae are weak swimmers and act as passive particles on horizontal scales, they may be able to regulate their vertical position in response to different factors, including increased food concentration. We examined the effect of the quality of food patches on larval vertical distribution for the sea urchins Lytechinus variegatus and Strongylocentrotus droebachiensis, and determined the effect of dietary conditioning on that response in the laboratory. We reared larvae on a mixed algal diet of Dunaliella tertiolecta and Isochrysis galbana under low (500 cells ml⁻¹) and high (5000 cells ml⁻¹) rations. Food patches were maintained in Plexiglas rectangular columns (30×10×10 cm) using a density gradient, where practical salinity in the bottom layer was 33, in the middle layer 30, and in the top layer 27. We examined the magnitude and mechanism of a behavioural response of larvae of L. variegatus in the four-arm stage, and on two developmental stages of S. droebachiensis (four- and six-arm), by manipulating patch quality. In the absence of a patch, larvae of both species and developmental stages swam through to the surface of the experimental columns. In the presence of algae, fewer larvae were present above the patch and more were at the patch than in control columns. More larvae swam through patches of “unflavoured” algal mimics than algal patches, and aggregated at the surface. Larval distribution relative to patches of algal filtrate without algal cells or of “flavoured” algal mimics in algal filtrate was not consistently different from that in either control or algal patches; thus, the magnitude of larval response to filtrate (with or without particles) was intermediate between that to control and algal patches. For L. variegatus, more larvae crossed the patches when reared on low than high rations, indicating that poorly conditioned larvae may be less responsive to environmental cues. Our results suggest that larvae can actively aggregate and maintain a vertical position in response to a food patch that depends on the quality and quantity of food. The response appears to be based mostly on a chemosensory rather than a mechanosensory mechanism.     

Short-term fluctuation in salinity promotes rapid larval development and metamorphosis in Dendraster excentricus

The effect of constant and fluctuating salinity on larval development and metamorphosis of the sand dollar Dendraster excentricus was investigated in the laboratory. Sand dollar larvae at different stages of development were kept either at 32‰ (controls), exposed to constant low salinity (22‰) throughout development, or exposed to fluctuating salinity (i.e. transferring larvae from 32‰ to 22‰ for 7 days then back to 32‰ for the rest of their development). Larvae exposed to constant low salinity were significantly smaller but developed all larval arms at a slower rate than larvae in all other treatments. Larvae exposed to fluctuating salinity recovered and developed significantly longer larval arms and bigger rudiments than larvae kept at constant low salinity. Larvae exposed to fluctuating salinity produced more juveniles than larvae at constant high salinity (32‰), while those at constant low salinity produced few or no juveniles. Four-arm larvae exposed to fluctuating salinity produced significantly more juveniles than six-arm larvae exposed to the same treatment. Transferring competent 8-arm larvae from 31‰ to 15‰ for 2 days then back to 31‰, induced metamorphosis with juvenile production being significantly higher than for those kept at a constant salinity of 20, 25 and 31‰. This study indicates that a short-term decrease in salinity might induce metamorphosis for this species.     

Survivorship, development, and DNA damage in echinoderm embryos and larvae exposed to ultraviolet radiation (290-400 nm)

Laboratory experiments utilizing ecologically relevant irradiances of ultraviolet radiation (UVR) known to occur in shallow Gulf of Maine waters were conducted on the planktonic embryos and larvae of two common benthic echinoids; the green sea urchin Strongylocentrotus droebachiensis and the sand dollar Echinarachnius parma. Significant decreases in survivorship were observed in freshly fertilized embryos of both species with greater mortality in E. parma that was associated with the absence of UVR-absorbing compounds, the mycosporine-like amino acids. Experiments on blastula, gastrula, and prism larval stages of S. droebachiensis also showed significant decreases in survivorship, delays in development, and abnormal embryos and larvae associated with exposure to UVR. Additionally, all developmental stages of S. droebachiensis experimentally exposed to UVR resulted in significant increases in DNA damage, measured as cyclobutane pyrimidine dimer photoproducts. The observed delays in early cleavage and subsequent developmental stages for S. droebachiensis are correlated with DNA damage. It is postulated that cell cycle arrest at critical checkpoints after DNA damage, mediated by a suite of cell cycle genes, is a component of the observed UVR induced developmental delays.     

Species status and population structure of mussels (Mollusca: Bivalvia: Mytilus spp.) in the Wadden Sea of Lower Saxony (Germany)

Three species of mussel (genus Mytilus) occur in Europe: M. edulis (Linnaeus 1758), M. galloprovincialis (Lamarck 1819) and M. trossulus (Gould, Boston Society of Natural History 3: 343?C348, 1850). Although these species are indigenous to the North Sea, the Mediterranean and the Baltic Sea, respectively, they form an extended patchy species complex along the coasts of Europe (??the Mytilus edulis complex??) and are able to hybridize where their distributions overlap. Recent studies examining the taxonomic status and genetic composition of Mytilus populations in the Netherlands and the British Isles have revealed introgressive hybridization processes within this species complex, with hints of an invasion of nonindigenous M. galloprovincialis into the North Sea. Furthermore, an extensive international mussel fishery industry in Europe (i.e., Great Britain, the Netherlands, Denmark, and Germany) is also in discussion for a possibly anthropogenically induced bioinvasion of nonindigenous Mytilus traits into the Wadden Sea area. Although it is assumed that the Wadden Sea of Germany comprises M. edulis only, this has never been confirmed in a molecular genetic study. To assess the situation for the Wadden Sea of Lower Saxony, we conducted the first molecular study of the Mytilus genus in the region. Taxonomic identification of 504 mussels from 13 intertidal mussel banks using the nDNA marker Me15/16 revealed a population composition of 99% M. edulis and 1% M. edulis X M. galloprovincialis hybrids. Hence, the Wadden Sea population is unaffected by range expansion of nonindigenous Mytilus traits. The genetic structure of the M. edulis populations was investigated using the phylogenetic and population genetics analyses of the mitochondrial DNA cytochrome-c-oxidase subunit I (COI) and the first variable domain of the control region (VD1), which were sequenced for >120 female individuals. These results showed a heterogeneous, panmictic population due to unrestricted gene flow. This can be attributed to extensive larval dispersal linked to the tidal circulation system in the back barrier basins of the Wadden Sea.     

Feeding preference of Strongylocentrotus droebachiensis (Echinoidea) for a dominant native ascidian, Aplidium glabrum, relative to the invasive ascidian Botrylloides violaceus

Subtidal benthic communities show distinct patterns of community structure related to substrate angle. Suspension-feeding invertebrate communities often dominate vertical and undercut rock surfaces, while macroalgae dominate horizontal surfaces. One factor that may shape this pattern is sea urchin grazing, which is often more intense on horizontal surfaces. The native Gulf of Maine ascidian Aplidium glabrum, like other native ascidians, is generally restricted to vertical and undercut rock walls, whereas the introduced ascidian Botrylloides violaceous is often abundant on horizontal surfaces. We tested the hypothesis that this pattern could be due to differing predation intensity on these two ascidians by Strongylocentrotus droebachiensis, a dominant omnivore in the Gulf of Maine. Feeding preference of S. droebachiensis on the native A. glabrum vs. B. violaceous was estimated in the laboratory and in field experiments. Laboratory results show that S. droebachiensis prefers to feed on the native ascidian A. glabrum over B. violaceous. In the field, potential differences in the impact of S. droebachiensis on the two species were masked by the much greater growth rate of B. violaceus compared to A. glabrum. These results may help explain observed patterns in ascidian distribution in the Gulf of Maine, and ultimately the overall success of B. violaceus as a major invader throughout New England.     

A laboratory investigation of temperature and salinity tolerances of juvenile Metapenaeus bennettae Racek and Dall (Crustacea: Penaeidae)

Temperature and salinity tolerances of juvenile Metapenaeus bennetlae Racek and Dall were estimated by abrupt exposure to critically high or low levels of each factor following acclimation to 12 combinations of temperature (17, 22, 27 and 32°C) with salinity (5, 20 and 35‰.). No significant differences were found between tolerances of males and females. Acclimation temperature influenced both temperature and salinity tolerances, while acclimation salinity affected only the salinity tolerance. Irrespective of temperature and salinity acclimation levels, juvenile M. bennettae were able to tolerate temperatures from 8.1 to 32.9°C and salinities from 1.0 to 62.0‰ These findings are discussed in relation to similar published studies.     

A study on early tolerance of Mactra chinensis philippi to salinity

In order to evaluate the early tolerance of Mactra chinensis to salinity, the treatments of salinity gradients and salinity gradual changes were set in this study, and the post growth and development of juveniles were analyzed in recovery experiment, respectively. The result showed that the optimum hatching of zygotes was found at a salinity from 24 to 32, which is narrower than that of larvae (20–32); a slight of low salinity (16–32) will benefit the early growth and development of M. chinensis; at planktonic and creeping stages, low salinity stress (20) was conducive to promoting the growth of juvenile M. chinensis philippi; 4, 48 was the ultimate salinity of M. chinensis; The range of early tolerance of larvae M. chinensis philippi to salinity can be widened through a short period of salinity acclimation.     

Anammox bacterial populations in deep marine hypersaline gradient systems

To extend the knowledge of anaerobic ammonium oxidation (anammox) habitats, bacterial communities were examined in two hypersaline sulphidic basins in Eastern Mediterranean Sea. The 2 m thick seawater–brine haloclines of the deep anoxic hypersaline basins Bannock and L’Atalante were sampled in intervals of 10 cm with increasing salinity. ¹⁵N isotope pairing incubation experiments showed the production of ²⁹N₂ and ³⁰N₂ gases in the chemoclines, ranging from 6.0 to 9.2 % salinity of the L’Atalante basin. Potential anammox rates ranged from 2.52 to 49.65 nmol N₂ L^?1 day^?1 while denitrification was a major N₂ production pathway, accounting for more than 85.5 % of total N₂ production. Anammox-related 16S rRNA genes were detected along the L’Atalante and Bannock haloclines up to 24 % salinity, and the amplification of the hydrazine synthase genes (hzsA) further confirmed the presence of anammox bacteria in Bannock. Fluorescence in situ hybridisation and sequence analysis of 16S rRNA genes identified representatives of the marine anammox genus ‘Candidatus Scalindua’ and putatively new operational taxonomic units closely affiliated to sequences retrieved in marine environments that have documented anammox activity. ‘Scalindua brodae’ like sequences constituted up to 84.4 % of the sequences retrieved from Bannock. The anammox community in L’Atalante was different than in Bannock and was stratified according to salinity increase. This study putatively extends anammox bacterial habitats to extremely saline sulphidic ecosystems.     

Salinity stratified benthic macrofaunal communities and long-term monitoring along the west coast of Sweden

Benthic macrofauna was quantitatively sampled in 12 coastal areas along the west coast of Sweden in 1971–1976. The Baltic Current creates a halocline at ≈15m depth which acts as a barrier between two differently structured benthic communities, one of which has a wide extension along the coast below the halocline and the other above the halocline. Physical and chemical factors, e.g., variations in salinity and temperature, have a great influence on the faunal structure above the halocline, whereas biological processes are the main determinating factors of the faunal composition in the comparatively more stable environment below the halocline. The sub-halocline community contains significantly more species (60%), a significantly higher mean abundance (4200 compared with 2000 ind. m^?2) and a significantly greater mean biomass (146 compared with 71 g m^?2 wet wt) than that above the halocline. Diversity measured by the Shannon-Wiener formula and its evenness had approximately the same means in both these habitats, 3.3–3.7 and 0.62, respectively.Four of the areas investigated in 1976 were also studied in the 1920's by approximately the same methods. The number of species, abundance, and biomass were significantly greater in the recent samples than in those taken half a century ago. and the community structures differed between the two periods. These differences are attributed to seasonal and natural long-term changes and different methods in processing the samples obtained. It is concluded that the halocline is a habitat divider and creates a vertical discontinuity for benthic communities on the west coast of Sweden.     

The Influence of Salinity on Early Ontogeny of the Mussel Mytilus edulisand the Starfish Asterias rubensfrom the White Sea

The influence of increased and decreased salinity on early larvae of the mussel Mytilus edulisL. and the starfish Asterias rubensfrom the White Sea is studied. Fertilized egg cells, embryos, larvae, and yearlings of the two species were compared. Salinity tolerance changes with the stage of development of mussels and starfish. Morphological changes occurring in eggs and embryos exposed to extremely high and low salinities are reported.     

The effect of environmental salinity on the level of heat shock proteins in gill epithelium of Mytilus edilis L. mussel

The composition and the level of heat shock proteins in the gill epithelium cells of mussels Mytilus edulis L. from the White Sea under different levels of environmental salinity were studied by the method of immunoblotting. In mussels maintained under normal salinity (26%), constitutive Hsp70 and protein of about 40 kDa were revealed. After long-term (11?C14 days) acclimation to 14 and 35?? of the level Hsp70 in gill epithelium cells increased. Hsp70 induction was also observed in cells of isolated gills after salinity shock at 14% for 3 and 24 h.     

Effect of the excretory-secretory products of some marine invertebrates on byssus production of the blue mussel <Emphasis Type="Italic">Mytilus edulis</Emphasis> (Bivalvia: Mytilidae)

Under laboratory conditions, we investigated byssus production in the blue mussel Mytilus edulis, as affected by the excretory-secretory products (ESPs) of the mussel itself and some marine invertebrates: the predatory starfish Asterias rubens, and organisms competing with mussels in White Sea fouling communities—a bivalve Hiatella arctica, the solitary ascidians Styela rustica and Molgula citrine, and a sponge Halichondria panicea. The number of attachment disks produced by a mussel per day and the thickness of byssal threads were estimated. Excretory-secretory products of H. arctica and M. citrine had no effect on the number of attachment disks, while ESPs of S. rustica, H. panacea and A. rubens stimulated mussels to produce attachment plaques. The activity of the mussel was slightly increased at low levels of its own ESPs in seawater. The thickness of byssal threads decreased with an increase in the ESPs of mussels in seawater, but it increased in experiments with the ESPs of any other species tested.     

Effects of Low Salinity on Adult Behavior and Larval Performance in the Intertidal Gastropod Crepipatella peruviana (Calyptraeidae)

Shallow-water coastal areas suffer frequent reductions in salinity due to heavy rains, potentially stressing the organisms found there, particularly the early stages of development (including pelagic larvae). Individual adults and newly hatched larvae of the gastropod Crepipatella peruviana were exposed to different levels of salinity stress (32(control), 25, 20 or 15), to quantify the immediate effects of exposure to low salinities on adult and larval behavior and on the physiological performance of the larvae. For adults we recorded the threshold salinity that initiates brood chamber isolation. For larvae, we measured the impact of reduced salinity on velar surface area, velum activity, swimming velocity, clearance rate (CR), oxygen consumption (OCR), and mortality (LC50); we also documented the impact of salinity discontinuities on the vertical distribution of veliger larvae in the water column. The results indicate that adults will completely isolate themselves from the external environment by clamping firmly against the substrate at salinities ≤24. Moreover, the newly hatched larvae showed increased mortality at lower salinities, while survivors showed decreased velum activity, decreased exposed velum surface area, and decreased mean swimming velocity. The clearance rates and oxygen consumption rates of stressed larvae were significantly lower than those of control individuals. Finally, salinity discontinuities affected the vertical distribution of larvae in the water column. Although adults can protect their embryos from low salinity stress until hatching, salinities <24 clearly affect survival, physiology and behavior in early larval life, which will substantially affect the fitness of the species under declining ambient salinities.     

Characteristics of fluctuations in salinity and water quality in brackish Lake Obuchi

Fluctuations in the salinity and physicochemical characteristics of water quality were surveyed in brackish Lake Obuchi on the Shimokita Peninsula in Aomori, Japan. The mean salinity in the surface layer in all regions of Lake Obuchi was about 10 psu, whereas in the basin region at depths of greater than 3 m it was 20 psu. Furthermore, all the year round the halocline was formed at depths of 1–4 m. The maximum density gradient along a vertical axis in the center of the lake was observed at depths of 1–2 m in summer and 2–4 m in spring and fall. The depth of the maximum density gradient fluctuated with the seasons. In summer the water in the bottom layer was anoxic, and Fe, Mn, PO₄ ³⁻-P, and NH₄ ⁺-N supplied from the bottom sediment accumulated at high concentrations below the halocline. Thus, it was observed that the transfer of substances between the layers above and below the barrier formed by the halocline is suppressed. Although Lake Obuchi is small and shallow, the inflowing seawater easily resides, and a stable halocline readily forms because of the shape of its basin, which suddenly deepens on the Pacific Ocean side. Received: May 24, 1999 / Accepted: September 25, 1999