Ecological classification of macroalgae and angiosperm communities of inner coastal waters in the southern Baltic Sea

A classification system of macroalgae and angiosperms for the inner coastal waters of the Baltic coast of Mecklenburg-Vorpommern (Germany) was developed according to the guidelines of the European Water Framework Directive. These guidelines ask for a five-step classification scheme (high, good, moderate, poor, bad) of the ecological state based on submerged macrophytes.The approach is based on the comparison of the occurrence and distribution of plant communities in 1999–2003 with historical data of submerged macrophytes and their community structure. Due to the lack of appropriate data, historical depth distribution limits of plant communities were calculated based on modelled historical underwater light climate.Despite the fact of very heterogeneous salinities (0.5–20.0 practical salinity units, psu) and substrate types (silty–sandy) in the study area, two general main criteria were identified as appropriate indicators of eutrophic degradation of the inner coastal waters: (1) the lower distribution depth of plant communities and (2) the loss of charophyte-dominated plant communities. Based on these two criteria, a five-step classification scheme is proposed for the characterisation of the ecological state. The depth limit boundaries between the ecological classes were calculated by 5, 25, 50 and 75% reduction of the water transparency. Based on this calculation, the ecological status is described by the gradual upward shift of the vegetation depth limit. Complementary to the decrease of depth limits the loss of charophyte-dominated plant communities characterises the border between moderate and poor ecological conditions.     

Diffuse nutrient reduction in the German Baltic Sea catchment: Cost-effectiveness analysis of water protection measures

Water quality of groundwater, rivers, and seas is still negatively influenced by high diffuse nitrogen and phosphorus discharge. Increasing importance is attached to cost-effectiveness to indicate sufficient measures to reduce nutrient input of land use. The aim of this paper is to find cost-effective solutions for diffuse nutrient load reductions for the German Baltic Sea region (by formulating scenarios for a 25% and 50% reduction of nitrogen and phosphorus). I combined nutrient emission and land use modelling (literature analyses, MONERIS-data) with economic land use analysis (e.g. opportunity costs, costs of the production methods) in a cost-effectiveness analysis.The results show that total reduction costs need not be as expensive as often anticipated (e.g. 25% simultaneous reduction of nitrogen and phosphorus costs about 8–20 € (ha y)⁻¹).Advisory service turned out as the most cost-effective measure and the most important indicator for the cost-saving potential. The measure included guidelines to the farmers on land use and water quality optimization. For example, if a 100% advisory service is successful, total costs for a 25% simultaneous reduction of nitrogen and phosphorus could drop by 65%, corresponding to 23 million € y⁻¹ compared to a 50% advisory service. Due to a lack of data, I could not consider the additional nutrient retention capacity of wetlands and buffer strips in the analysis.The policy recommendations derived from this study deliver insights for the strategic orientation of water protection policy and useful indicators to measure the performance. The results are of interest not only in the Baltic Sea context, but also for supporting and facilitating the application of the cost-effectiveness analysis postulated in the Water Framework Directive. Future research should concentrate on improving data availability (e.g. livestock husbandry) and models and on implementing further transaction costs and additional indicators like nutrient retention of wetlands.     

Factors controlling Zostera marina L. growth in the eastern and western Pacific Ocean: Comparisons between Korea and Oregon,USA

Zostera marina distribution is circum-global and tolerates a wide range of environmental conditions. Consequently, it is likely that populations have adapted to local environmental conditions of light, temperature and nutrient supply. We compared Z. marina growth dynamics over a 2-year period in relation to environmental characters at Jindong Bay, South Korea and Yaquina Bay, Oregon, USA. Water temperature in Jindong Bay showed stronger seasonal variation (summer–winter ΔT = 20 °C) than in Yaquina Bay (summer–winter ΔT < 5 °C). Underwater irradiance in Jindong Bay exhibited a winter maximum, while in Yaquina Bay underwater light exhibited a summer maximum. Integrated annual underwater irradiance during 2003 was 2200 and 1200 mol photons m⁻² year⁻¹ in Korea and Oregon, respectively. Z. marina shoot density, biomass and integrated production were not significantly different between the two study sites. Seasonal Z. marina growth in Jindong Bay appeared to be controlled by temperature and light, while the growth pattern in Yaquina Bay suggested light regulation. Several seagrass parameters were correlated to phosphate concentrations, even though nutrients did not appear limiting. Despite differences in environmental factors, relative growth rates and temporal growth dynamics between study sites, integrated annual leaf production was quite similar at 335 and 353 g DW m⁻² year⁻¹ in the Jindong and Yaquina Bay study sites. We suggest that Z. marina net productivity is acclimated to the local environmental conditions and may be a general characteristic of temperate seagrass populations.     

Improved retrieval of Secchi depth for optically-complex waters using remote sensing data

Water transparency is one of the ecological indicators for describing water quality and the underwater light field which determines its productivity. In the European Water Framework Directive (WFD) as well as in the European Marine Strategy Framework Directive (MSFD) water transparency is used for ecological status classification of inland, coastal and open sea waters and it is regarded as an indicator for eutrophication in Baltic Sea management (HELCOM, 2007). We developed and compared different empirical and semi-analytical algorithms for lakes and coastal Nordic waters to retrieve Secchi depth (Z_SD) from remote sensing data (MERIS, 300 m resolution). The algorithms were developed in water bodies with high coloured dissolved organic matter absorption (a_CDOM(442) ranging 1.7–4.0 m⁻¹), Chl a concentration (0.5–73 mg m⁻³) and total suspended matter (0.7–37.5 g m⁻³) and validated against an independent data set over inland and coastal waters (0.6 m < Z_SD < 14.8 m). The results indicate that for empirical algorithms, using longer wavelengths in the visible spectrum as a reference band decreases the RMSE and increases the coefficient of determination (R²). The accuracy increased (R² = 0.75, RMSE = 1.33 m, n = 134) when Z_SD was retrieved via an empirical relationship between Z_SD and K_d(490). The best agreement with in situ data was attained when Z_SD was calculated via both the diffuse and the beam attenuation coefficient (R² = 0.89, RMSE = 0.77 m, n = 89). The results demonstrate that transparency can be retrieved with high accuracy over various optical water types by the means of ocean color remote sensing, improving both the spatial and temporal coverage. The satellite derived Z_SD product could be therefore used as an additional source of information for WFD and MSFD reporting purposes.     

Development and application of an ecological classification tool for fish in lakes in Ireland

A classification tool suitable for establishing the ecological status of lakes based on fish population parameters has been developed for the Republic of Ireland and Northern Ireland (EU Water Framework Directive Ecoregion 17). A lake typology relevant to fish populations in lakes from Ecoregion 17 was produced as part of the ecological classification tool development. Four lake types were determined based on fish metrics and abiotic variables from 43 “reference” lakes. The specific lake fish typology categorised lakes into low (≤67 CaCO₃ mg L⁻¹) or high (>67 CaCO₃ mg L⁻¹) alkalinity, and shallow (≤17 m) or deep (>17 m) maximum depth. The fish in lakes classification tool (FIL2) follows a novel multimetric predictive approach, assigning ecological status to a lake using two independent methods. FIL2 qualitatively defines a lake's ecological status based on fish metrics using discriminant classification rules and, using a generalised linear model, quantitatively derives an Ecological Quality Ratio (EQR, 0 < EQR < 1), along with associated confidence intervals. It is recommended that both methods are used to validate output and cross-check and highlight potential misclassification.     

Measurement of δ15N in macroalgae stored in an environmental specimen bank for regional scale monitoring of eutrophication in coastal areas

The use of stored samples of macroalgae (Fucus ceranoides, Fucus spiralis and Fucus vesiculosus) to monitor contamination of coastal areas was evaluated. The samples, collected from sampling sites (SS) along the entire coast of Galicia (NW Spain) in 5 sampling surveys carried out between 1990 and 2007, were stored in an Environmental Specimen Bank (ESB). The δ¹⁵N was measured in the macroalgal tissues as an indicator of eutrophication. A decrease in δ¹⁵N (p < 0.05) was observed in all species (except for F. vesiculosus in 2007) in successive surveys. δ¹⁵N values were higher in macroalgae from interior zones of the estuaries than in macroalgae collected from the open sea. The method was sensitive enough to detect variations in δ¹⁵N signatures and SS with anomalous values, and to compare trends in contamination on a regional scale. The method is therefore a useful tool for the establishment of environmental improvement programmes (e.g. European Framework Directives) and monitoring surveys.     

Abundance and distribution of Ulva microscopic propagules associated with a green tide in the southern coast of the Yellow Sea

The presence of Ulva microscopic propagules may play an important role in the rapid development of high-biomass blooms of green algae in the Yellow Sea. Six cruises were conducted, to determine the abundance and distribution of Ulva microscopic propagules associated with a green tide that developed in the southern coastal waters of the Yellow Sea from April to August, 2012. Results indicated that Ulva microscopic propagules were widespread in these waters, with the highest density being up to 4800 ind. L⁻¹, prior to the appearance of the green tide in April. High densities were also widely distributed along the coast during May and June, after the appearance of the floating green tide. The quantity of Ulva microscopic propagules significantly decreased when the floating green tide declined in July, reaching densities of up to 162 ind. L⁻¹, following the disappearance of the floating green tide in August. Quantitative studies on the distribution patterns of Ulva microscopic propagules along the southern coast of the Yellow Sea indicated a significant correlation between density and salinity, turbidity and nutrient concentrations. Temporal and geographical distribution patterns of Ulva microscopic propagules were also significantly affected by the presence of a large biomass of attached, or floating, Ulva species algae.     

Intertidal seagrass in Ireland: Pressures,WFD status and an assessment of trace element contamination in intertidal habitats using Zostera noltei

Intertidal seagrass has been selected as a Biological Quality Element for the assessment of ecological status under the Water Framework Directive. In Ireland, two species of seagrass, Zostera marina and Z. noltei occur in intertidal habitats. This study presents the first comprehensive assessment of the distribution and Water Framework Directive status of intertidal seagrass in the Republic of Ireland and Northern Ireland. Most of the areas assessed, using the Water Framework Directive-compliant assessment tool, have a status of HIGH or GOOD. Only two areas were found to have a status less than GOOD and in both, the cause for the decline was smothering by opportunistic foliose green macroalgae. Linear regression showed a relationship between pressure index scores and Ecological Quality Ratio, showing the relevance of the index as a metric of anthropogenic pressure. Trace element concentrations were examined in Z. noltei tissues and Trace Element Pollution Index values were calculated. The relationship between Trace Element contamination and Water Framework Directive status was examined but the results showed little correlation. However, a relationship between the pressure index and trace element contamination was obtained. This assessment provides the most comprehensive overview of intertidal seagrass beds in Ireland and establishes a strong baseline for ongoing monitoring and assessment under the Water Framework Directive. The data provide key information on the pressures affecting these valuable habitats which will assist in the development of measures to improve and protect our transitional and coastal waters.     

Restoration experiment of Zostera marina L. in a subtropical coastal lagoon

Zostera marina is the dominant seagrass species in coastal lagoons on the western coast of Baja California Peninsula, and due to its coastal location it is threatened by natural and anthropogenic factors, as is happening in Puerto San Carlos, B.C.S., where a fish cannery unloads its wastewater to the beach. Apparently an extensive intertidal meadow replacement was established by great amounts of green macroalgae. We evaluated the possibility to mitigate the impacts of this cannery with transplants of Z. marina meadow using adult plants. The transplant experiment was made in two different seasons for which two undisturbed donor meadows were chosen: El Cuervo and San Carlitos. The winter one obtained a 30% and in San Carlitos 90% after 13 months and the autumn transplant in San Carlos obtained a 0% of survival after 3 months. The results of these transplant activities were reflected in the shoot density at the end of the experiment (San Carlos was of 482 shoots/m² and San Carlitos of 818 shoots/m²s and agree with the density of the natural meadows. This experiment shows that it is possible to develop a small-scale seagrass restoration as mitigation for Baja California coastal lagoons which are under severe threat for coastal development.     

Density and distribution of seeds in bottom sediments in Zostera marina beds in Ago Bay,central Japan

The density of Zostera marina L. seeds in bottom sediments was examined to study the reproductive patterns of the Z. marina population in Ago Bay, Mie Prefecture, central Japan.Seeds and seed coats were numerous in Tategami, where the annual type of Z. marina grows. In contrast, seeds were scarce in Hamajima, where the perennial type of Z. marina grows. Bottom sediment was sampled with sediment cores at Tategami in November 2004 and March 2005 to examine density and depth distribution of the seeds. Seeds were found as deep as 8 cm, but no deeper. On the other hand, empty seed coats were found as deep as 16 cm in both months. In the upper layers of the sediment to a depth of 8 cm, the average number of seed coats was 7960 ± 2997 m⁻² in November and 16,318 ± 2922 m⁻² in March. Deeper than 8 cm, the number of seed coats gradually decreased owing to decomposition, and none was found below 16 cm. We used the density of reproductive shoots and number of seeds per spadix in Tategami to estimate the fate of seeds and seed coats of the annual type of Z. marina in bottom sediments: out of the 6000 seeds m⁻² produced annually, 72% disappears from the stand and 28% is buried locally. The density and distribution of Z. marina seeds are among the most important factors in maintenance and propagation of the annual population at Tategami.     

Bloom forming Alexandrium ostenfeldii (Dinophyceae) in shallow waters of the Åland Archipelago,Northern Baltic Sea

In the past years, late summer blooms of the bioluminescent dinoflagellate Alexandrium ostenfeldii have become a recurrent phenomenon in coastal waters of the central and Northern Baltic Sea. This paper reports exceptionally high cell concentrations (10⁵ to 10⁶ cells L^?1) of the species found during bioluminescent blooms in 2003 and 2004 in a shallow embayment of the Åland archipelago at the SW coast of Finland. Clonal cultures were established for morphological, molecular, toxicological and ecophysiological investigations to characterize the Finnish populations and compare them to other global A. ostenfeldii isolates. The Finnish isolates exhibited typical morphological features of A. ostenfeldii such as large size, a prominent ventral pore and an orthogonally bent first apical plate. However, unambiguous differentiation from closely related Alexandrium peruvianum was difficult due to considerable variation of sulcal anterior plate shapes. The Finnish strains were genetically distinct from other isolates of the species, but phylogenetic analyses revealed a close relationship to isolates from southern England and an A. peruvianum morphotype from the Spanish Mediterranean. Together these isolates formed a distinct clade which was separated from a clade containing other Northern European, North American and New Zealand populations. Toxin analyses confirmed the presence of the PSP toxins GTX2, GTX3 and STX in both Finnish isolates with GTX3 being the dominant toxin. Total relative PSP toxin contents were moderate, ranging from approximately 6 to 15 fmol cell^?1 at local salinities of 5 and 10 psu, respectively. Spirolides were not detected. Salinity tolerance experiments showed that the Finnish isolates were well adapted to grow at the low salinities of the Baltic Sea. With a salinity range of approximately 6 to 20–25 psu, Baltic populations are physiologically distinct from their marine relatives. Vigorous production of different cyst types in the cultures suggest that cysts may play a crucial role in the survival and retainment of A. ostenfeldii populations in the Baltic Sea.     

Evaluating the influence of off-shore cage aquaculture on the benthic ecosystem in Alghero Bay (Sardinia,Italy) using AMBI and M-AMBI

The impact of an off-shore fish farm in Alghero Bay (northwest Sardinia, Italy) on the benthic ecosystem was investigated in 2007 and 2008. In addition to studying the chemical and physical characteristics of the area (i.e., currents and sediment analyses), some biological analyses were also performed. The AZTI's Marine Biotic Index (AMBI) and the multivariate AMBI (M-AMBI) were calculated, which are being used in assessing the ecological status of benthic communities within the European Water Framework Directive (WFD). Clear impact gradients were detected according to both methods; they are related to farm production, prevailing currents, and characteristics of the area (i.e., water depth and distance to the cages). The site affected most was detected within 84 m from the cages; the area that no longer showed effects was over 907 m from the cages. The gradient is shown by decreasing AMBI values and percentage of opportunistic species and increasing richness, diversity, and the presence of sensitive species. This study highlights the importance of setting reference conditions for different areas when calculating M-AMBI. These reference conditions correspond to those in undisturbed sites in the opposite direction of the prevailing currents within the area.     

A mutualistic interaction between the bacterium Pseudomonas asplenii and the harmful algal species Chattonella marina (Raphidophyceae)

Several studies on various Chattonella species have reported that bacteria may play an important role in Chattonella bloom initiation, however, no studies have described how these bacteria promote the growth of C. marina. The interaction between C. marina and bacteria was investigated for identification and characterization of potential growth-promoting bacteria. In preliminary tests, the growth promoting effect of Pseudomonas species (25 strains) was investigated and P. asplenii (≥2.27) was determined as a growth-promoting bacteria for both C. marina strains (CCMP 2049 and 2050). This bacterium exerted optimal growth-promoting effects on C. marina, causing an increase in the initial density of P. asplenii to approximately 1 × 10⁷ cells mL⁻¹, which was used as the initial density in this study. To determine whether the growth-promoting activity was direct or indirect, P. asplenii was incubated in the algal media and then a filtrate of this culture was added to both C. marina strains. The P. asplenii filtrate stimulated the growth of C. marina and maintained the growth-promoting effects after high temperature (121 °C for 20 min) and pressure (15 psi) treatment. Thus, P. asplenii is able to promote C. marina growth through the release of a heat-resistant substance, such as inorganic nutrients. A nutrient analysis indicated that this bacterium elevated the phosphate concentration. Interestingly, P. asplenii was unable to survive in phosphate-limited media but could grow in phosphate-limited media incubating C. marina. Moreover, this bacterium could secrete significantly more phosphate in the presence of C. marina (p < 0.0001). These results suggested that P. asplenii and C. marina may have a mutualistic interaction.     

Effects of filamentous algae and deposited matter on the survival of Fucus vesiculosus L. germlings in the Baltic Sea

As a result of increased nutrient levels in the Baltic Sea during thepast 50 years, mass developments of filamentous algae have become a commonfeature along the Swedish east coast and deposition of organic matter has alsoincreased. To test whether these two factors have any effects on the early lifestages of Fucus vesiculosus a number of laboratory andfield studies were conducted. The amount of epilithic and epiphytic filamentousalgae on F. vesiculosus and the amount of deposited matterin the littoral zone were quantified during the two reproductive periods ofF. vesiculosus, early summer (May–June) and lateautumn (September–October). Both filamentous algae (Cladophoraglomerata) and deposited matter (introduced either before or aftersettlement of fertilized eggs) were shown to significantly decrease the numberof surviving germlings. The survival of germlings seeded on stones withfilamentous algae, or seeded on culture dishes concurrently with the lowestconcentration of deposited matter (0.1 g dm^–2),was 5% or less. In the field, the amount of filamentous algae was significantlyhigher during F. vesiculosus summer reproduction, whereasthe amount of deposited matter collected in traps was significantly higherduring the period of autumn reproduction. The greatest biomass of filamentousalgae was observed at sheltered sites. Based on the negative effects offilamentous algae and deposited matter on Fucusrecruitmentand the observation of local and seasonal differences in abundance offilamentous algae and deposition, we suggest that the prerequisites for thesurvival of either summer or autumn-reproducing populations of F.vesiculosus in the Baltic Sea may differ locally.     

Phylogenetic relationships of Cochlodinium polykrikoides Margalef (Gymnodiniales,Dinophyceae) from the Mediterranean Sea and the implications of its global biogeography

Although the diversity of dinoflagellates has been intensively studied in several locations in the Mediterranean Sea since the 1950s, it is only during the last two decades that the morphotype of the toxic unarmoured dinoflagellate Cochlodinium polykrikoides Margalef has been detected, coinciding with its apparent worldwide expansion in marine coastal waters. In this study, vegetative cells of C. polykrikoides morphotype from the Catalan coast (NW Mediterranean Sea) were detected and isolated, and the DNA from collected cells was sequenced. While in the Mediterranean Sea, detections are scarce and C. polykrikoides is consistently present at low concentrations, we reported exceptional blooms of this species, in which the maximum abundance reached 2 × 10⁴ cells L⁻¹. Partial LSU rDNA region sequences showed that most C. polykrikoides populations from the Catalan coast formed a new differentiated ribotype, but others were included within the ‘Philippines’ ribotype, demonstrating their coexistence in the Mediterranean Sea. Thus, the current biogeographic nomenclature of the ribotypes is likely to be invalid with respect to the available information from populations comprising the ‘Philippines’ ribotype. The phylogeny suggests the existence of cryptic species that should be evaluated for species-level status. Accordingly, the ribotype determination must be carefully evaluated for all detections and bloom events, since accurate characterization of the morphology, ecophysiology and distribution of the ribotypes are not well resolved.     

Abundance and distribution of toxic Alexandrium tamarense resting cysts in the sediments of the Chukchi Sea and the eastern Bering Sea

Abundance and distribution of the toxic dinoflagellate Alexandrium tamarense species complex resting cyst were investigated in the eastern Bering Sea and the Chukchi Sea for the first time. Sediment samples (top 0–3 cm depth) were collected from the continental shelf of the eastern Bering Sea (17 stations) and the Chukchi Sea (13 stations) together with a long core sample (top 0–21 cm depth) from one station in the Chukchi Sea during 2009–2012. The cysts were enumerated using the primuline staining method. Species identification of the cysts was carried out with multiplex PCR assay and the plate morphology of vegetative cells germinated from cysts in the both areas. Alexandrium cysts were widely detected in the both areas, ranging from not detected (<1 cysts cm⁻³) to 835 cysts cm⁻³ wet sediment in the eastern Bering Sea and from not detected (<1 cysts cm⁻³) to 10,600 cysts cm⁻³ in the Chukchi Sea, and all isolated cysts were genetically and morphologically identified as the North American clade A. tamarense. Their cysts were mainly distributed in the shallow continental shelf where the water depth was less than 100 m in both areas. The cysts were detected from the deep layer (18–21 cm depth of sediment core) of the long core sample. The present study confirmed the abundant existence of A. tamarense with wide range of distribution in these areas. This fact suggests that A. tamarense vegetative cells have appeared in the water column in the both areas. Furthermore, these abundant cyst depositions indicate that this species originally distributed in the Arctic and subarctic regions and well adapted to the environments in the marginal ice zone.     

An overview of Iranian mangrove ecosystems, northern part of the Persian Gulf and Oman Sea

Iranian mangrove forests occur between longitude 25°19′ and 27°84′, in the north part of the Persian Gulf and Oman Sea. In 2002, it was estimated that 93.37 km² of Iranian shorelines were covered with mangrove forests, with the largest area (67.5 km²) occurring between the Khamir Port and the northwest side of Qeshm Island, and the smallest area (0.01 km²) in the Bardestan estuary. Only two species of mangrove are found in the Persian Gulf: Avicennia marina from Avicenniaceae and Rhizophora macrunata from Rhizophoraceae. A. marina is the dominant specie in these forests whereas Rh. macrunata is found only in the Sirik region. Overexploitation of mangrove leaves and oil pollution are the main causes of mangrove destruction in this region.     

Deposit-feeders accumulate the cyanobacterial toxin nodularin

Blooms of toxic cyanobacteria may potentially affect food web productivity and even be a human health hazard. In the Baltic Sea, regularly occurring summer blooms of nitrogen-fixing cyanobacteria are often dominated by Nodularia spumigena, which produces the potent hepatotoxin nodularin. Evidence of sedimentation of these blooms indicates that benthic fauna can be exposed to nodularin. In a one month experiment, we simulated the settling of a summer bloom dominated by N. spumigena in sediment microcosms with three species of sediment-dwelling, deposit-feeding macrofauna, the amphipods Monoporeia affinis and Pontoporeia femorata and the bivalve Macoma balthica, and analyzed nodularin in the animals by HPLC–ESI–MS (high-performance liquid chromatography–electrospray ionization–mass spectrometry). We found nodularin in quantities of 50–120 ng g⁻¹ DW. The results show that deposit-feeding macrofauna in the Baltic Sea may contribute to trophic transfer of nodularin.     

Evaluation of the effects of light intensity on growth of the benthic dinoflagellate Ostreopsis sp. 1 using a newly developed photoirradiation-culture system and a novel regression analytical method

Benthic dinoflagellates of the genus Ostreopsis are found all over the world in temperate, subtropical, and tropical coastal regions. Our recent studies revealed that a putative “cryptic” species of Ostreopsis ovata is present widely along Japanese coasts. This organism, Ostreopsis sp. 1, possesses palytoxin analogs and thus its toxic blooms may be responsible for potential toxification of marine organisms. To evaluate the bloom dynamics of Ostreopsis sp. 1, the present study examined the growth responses of Ostreopsis sp. 1 strain s0716 to various light intensities (photon flux densities: μmol photons m⁻² s⁻¹) using a newly devised photoirradiation-culture system. This novel system has white light-emitting diodes (LEDs) capable of more closely simulating the wavelength spectrum of light entering the oceanic water column than do fluorescent tubes and halogen lamps. In this system, the light intensity of the white LEDs was reduced through two polarizing filters by varying the rotation angles of the filters. Thereby, the new system was capable of culturing microalgae under well-controlled light intensity conditions. Ostreopsis sp. 1 grew proportionally when light intensity was increased from 49.5 to 199 μmol photons m⁻² s⁻¹, but its growth appeared to be inhibited slightly at ≥263 μmol photons m⁻² s⁻¹. The relationship between observed growth rates and light intensity was calculated at R > 0.99 (P < 0.01) using a regression analysis with a modified equation of the photosynthesis-light intensity (P-L) model. The equation determined the critical light intensities for growth of Ostreopsis sp. 1 and the organism's growth potential as follows: (1) the threshold light intensity for growth: 29.8 μmol photons m⁻² s⁻¹; (2) the optimum light intensity (L_m) giving the maximum growth rate (μ_max = 0.659 divisions day⁻¹): 196 μmol photons m⁻² s⁻¹; (3) the optimum light intensity range (L_opt) giving ≥95% μ_max: 130–330 μmol photons m⁻² s⁻¹; (4) the semi-optimum range (L_sopt) giving ≥80% μ_max: 90 to over 460 μmol photons m⁻² s⁻¹. The L_sopt represents 4.5–23% ambient light intensity present in surface waters off of a temperate region of the Japanese coast, Tosa Bay; putatively, this semi-optimum range of light intensity appears at depth of 12.9–27.8 m. Considering these issues, our data indicate that Ostreopsis sp. 1 in coastal environments may form blooms at ca. ∼28 m depth in regions along Japanese coasts.