Benthic microalgae in coral reef sediments of the southern Great Barrier Reef,Australia

The abundance and productivity of benthic microalgae in coral reef sediments are poorly known compared with other, more conspicuous (e.g. coral zooxanthellae, macroalgae) primary producers of coral reef habitats. A survey of the distribution, biomass, and productivity of benthic microalgae on a platform reef flat and in a cross-shelf transect in the southern Great Barrier Reef indicated that benthic microalgae are ubiquitous, abundant (up to 995.0 mg chlorophyll (chl) a m^–2), and productive (up to 110 mg O₂ m^–2 h^–1) components of the reef ecosystem. Concentrations of benthic microalgae, expressed as chlorophyll a per surface area, were approximately 100-fold greater than the integrated water column concentrations of microalgae throughout the region. Benthic microalgal biomass was greater on the shallow water platform reef than in the deeper waters of the cross-shelf transect. In both areas the benthic microalgal communities had a similar composition, dominated by pennate diatoms, dinoflagellates, and cyanobacteria. Benthic microalgal populations were potentially nutrient-limited, based on responses to nitrogen and phosphorus enrichments in short-term (7-day) microcosm experiments. Benthic microalgal productivity, measured by O₂ evolution, indicated productive communities responsive to light and nutrient availability. The benthic microalgal concentrations observed (92–995 mg chl a m^–2) were high relative to other reports, particularly compared with temperate regions. This abundance of productive plants in both reef and shelf sediments in the southern Great Barrier Reef suggests that benthic microalgae are key components of coral reef ecosystems.Communicated by Environmental Editor, B.C. Hatcher     

Nutrient content of seagrasses and epiphytes in the northern Gulf of Mexico: Evidence of phosphorus and nitrogen limitation

We examined C:N:P ratios of seagrass leaves and epiphytic algae from the eastern shoreline of Grand Bay (Alabama, USA) and the entire shoreline of Big Lagoon (Florida, USA) during the summer of 2001 and March 2003, and used contour plotting of N:P ratios in both locations to examine spatial trends in our data. Results indicated phosphorus limitation for seagrass and epiphytes in each bay. In addition, C:N, C:P, and N:P ratios in both locations showed differences between summer and wintertime values for seagrasses; however, the only epiphytic elemental ratios to differ were C:P and N:P ratios in Grand Bay. Within Grand Bay, phosphorus limitation was stronger in epiphytes than seagrasses, with the largest amount of variation in N:P ratios occurring adjacent to the only developed land on the shoreline. In Big Lagoon, two distinct areas were present in N:P contour plots: the eastern end of the bay that was influenced by water from the Gulf of Mexico and Santa Rosa Sound, and the western end of the bay that was most influenced by Perdido Bay and a developed area along the northern shoreline. Detection of phosphorus limitation within Big Lagoon was not surprising, as both input sources to Big Lagoon are known to be low in phosphorus. However, phosphorus limitation in Grand Bay was unexpected, as both “feeder systems” (Mobile Bay and the Mississippi Sound) have high ambient phosphorus levels. As a result, C:N:P ratios from seagrasses and epiphytes may not accurately reflect ambient nutrient levels in Grand Bay due to decreased availability of some forms of phosphorus or increased competition for the uptake of phosphorus. Overall, our C:N:P analysis suggested that not only was P limitation greater than N limitation in Grand Bay and Big Lagoon, but patterns of nutrient limitation varied both temporally and geographically for inter- and intra-bay comparisons.     

Epiphyte grazing enhances productivity of remnant seagrass patches

Anthropogenic nutrient enrichment is increasingly modifying community structure and ecosystem functioning in terrestrial and aquatic ecosystems. In marine ecosystems, the paradigm is that nutrient enrichment leads to a decline of seagrasses by stimulating epiphytic algal growth, which shades and overgrows seagrasses. This ignores the potential for herbivores, which graze upon epiphytic algae, to partially or wholly counter such nutrient effects. We conducted a field experiment to assess the role that the trochid gastropod Calthalotia fragum plays in reducing nutrient impacts on the seagrass, Posidonia australis, in an urbanized Australian estuary, Botany Bay, Sydney. In a field experiment, where nutrient loading and grazer density were orthogonally manipulated, nutrient enrichment failed to promote epiphyte biomass or diminish growth and primary productivity of P. australis. To the contrary, nutrient enrichment enhanced photosynthesis of the seagrass in plots where the grazer was present at higher density. Epiphytic growth was negatively affected by increased C. fragum density, while P. australis shoot growth was positively influenced. Thus, in this study system, grazing appears to play a much greater role in determining seagrass primary productivity and above‐ground growth than moderate nutrient loading, suggesting that the interaction between grazers and nutrients depends on the relative levels of each. Our study contributes to a growing body of literature suggesting that effects of nutrient loading on benthic assemblages are not universally negative, but are dependent on the biotic and abiotic setting.     

Food habits of fishes in the surf zone of a sandy beach at Sanrimatsubara, Fukuoka Prefecture, Japan

To clarify the feeding habits of fishes in surf zones, the gut contents of 19 fish species collected in the surf zone of a sandy beach at Sanrimatsubara, western Japan, were examined. Ontogenetic changes in food preference were recognized in seven species (Mugil cephalus cephalus, Lateolabrax latus, Sillago japonica, Paralichthys olivaceus, Paraplagusia japonica, Takifugu poecilonotus, and Takifugu niphobles). A cluster analysis based on dietary overlaps showed that the surf zone fish assemblage comprised six trophic groups (zooplankton, benthic and epiphytic crustacean, detritus, polychaete, fish, and insect feeders). Of these, the most abundant trophic group was zooplankton feeders, along with benthic and epiphytic crustacean feeders.     

FIELD MEASUREMENTS OF INORGANIC NITROGEN UPTAKE BY EPIFLORA COMPONENTS OF THE SEAGRASS POSIDONIA OCEANICA (MONOCOTYLEDONS,POSIDONIACEAE)1

Crustose corallines, crustose and erect brown algae, and sessile animals are major components of the epiphytic community of the Mediterranean seagrass Posidonia oceanica (L.) Delile. Production, biomass, and specific composition of this epiphyte–seagrass association are impacted by anthropogenic increase of nutrient load in this oligotrophic area. In this context, nitrogen uptake by P. oceanica and its epiflora was measured using the isotope ¹⁵N at a 10 m depth in the Revellata Bay (Corsica, Mediterranean Sea). Epiflora components showed various seasonal patterns of biomass and abundance. The epiphytic brown algae appeared at the end of spring, later than the crustose corallines, and after the nitrate peak in the bay. Because of their later development in the season, epiphytic brown algae mostly rely on ammonium for their N needs. We hypothesize that the temporal succession of epiphytic organisms plays a crucial role in the N dynamics of this community under natural conditions. The epiphytic brown algae, which have a growth rate one order of magnitude greater than that of crustose corallines, showed lower N‐uptake rates. The greater N‐uptake rates of crustose corallines probably reflect the greater N requirements (i.e., lower C/N ratios) of red algae. We determined that the epiflora incorporated ammonium and nitrate more rapidly than their host. Nevertheless, when biomass was taken into account, P. oceanica was the most important contributor to N uptake from the water column by benthic macrophytes in this seagrass bed.     

Spatial and temporal distributions of epiphytic diatoms growing on Thalassia testudinum Banks ex König: relationships to water quality

The spatial and temporal distributions of the epiphytic diatom flora on Thalassia testudinum was described within the Florida Bay estuary and at one Atlantic site east of the Florida Keys over a 1-year period. Species of the genus Mastogloia dominated the epiphytic diatom flora (82 out of 332 total species). Nonmetric Multidimensional Scaling (NMDS) and Analysis of Similarity (ANOSIM) revealed four distinct spatial assemblages and two temporal assemblages. Eastern and western Florida Bay assemblages were identified within the estuary. The eastern diatom assemblage was characterized by high relative abundances of Brachysira aponina and Nitzschia liebetruthii, while the western assemblage was characterized by the abundance of Reimerothrix floridensis, particularly during summer. Two diverse and distinct marine assemblages, one located in the Gulf of Mexico along the western edge of Florida Bay and the other behind the Florida reef tract in the Atlantic Ocean, were also identified. Analysis of the spatial distribution of diatoms and water quality characteristics within Florida Bay suggest that these assemblages may be structured by salinity and nutrient availability, particularly P. The Gulf of Mexico and the western Florida Bay assemblages were associated with higher water column salinities and TP concentrations and lower DIN concentrations and TN:TP ratios relative to the eastern Florida Bay assemblage. The temporal variation in diatom assemblages was associated with water temperature, though temporal indicator species were few relative to the number of spatial indicators. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Diatom composition and biomass variability in nearshore waters of Maxwell Bay, Antarctica, during the 1992/1993 austral summer

 Diatom composition and biomass were investigated in the nearshore water (<30 m in depth) of Maxwell Bay, Antarctica during the 1992/1993 austral summer. Epiphytic or epilithic diatoms such as Fragilaria striatula, Achnanthes brevipes var. angustata and Licmophora spp. dominated the water column microalgal populations. Within the bay, diatom biomass in surface water was several times higher at the nearshore (2.4–14 μg C l^-1) than at the offshore stations (>100 m) (1.2–3.2 μg C l^-1) with a dramatic decrease towards the bay mouth. Benthic forms accounted for >90% of diatom carbon in all nearshore stations, while in the offshore stations planktonic forms such as Thalassiosira antarctica predominated (50–>90%). Microscopic examination revealed that many of these diatoms have become detached from a variety of macroalgae growing in the intertidal and shallow subtidal bottoms. Epiphytic diatoms persistently dominated during a 19-day period in the water column at a fixed nearshore station, and the biomass of these diatoms fluctuated from 0.86 to 53 μg C l^-1. A positive correlation between diatom biomass and wind speed strongly suggests that wind-driven resuspension of benthic forms is the major mechanism increasing diatom biomass in the water column. Received: 28 April 1995/Accepted: 1 April 1996     

Estuarine intertidal sandflat benthic microalgal responses to in situ and mesocosm nitrogen additions

Benthic microalgal communities are important components of estuarine food webs and make substantial contributions to coastal materials cycling. Nitrogen is generally the limiting factor for marine primary production; however other factors can limit benthic primary producers because of their access to the additional nutrients found in sediment porewater. Field and laboratory experiments were conducted to test the hypothesis that water column nitrogen supply affects estuarine sandflat benthic microalgal community structure and function. Our field and mesocosm experiments assessed changes at both the population and functional group levels. Simulated water column nitrogen additions increased maximum community photosynthesis in most cases (Pb_max from photosynthesis vs. irradiance curves). Additional changes that resulted from nitrogen additions were decreases in porewater phosphate, increases in porewater ammonium, shifts in community composition from N₂ fixing cyanobacteria toward diatoms, and detectable, though not statistically significant increases in biomass (as chlorophyll a). Results from field and laboratory experiments were quite similar, suggesting that laboratory experiments support accurate predictions of the response of intertidal benthic microalgae to changes in water column nutrient conditions.     

BENTHIC AND PLANKTONIC ALGAL COMMUNITIES IN A HIGH ARCTIC LAKE: PIGMENT STRUCTURE AND CONTRASTING RESPONSES TO NUTRIENT ENRICHMENT1

We investigated the fine pigment structure and composition of phytoplankton and benthic cyanobacterial mats in Ward Hunt Lake at the northern limit of High Arctic Canada and the responses of these two communities to in situ nutrient enrichment. The HPLC analyses showed that more than 98% of the total pigment stocks occurred in the benthos. The phytoplankton contained Chrysophyceae, low concentrations of other protists and Cyanobacteria (notably picocyanobacteria), and the accessory pigments chl c₂, fucoxanthin, diadinoxanthin, violaxanthin, and zeaxanthin. The benthic community contained the accessory pigments chl b, chl c₂, and a set of carotenoids dominated by glycosidic xanthophylls, characteristic of filamentous cyanobacteria. The black surface layer of the mats was rich in the UV‐screening compounds scytonemin, red scytonemin‐like, and mycosporine‐like amino acids, and the blue‐green basal stratum contained high concentrations of light‐harvesting pigments. In a first bioassay of the benthic mats, there was no significant photosynthetic or growth response to inorganic carbon or full nutrient enrichment over 15 days. This bioassay was repeated with increased replication and HPLC analysis in a subsequent season, and the results confirmed the lack of significant response to added nutrients. In contrast, the phytoplankton in samples from the overlying water column responded strongly to enrichment, and chl a biomass increased by a factor of 19.2 over 2 weeks. These results underscore the divergent ecophysiology of benthic versus planktonic communities in extreme latitudes and show that cold lake ecosystems can be dominated by benthic phototrophs that are nutrient sufficient despite their ultraoligotrophic overlying waters.     

Response of epiphytic bryophytes to simulated N deposition in a subtropical montane cloud forest in southwestern China

A field manipulation experiment was conducted in a subtropical montane cloud forest in southwestern China to determine the possible responses of epiphytic bryophytes to increasing nitrogen (N) deposition from community to physiology level, and to find sensitive epiphytic bryophytes that may be used as indicators for assessing the degree of N pollution. N addition had significantly negative effects on species richness and cover of the epiphytic bryophyte community. Harmful effects of high N loads were recorded for chlorophyll, growth, and vitality of the species tested. The decline of some epiphytic bryophytes may result from detrimental effects on degradation to photosynthetic pigments. Bazzania himalayana (Mitt.) Schiffn., Bazzania ovistipula (Steph.) Mizut., and Homaliodendron flabellatum (Sm.) Fleisch. are candidates in atmospheric nitrogen monitoring. Epiphytic bryophytes in the montane cloud forest are very sensitive to increasing N deposition and often difficult to recover once they have been destroyed, providing early detection of enhanced N pollution for trees or even the whole forest ecosystem. The inference that increasing N pollution may lead to loss of biodiversity is a concern to the developing economy in western China, and should alert the government to the adverse impacts caused by increased industrial pollution during the process of China’s West Development.     

Phosphorus cycling in a dense Potamogeton pectinatus L. Bed

Summary An analysis of phosphorus concentrations and standing stocks in the various components of a dense Potamogeton pectinatus L. community in a brackish lake showed that the amounts of P per unit area varied in the following order: Sediments > Above ground macrophytes > Detritus > Benthic Invertebrates > Below ground macrophyte tissue > Epiphytic algae > Water soluble P > Water borne particulate P. Seasonal changes in the variability of these stocks are described, and shown to be controlled by the annual growth and decomposition pattern of the Potamogeton. The sediments were suggested as the major source of P for the plant biomass. Studies using ³²P tracer showed that of a given input of P to the water, 32% went to large epiphytic algae, 17% to the Potamogeton, 16% to the benthic fauna (mostly filter feeding bivalves), 28% to the sediments (almost all incorporated in the top 1 cm), and the remaining 7% was adsorbed on to, or absorbed by microorganisms associated with detritus. Analysis of ³²P uptake curves indicated that of the P absorbed by the Potamogeton, a significant proportion went to the complex adnate periphyton on the leaf surface. We were unable to separate this fraction. Movement of P in the community was shown to be a closed cycle, and any release of P from decaying macrophytes would be rapidly reabsorbed by epiphytic algae. It is unlikely that phosphorus, once cycling in the macrophyte community, would become transferred to the circulation in the open lake.     

Biodiversity of settled material in a sediment trap in the Gulf of Trieste (northern Adriatic Sea)

Phytoplankton succession and sinking rates were studied from January to December 2003 at a coastal station in the Gulf of Trieste (northern Adriatic Sea), 200 m offshore, in a relatively undisturbed area. A conical sediment trap, moored at 15 m depth (water depth 17 m), was used. The hypothesis if the presence of benthic and epiphytic diatoms can lead to an overestimation of the vertical fluxes was tested. To evaluate primary and secondary sedimentation contributions, planktonic, benthic and epiphytic diatoms were distinguished. Benthic species abundance varied throughout the year and it was related to resuspension that strongly influenced sinking rates. All over the year, diatoms were the prevailing class in the trap material accounting for 75.32% of the settled cells, while flagellates represented 24.11%. Dinophyceae and resting cells constituted minor components, accounting for 0.43% and 0.14%, respectively. The gross sedimentation rates ranged from 0.006 × 10⁸ cell m⁻² d⁻¹ in the second week of May to 6.30 × 10⁸ cell m⁻² d⁻¹ in the third week of January with a mean annual value of 1.09 ± 1.43 × 10⁸ cell m⁻² d⁻¹. To the primary sedimentation rate Pseudo-nitzschia seriata of the group “Nitzschia seriata complex” contributed for 49.77% followed by Chaetoceros spp. (23.88%). The major contributor to the secondary sedimentation rate was the diatom Paralia sulcata, accounting for 24.76%. Epiphytic diatoms contributed for 11.19% and 12.27% on annual average gross abundance and biomass, respectively, reaching even 72.04% of gross abundance and 56.06% of gross biomass in the second week of August. The correlation between temperature and the logarithm of the epiphytic biomass was statistically significant, with r = 0.66 and P < 0.001. Both in the cluster analysis and in the PCA four main groups were formed, where benthic and epiphytic species were separately gathered. Planktonic, benthic and epiphytic forms accounted for 50.78%, 36.95% and 12.27%, respectively, calculated on the annual average biomass. Therefore, vertical fluxes can be overestimated of 50% or more if benthic and epiphytic species are not rejected.     

Composition of inorganic and organic nutrient sources influences phytoplankton community structure in the New River Estuary, North Carolina

The New River Estuary, NC, is a nutrient-sensitive, eutrophic water body that is prone to harmful algal blooms. High annual loading from the watershed of varying nutrient forms, including inorganic phosphorus and inorganic and organic nitrogen, may be linked to the persistence of algal blooms in the estuary. In order to evaluate phytoplankton response to nutrient inputs, a series of in situ nutrient addition experiments were carried out during June 2010 to July 2011 on water from an estuarine site known to support algal blooms. Estuarine water was enriched with nutrients consisting of individual and combined sources of dissolved inorganic nitrogen, orthophosphate, urea, and a natural dissolved organic nitrogen (DON) addition derived from upstream New River water. The combined inorganic N and P addition most frequently stimulated phytoplankton biomass production as total chlorophyll a. The responses of diagnostic (of major algal groups) photopigments were also evaluated. Significant increases in peridinin (dinoflagellates), chlorophyll b (chlorophytes), and myxoxanthophyll (cyanobacteria) were most frequently promoted by additions containing riverine DON. Significant increases in zeaxanthin (cyanobacteria) were more frequently promoted by inorganic nitrogen additions, while increases in fucoxanthin (diatoms) and alloxanthin (cryptophytes) were not promoted consistently by any one nutrient treatment. Evaluating the impact of varying nutrient forms on phytoplankton community dynamics is necessary in order to develop strategies to avoid long-term changes in community structure and larger-scale changes in ecosystem condition.     

Upward cascading effects of nutrients: shifts in a benthic microalgal community and a negative herbivore response

We evaluated the effects of nutrient addition on interactions between the benthic microalgal community and a dominant herbivorous gastropod, Cerithidea californica (California horn snail), on tidal flats in Mugu Lagoon, southern California, USA. We crossed snail and nutrient (N and P) addition treatments in enclosures on two tidal flats varying from 71 to 92% sand content in a temporally replicated experiment (summer 2000, fall 2000, spring 2001). Diatom biomass increased slightly (~30%) in response to nutrient treatments but was not affected by snails. Blooms of cyanobacteria (up to 200%) and purple sulfur bacteria (up to 400%) occurred in response to nutrient enrichment, particularly in the sandier site, but only cyanobacterial biomass decreased in response to snail grazing. Snail mortality was 2–5 times higher in response to nutrient addition, especially in the sandier site, corresponding to a relative increase in cyanobacterial biomass. Nutrient-related snail mortality occurred only in the spring and summer, when the snails were most actively feeding on the microalgal community. Inactive snails in the fall showed no response to nutrient-induced cyanobacterial growths. This study demonstrated strongly negative upward cascading effects of nutrient enrichment through the food chain. The strength of this upward cascade was closely linked to sediment type and microalgal community composition.     

The importance of nutrient co‐limitation in regulating algal community composition,productivity and algal‐derived DOC in an oligotrophic marsh in interior Alaska

1. Compared to lakes and streams, we know relatively little about the factors that regulate algae in freshwater wetlands. This discrepancy is particularly acute in boreal regions, where wetlands are abundant and processes related to climate change (i.e. increased permafrost collapse and soil weathering) are expected to increase nutrient inputs into aquatic systems. To investigate how accelerated nutrient inputs might affect algal structure and function in northern boreal wetlands, we added nitrogen, phosphorus and silica to mesocosms in an oligotrophic marsh in interior Alaska. 2. We conducted two in situ mesocosm enrichment experiments during consecutive summer growing seasons, each lasting 24 days. In 2007, we investigated the effects of +N, +P, +Si and +N+P+Si enrichment on benthic algal biomass (chlorophyll‐a, ash‐free dry mass, biovolume), chemistry (N : P ratio) and community composition. In 2008, we expanded our first experiment to investigate the effects +N+P, +N+Si, +P+Si and +N+P+Si on the same algal parameters as well as productivity (mg C m^?2 h^?1). 3. In both experiments, we measured water‐column dissolved organic carbon (DOC) inside treatment enclosures and related changes in DOC to standing algal biomass. 4. Benthic algal accrual did not increase following 24 days of enrichment with any nutrient alone or with P and Si together (+P+Si), but increased significantly with the addition of N in any combination with P and Si (+N+P, +N+Si, +N+P+Si). 5. Algal productivity (20 mg C m^?2 h^?1) increased between three‐ and seven‐fold (57–127 mg C m^?2 h^?1) with the addition of N in combination with any other nutrient (+N+P, +N+Si, +N+P+Si). Water‐column DOC concentration was significantly higher inside N‐combination treatments compared to the control during each season, and DOC increased linearly with benthic algal biomass in 2007 (r² = 0.89, P < 0.0001) and 2008 (r² = 0.74, P < 0.0001). 6. Taxonomic composition of the wetland algal community responded most strongly to N‐combination treatments in both seasons. In 2007, there was a significant shift from Euglena and Mougeotia in the control treatment to Chroococcus and Gloeocystis with +N+P+Si enrichment, and in 2008, a Mougeotia‐dominated community was replaced by Gloeocystis in the +N+P treatment and by Nitzschia in +N+Si and +N+P+Si treatments. 7. Together, these data provide several lines of evidence for co‐limitation, and the central importance of N as a co‐limiting nutrient for the wetland algal community. Changes in algal dynamics with increased nutrient concentrations could have important implications for wetland food webs and suggest that algae may provide a functional link between increasing nutrient inputs and altered wetland carbon cycling in this region.     

Horizontal dinoflage11ate cyst distribution, sediment characteristics and benthic flux in Manila Bay, Philippines

The lateral variation of sediment properties and associated cyst content of sediment in Manila Bay were determined and their possible role/s in the occurrences of Pyrodinium bahamense Plate var. compressum (Bohm) Steidinger, Tester et Taylor toxic blooms were assessed. Manila Bay's surface sediment was determined to be silt dominated. Clay generally increased towards the coast, probably as a result of flocculation and rapid deposition upon entry of sediments from the rivers. High sand content characterized the southeastern part of the bay attributed to the greater sand inputs and relatively strong currents in this area. Bulk densities were lower in the eastern side of the bay from dilution by high organic load from sewage and urban areas. Benthic flux calculations, particularly NH3, suggest more than 50% nutrient contribution comes from sediments. In general, dinoflagellate cyst density increased from the center of the bay towards the coast, except in Pampanga Bay where it deceased near the coasts. A maximum of 23 dinoflagellate species were identified: 5 were autotrophic (Lingulodinium polyedrum (Stein) Dodge, Gonyaulax spp., Pyrophacus steinii (Schiller) Wall et Dale, Protoceratium reticulatum (Claparede et Lachmann) Butschli, and Pyrodinium bahamense var. compressum), and the rest were predominantly composed of Protoperidinium spp. and Diplopsalis spp. Heterotrophs comprised about 70% of the total cyst counts. Pyrodinium counts increased towards the northwestern part of the bay where it was the dominant autotroph species. Negative correlations were observed for live Pyrodinium cyst density and N flux, P flux, ratio of N to P and total organic carbon (TOC) content. However, areas with high N:P ratio contain abundant Pyrodinium live cysts.     

SHIFT FROM CHLOROPHYTES TO CYANOBACTERIA IN BENTHIC MACROALGAE ALONG A GRADIENT OF NITRATE DEPLETION1

A survey of the spatial distribution of benthic macroalgae in a fluvial lake of the St. Lawrence River (Lake Saint‐Pierre, Quebec, Canada) revealed a shift in composition from chlorophytes to cyanobacteria along the flow path of nutrient‐rich waters originating from tributaries draining farmlands. The link between this shift and changes in water quality characteristics was investigated by sampling at 10 sites along a 15 km transect. Conductivity, current, light extinction, total phosphorus (TP; >25 μg P · L^?1), and ammonium (8–21 μg N · L^?1) remained fairly constant along the transect in contrast to nitrate concentrations, which fell sharply. Filamentous and colonial chlorophytes [Cladophora sp. and Hydrodictyon reticulatum (L.) Bory] dominated in the first 5 km where nitrate concentrations were >240 μg N · L^?1. A mixed assemblage of chlorophytes and cyanobacteria characterized a 1 km transition zone where nitrate decreased to 40–80 μg N · L^?1. In the last section of the transect, nitrate concentrations dropped below 10 μg N · L^?1, and cyanobacteria (benthic filamentous mats of Lyngbya wollei Farl. ex Gomont and epiphytic colonies of Gloeotrichia) dominated the benthic community. The predominance of nitrogen‐fixing, potentially toxic cyanobacteria likely resulted from excessive nutrient loads and may affect nutrient and trophic dynamics in the river.     

Pigment Content of Selected Planktonic Algae in Response to Simulated Natural Light Fluctuations and a Short Photoperiod

The lipophilic photosynthetic pigments in Limnothrix redekei, Planktothrix agardhii (cyanobacteria), Stephanodiscus minutulus, Synedra acus (diatoms), Scenedesmus acuminatus, and Scenedesmus armatus (chlorophycean) all isolated from an eutrophic lake were quantitatively determined by HPLC. The algae were grown semi-continuously under nutrient sufficient conditions at 20°C at a 12/12 h light/dark cycle with constant irradiance or with simulated natural light fluctuations as well as at a 6/18 h light/dark cycle with constant irradiance, all at the same daily light exposure. The zeaxanthin and the myxoxanthophyll contents of cyanobacteria were not influenced by fluctuating light, a short photoperiod or a different sampling time. The chlorophyll b/a ratio, the lutein/chlorophyll a ratio, and the neoxanthin content of chlorophycean as well as the chlorophyll c/a and the fucoxanthin/chlorophyll a ratio of diatoms were only slightly influenced by these factors. Therefore in some cases marker pigment contents and in other cases marker pigment/chlorophyll a ratios may be more useful for quantifying the relative importance of different taxonomic groups in natural phytoplankton. Simulated natural light fluctuations or the length of the photoperiod only slightly influenced the pigment content or the marker pigment/chlorophyll a ratio.     

Pigments and fatty acids of marine raphidophytes: A chemotaxonomic re-evaluation

The patterns of occurrence of photosynthetic pigments and fatty acids among seven available species (11 strains) of marine raphidophytes were determined and used as chemotaxonomic markers. All currently recognized genera of marine raphidophytes were included for analysis: that is, Chattonella, Fibrocapsa, Heterosigma, Olisthodiscus and Haramonas. The characteristic pigment composition was shown to be chlorophyll a, chlorophylls c₁ and/or c₂, fucoxanthin as the major carot-enoid, β,β-carotene and any or all of zeaxanthin, violaxanthin and an auroxanthin-like pigment as the minor carotenoids. The carotenoid composition of all marine raphidophyte genera investigated was virtually the same, except in Fibrocapsa and Haramonas, which differed due to the occurrence of fucoxanthinol and 19′-butanoyloxyfucoxanthin, respectively. These fucoxanthin derivatives, in addition to fucoxanthin, have potential chemotaxonomic use for differentiating the two species. In all 11 strains, 15 fatty acids (saturated, mono-unsaturated and polyunsaturated) were determined. Significant taxonomic distinctions between genera were reflected by their fatty acid profiles. A rapid key for the differentiation of genera, in addition to morphological features, may be the absence of the 18:4 fatty acid in Olisthodiscus; presence of 18:5 in Heterosigma; the presence of fucoxanthinol in Fibrocapsa and presence of 19′-butanoyloxyfucoxanthin in Haramonas.     

THE EFFECT OF NITROGEN SOURCE ON THE GROWTH AND TOXICITY OF THREE POTENTIALLY HARMFUL DINOFLAGELLATES

Increases in population and agriculture in coastal areas can result in increased nutrient inputs and alterations in the ratios of organic to inorganic nutrients in coastal waters. Such changes in coastal nutrient regimes can affect phytoplankton community structure by creating conditions favorable for growth and dominance of algae that were not dominant before. The effect that changes in ratios and concentrations of nutrients have on toxicity of harmful algal species is not well known. There seems to be a relationship; however, between nutrient stress and toxin production among harmful phytoplankton producing low‐N toxins, e.g. Diarrhetic Shellfish Poisoning (DSP) toxins. Even less is known about the relationship between organic nutrient uptake and toxin production. Benthic species and species in coastal areas are probably exposed to greater fluxes of dissolved organic nitrogen (DON). In this study, benthic and planktonic species of Prorocentrum were grown on L1 media with the sole N‐source varying among treatments as nitrate, ammonium, urea, L‐glutamic acid, and high molecular weight natural DON. An ELISA specific to the DSP toxins, okadaic acid and 35‐methylokadaic acid, was used to determine toxin production by each species when grown on the different N sources. Preliminary results indicate that some organic forms of N support growth as well as inorganic forms for Prorocentrum minimum, P. mexicanum, and P. hoffmannianum.