COMPARISON OF IN SITU PHOTOSYNTHETIC ACTIVITY OF EPIPHYTIC,EPIPELIC AND PLANKTONIC ALGAL COMMUNITIES IN AN OLIGOTROPHIC LAKE,SOUTHERN FINLAND1

The photosynthetic activity of different algal communities at the outer edge of an Equisetum fluviatile L. stand in an oligotrophic lake (Pääjärvi, in southern Finland) was investigated. Production by the algal communities was measured simultaneously using a modified ¹⁴C-method, and the results were related to the volume of algae and the available irradiance. The relative production rate (P/B quotient) of phytoplankton was ca. 3 × that of epiphyton and ca. 20 × that of epipelon. Epiphyton productivity remained almost constant although the algal volume varied greatly, suggesting that the surface layer of the algal community was mainly responsible for the photosynthetic activity. In the littoral area (at 1 m depth) primary production/m² of lake surface by phytoplankton, epiphyton and epipelon was similar but in the littoriprofundal area (2–4 m) phytoplankton production was twice that of epipelon. Primary productivity of epiphyton and epipelon/m² of substratum was about equal to phytoplankton productivity/m³ of water at the same irradiance. This relation provided a means of estimating the relative contributions of the different algal communities to the total algal production in the lake.     

PERIPHYTIC DIATOM ASSEMBLAGES FROM BATHURST ISLAND, NUNAVUT, CANADIAN HIGH ARCTIC: AN EXAMINATION OF COMMUNITY RELATIONSHIPS AND HABITAT PREFERENCES

Diatoms are potentially the most important biomonitors of environmental change in high arctic lakes and ponds, but to date few autecological data are available. Because of the shallow nature of many of these water bodies, a large proportion of taxa are periphytic and planktonic diatoms are absent for the most part. By determining the microhabitat and substrate preferences of these benthic diatom taxa, the potential exists to infer past changes in available habitats from fossil diatom assemblages collected from sediment cores and ultimately to reconstruct past environmental and climatic changes responsible for these shifts in habitat availability. To refine our understanding of high arctic diatom habitat preference, the common diatom taxa found on submerged moss (bryophyte), sediment, and rock substrates from lakes and ponds on Bathurst Island, Nunavut, Canadian High Arctic were examined. The relationships among key limnological variables and the common taxa from each habitat were examined. Many diatom taxa exhibited varying degrees of microhabitat preference, with moss representing the more unique habitat. In addition, the following limnological variables significantly ( P ≤ 0.05) explained the species variance for each of the three substrates: Na ⁺ and total nitrogen for moss; total phosphorus (filtered) and pH for rock; and Fe^{3 +} , total phosphorus (unfiltered), total nitrogen, temperature, and pH for sediment. These data can be used to help interpret monitoring and paleolimnological studies in this environmentally sensitive region.     

CHARACTERIZING AND QUANTIFYING PHOTOINHIBITION IN INTERTIDAL MICROPHYTOBENTHOS1

This study characterizes the short‐term influence of the sustained saturating irradiance encountered by the microphytobenthos inhabiting intertidal mudflats. The kinetics of photoinhibition in epipelic microalgae from intertidal mudflats were investigated in the laboratory. Previously isolated benthic microalgae were exposed to a saturating photon flux density (PFD) for periods ranging from 0 to 180 min; every 30 min, a photosynthesis‐irradiance curve was established to quantify the effect of the saturating PFD on both parameters α^B, the photosynthetic efficiency, and P_m^B, the photosynthetic capacity. The α^B decreased from the beginning of light exposure until the end, whereas P_m^B first slightly increased and then diminished from 90 min exposure onward. It turned out that epipelic microphytobenthos undergoes photoinhibition after about 90 min of saturating PFD. The possible ecological consequences of these ecophysiological results are discussed.     

ENVIRONMENTAL EFFECTS ON EXOPOLYMER PRODUCTION BY MARINE BENTHIC DIATOMS: DYNAMICS,CHANGES IN COMPOSITION,AND PATHWAYS OF PRODUCTION1

Marine benthic diatoms excrete large quantities of extracellular polymeric substances (EPS), both as a function of their motility system and as a response to environmental conditions. Diatom EPS consists predominantly of carbohydrate‐rich polymers and is important in the ecology of cells living on marine sediments. Production rates, production pathways, and monosaccharide composition of water‐soluble (colloidal) carbohydrates, EPS, and intracellular storage carbohydrate (glucans) were investigated in the epipelic (mud‐inhabiting) diatoms Cylindrotheca closterium (Ehrenburg), Navicula perminta (Grün.) in Van Heurck, and Amphora exigua Greg. under a range of experimental conditions simulating aspects of the natural environment. Cellular rates of colloidal carbohydrate, EPS, and glucan production were significantly higher during nutrient‐replete compared with nutrient‐limited growth for all three species. The proportion of EPS in the extracellular carbohydrate pool increased significantly (to 44%–69%) as cells became nutrient limited. Cylindrotheca closterium produced two types of EPS differing in sugar composition and production patterns. Nutrient‐replete cells produced a complex EPS containing rhamnose, fucose, xylose, mannose, galactose, glucose, and uronic acids. Nutrient‐limited cells produced an additional EPS containing mannose, galactose, glucose, and uronic acids. Both EPS types were produced under illuminated and darkened conditions. ¹⁴C‐labeling revealed immediate production of ¹⁴C‐glucan and significant increases in ¹⁴C‐EPS between 3 and 4 h after addition of label. The glucan synthesis inhibitor 2,6‐dichlorobenzonitrile significantly reduced ¹⁴C‐colloidal carbohydrate and ¹⁴C‐EPS. The glucanase inhibitor P‐nitrophenyl β‐d ‐glucopyranoside resulted in accumulation of glucan within cells and lowered rates of ¹⁴C‐colloidal and ¹⁴C‐EPS production. Cycloheximide prevented glucan catabolism, but glucan production and EPS synthesis were unaffected.     

The Epipelic Algal Flora of the River Wye System,Wales, U. K. 2. Algal Phyla and Species Population Dynamics

The Bacillariophyta dominated over the other phyla and were mainly recorded in high densities during summer and autumn. The Chlorophyta and Myxophyta (Cyanobacteria) were represented by coccoid forms and by non-heterocystous, filamentous forms, respectively. Both were mainly recorded during summer and mostly absent during winter. Other phyla were occasionally recorded in low densities; the Chrysophyta being found in the River Elan and at one site on the River Wye during June 1980. Their members were considered as “contaminants” or “fall-out” from other communities. Pennate diatoms were the most “constant” species and either showed a general upstream or downstream increase or a general distribution throughout the study area. Populations of the same species colonising both sediments and stones were not correlated or insignificantly correlated with each other at most stations. It was concluded that sediments were unsuitable for algal colonisation. The River Elan and upper Wye were rated as oligosaprobic, the River Ithon and lower Wye as β-mesosaprobic and the River Lugg as α-mesosaprobic.     

The Epipelic Algal Flora of the River Wye System,Wales, U.K. 1. Productivity and Total Biomass Dynamics

The spatial and temporal fluctuations of the phytopigment content, “potential” primary productivity and total biomass of the epipelic algae of the River Wye System were studied during June 1979 to May 1981. Chlorophyll-a and productivity values showed a downstream increase, much less obvious for the total biomass. Phaeophytin-a values followed almost similar spatial and temporal fluctuations to those of chlorophyll-a. High chlorophyll-a productivity and total biomass values were recorded during warmer months due to favourable environmental conditions for algal growth, but lower values during unfavourable winter and flood periods.     

Epipelic diatoms blooming in Isahaya Tidal Flat in the Ariake Sea, Japan, before the drainage following the Isahaya-Bay Reclamation Project

A blooming diatom sample from Isahaya Tidal Flat in the Ariake Sea, south‐west Japan, before the bay was closed and drained following the Isahaya‐Bay Reclamation Project, is taxonomically examined. When the sample was collected on 25 May 1996, the area was a muddy tidal flat. Altogether, 103 diatom species were observed within 1000 counts of frustules. Both the dominant and subdominant diatom species were large raphid species. Light microscopy and scanning electron microscopy observation revealed that the dominant species was Haslea nipkowii (F.Meister) Poulin et G. Masse, which was originally described as a species of Gymsigma. So far, a report of correctly identified H. nipkowii has been presented only from China, because H. nipkowii sensu Poulin et al. from USA and France probably belongs to the other taxon, Gymsigma pallidum Riznyk. The subdominant species is described as a new species, Nitzschia gyrosigmasp. nov. This new species is similar to Nitzschia sigmoidea under light microscopy, but scanning electron microscopy observation reveals that their fine structures are quite different. Nitzschia granulata var. hyalina is transferred to the genus of Tryblionella and raised to an independent species, Tryblionella hyalina (Amosse) comb. nov.     

Responses of estuarine intertidal microphytobenthic algal assemblages to enhanced ultraviolet B radiation

As a result of ozone depletion, ground doses of ultraviolet B (UVB) radiation in the mid latitudes of the Northern Hemisphere have increased since the 1980s, and current predictions indicate no possible alleviation until at least post 2020. Mudflats and sandflats are important coastal-zone habitats, and support extensive biofilms of benthic microalgae (microphytobenthos). In intertidal situations, these assemblages are exposed to high levels of UVB radiation during periods of tidal exposure. Exposure of intertidal biofilms dominated by epipelic (mud-inhabiting) diatoms to 0, 0.18 or 0.35 W m⁻² UVB radiation for between 4 and 10 days resulted in no significant decreases in the maximum PSII quantum efficiency (F_v/F_m) throughout diel exposure periods. Although the quantum efficiency of electron transport (F_q′/F_m′) showed significant reductions early in some experiments, the major response was an increase in F_q′/F_m′ in UVB exposed biofilms. This increase in F_q′/F_m′ was suggestive of a protective vertical migration down into the sediment. Single-cell and whole biofilm fluorescence imaging demonstrated, for the first time, that motile diatoms are able to detect UVB radiation independently of UVA or photosynthetically active radiation (PAR) and migrate rapidly down (within 15 min) into the sediments to avoid it. This behavioural acclimation mechanism appears to prevent significant accumulation of UVB induced damage to the algae. UVB exposure had no significant effect of biofilm photosynthesis (measured by ¹⁴C carbon fixation), but did alter organic carbon allocation patterns, with significantly less new carbon allocated to intracellular storage (glucan) and extracellular colloidal carbohydrate fractions. Significant reductions in the sediment standing stocks of chlorophyll a (Chl a), colloidal carbohydrates, extracellular polymeric substances (EPS) were seen after 7 days of UVB exposure. This study showed that marine intertidal benthic diatoms use a behavioural strategy to avoid exposure to UVB and that this response is effective as a short-term protection mechanism against UVB damage. However, altered carbon allocation patterns feed forward over time into changes in biofilm biomass and sediment carbohydrate dynamics. This suggests that continual long-term exposure to UVB may impact on sediment carbon cycling and trophic interactions and on the stabilization of sediments by microalgal biofilms through their production of extracellular carbohydrates.     

EFFECTS OF LIGHT AND SUBSTRATE ON THE BENTHIC DIATOMS IN AN OLIGOTROPHIC LAKE: A COMPARISON BETWEEN NATURAL AND ARTIFICIAL SUBSTRATES1

Benthic diatoms form a particularly important community in oligotrophic lakes, but factors influencing their distribution are not well known. This study reports the depth distribution of living motile and total diatoms (living plus dead diatoms) on both natural (from sand to fine organic mud) and artificial substrates in an oligotrophic lake. On artificial substrates, motile diatom densities peaked in abundance (24–30 cells · mm^?2) between 0.6 and 1.9 m depth; on natural sediment surfaces, motile diatoms were generally more numerous and peaked in abundance (925 cells · mm^?2) at 1.3 m depth. Total diatom densities on artificial substrates were highest (1260 valves · mm^?2) at 0.6 m depth, with very low values below 3 m depth; on natural sediment surfaces, total diatom abundances were generally much higher (21600 valves · mm^?2) at 3 m depth and declined gradually with depth. Significant relationships were found between light and diatom densities on the artificial substrate. Ordination analysis indicated that substrate type significantly correlated with the variation of diatom composition on artificial and natural substrates. Our results suggest that in oligotrophic lakes, light influences benthic diatom abundance, whereas substrate type has more influence on benthic diatom composition.     

The production of extracellular carbohydrates by estuarine benthic diatoms: the effects of growth phase and light and dark treatment

Epipelic diatoms are important constituents of estuarine microphytobenthic biofilms. Field‐based investigations have shown that the production of carbohydrates by such taxa is ecologically important. However, limited information exists on the dynamics of carbohydrate production by individual species of epipelic diatoms. The production of low and high molecular weight extracellular carbohydrates in axenic cultures of five species of benthic estuarine diatoms, Cylindrotheca closterium (Ehrenberg), Navicula perminuta (Grun.) in Van Heurck, Nitzschia frustulum (Kütz.) Grunow, Nitzschia sigma (Kütz.) Grunow, and Surirella ovata (Kütz.) Grunow, were investigated. All species produced colloidal (water‐soluble) carbohydrates during growth, with maximal production occurring during stationary phase. During logarithmic growth, approximately 20% of extracellular carbohydrates consisted of polymeric material (extracellular polymeric substances [EPS]), but during stationary phase, EPS content increased to 34%–50%. Pyrolysis–mass spectrophotometry analysis showed differences in the composition of EPS produced during logarithmic and stationary phase. All species synthesized glucan as a storage carbohydrate, with maximum glucan accumulation during the transition from log to stationary phase. Short‐term labeling with ¹⁴C‐bicarbonate found that between 30 and 60% of photoassimilates were released as colloidal carbohydrate, with EPS consisting of approximately 16% of this colloidal fraction. When cells were placed in darkness, EPS production increased, and between 85 and 99% of extracellular carbohydrate produced was polymeric. Glucan reserves were utilized in dark conditions, with significant negative correlations between EPS and glucan for N. perminuta and S. ovata. Under dark conditions, cells continued to produce EPS for up to 3 days, although release of low molecular weight carbohydrates rapidly ceased when cells were dark treated. Three aspects of EPS production have been identified during this investigation: (1) production during rapid growth, which differs in composition from (2) EPS directly produced as a result of photosynthetic overflow during growth limiting conditions and (3) EPS produced for up to 3 days in the dark using intracellular storage reserves (glucans). The ecological implications of these patterns of production and utilization are discussed.