The pH regime sediments underlying acidified waters

The pH of sediments underlying acidified lake waters does not necessarily reflect the acidification of the water. Profiles of sediments in Lake Anna, Virginia showed interstitial pH values between 6.0 and 7.0 within the top 4 cm, even though the sediments are constantly exposed to overlying waters with pH values as low as 3.5. The amount of acidity neutralized by sediment processes is 2 orders of magnitude greater than previously reported observations. The results indicate that caution must be used in drawing conclusions about sediments biogeochemical processes based on the pH of overlying waters. This article originally appeared in an incorrect form in Biogeochemistry, Vol. 2, No. 1., pages 95–99     

Seasonal changes in the digestive enzyme levels of the amphipod Corophium volutator (Pallas) in relation to diet

The diet of the amphipod Corophium volutator (Pallas) living on extensive intertidal mud flats in the Bay of Fundy, eastern Canada, was investigated by direct and indirect methods. Microscopic observation of gut contents revealed macrophyte detritus but few diatom frustules. Laboratory cultures showed good survival on a diet of benthic diatoms, while on a diet of Spartina detritus survival was poor, although significantly better than the survival of starved animals. The ratios of gut enzyme activities, amylase to protease and amylase to laminarinase, changed markedly with the season. In summer the ratio was similar to that of animals cultured on a diet of benthic diatoms; in spring it resembled that of animals cultured on Spartina detritus, while in winter it was similar to that of starved animals. It is concluded that in summer, rapid growth is made possible by feeding on benthic diatoms, while Spartina detritus is an inferior food source which aids survival at other times of the year.     

The visual pigment sensitivity hypothesis: further evidence from fishes of varying habitats

Summary Visual pigments were extracted from the retinas of 8 species of marine teleosts and 4 species of elasmobranchs and a comparison was made of the pigment properties from these fishes, some inhabiting surface waters, others from the mesopelagic zone, and a few migrating vertically between these two environments. An association was found between the spectral position of the absorbance curve and the habitat depth or habitat behavior, with the blue-shifted chrysopsins being the pigments of the twilight zone fishes and the rhodopsins with fishes living near the surface. The retina of the swell shark (Cephaloscyllium ventriosum) yielded extracts with two photopigments; one, a rhodopsin at 498 nm; the second, a chrysopsin at 478 nm. This fish has been reported to practice seasonal vertical migrations between the surface and the mesopelagic waters. In addition to the spectral absorbance, several properties of these visual pigments were examined, including the meta-III product of photic bleaching, regeneration with added 11-cis and 9-cis retinals, and the chromophoric photosensitivity. The chrysopsin properties were found to be fundamentally similar to those of typical vertebrate rhodopsins. Correlating the spectral data with the habitat and habitat behavior of our fishes gives us confidence in the idea that the scotopic pigments have evolved as adaptations to those aspects of their color environment that are critical to the survival of the species.     

Long-term survival of marine planktonic diatoms and dinoflagellates in stored sediment samples

Sediment samples from Scottish coastal sites, taken over the last 9 years, were stored in closed containers at 5C. Slurry cultures were used to determine the survival of phytoplankton in these sediments. A range of diatom and dinoflagellate species survived for at least 27 months in these stored samples. A number of species grew for which no resting stage has yet been described: Thalassiosira angulata, T.pacifica, T.punctigera, T.eccentrica, T.minima and T.anguste-lineata. Notable results were survival times of 73 months for Skeletonema costatum, 96 months for Chaetoceros socialis, C.didymus and C.diadema, 109 months for Scrippsiella sp. and 112 months for Lingulodinium polyedrum. A single sample was stored and repeatedly cultured for diatoms over a period of 16 months. The number of species cultured from the sediment declined over this time. Lingulodinium polyedrum cysts isolated from sediments collected at least 18 months previously gave a hatching success of 97%, and cysts isolated from a 9-year-old sample gave a hatching success of 3%. The study indicated the potential importance of coastal sediments as a source of phytoplankton to their overlying waters. The validity of using marine planktonic diatoms and dinoflagellates for modelling geological events is discussed.      

The pH regime of sediments underlying acidified waters

The pH of sediments underlying acidified lake waters does not necessarily reflect the acidification of the water. Profiles of sediments in Lake Anna, Virginia showed interstitial pH values between 6.0 and 7.0 within the top 4 cm, even though the sediments are constantly exposed to overlying waters with pH values as low as 3.5. The amount of acidity neutralized by sediment processes is 2 orders of magnitude greater than previously reported observations. The results indicate that caution must be used in drawing conclusions about sediment biogeochemical processes based on the pH of overlying waters.     

THE ECOLOGY OF EPIPELIC ALGAE OF FIVE WELSH LAKES,WITH SPECIAL REFERENCE TO VOLVOCALEAN GREEN FLAGELLATES (CHLOROPHYCEAE)1

The ecology of epipelic algae on the marginal sediments of five Welsh lakes was studied over an annual cycle. The lakes, Llydaw, Cwellyn, Padarn, Maelog and Coron ranged from very oligotrophic to nutrient-rich. Attention was focussed on chlamydomonad flagellates, diatoms, blue-green algae and euglenoids and the different proportions of these in algae in the epipelon of lakes of contrasting water quality. A total of 75 algal taxa was found in the five lakes, 25 were species of volvocalean flagellates. Mean annual population density of these flagellates differed by an order of magnitude between the lakes. The greatest population density was recorded for Chlamydomonas anticontata Schiller in nutrient-rich Llyn Maelog. Twenty species of pennate diatoms were recorded frequently in the epipelon. In the nutrient-rich lakes, Maelog and Coron, pennate diatoms were dominant on the sediments, where they exhibited population maxima in spring and autumn. Increase in numbers of epipelic diatoms was recorded when silica concentrations were minimum in the overlying lake waters. Navicula hungarica Grun. achieved the maximum population density, 260 000 cells · cm^?2. Euglenoids formed large epipelic populations during late-summer and autumn in these nutrient-rich lakes. Blue-green algae were more important, proportionally, in the nutrient-poor mountain lakes, which had sediments of higher organic content. Chlamydomonads were the major algal component of the epipelon in the mountain lakes, Llydaw and Cwellyn, where the sediments were characterized by larger particle size, and higher organic content. In the nutrient-rich lakes, where the sediments had higher calcium content, chlamydomonads formed significant populations only during spring and summer, when nutrient levels were minimal in the overlying lakewaters.     

Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications

Diatoms are a major group of phytoplankton that account for approximately 40% of the ocean carbon fixation and the vast majority of biogenic silica production through the construction of their cell walls (termed frustules). These frustules accumulate and are partially preserved in the ocean sediments. Diatom growth and nutrient utilization in high‐nitrate, low‐chlorophyll regions of the world’s oceans are mostly regulated by iron availability. Diatoms acclimate to iron limitation by decreasing cell size. The associated increase in surface area‐to‐volume ratio and decrease in diffusive boundary layer thickness may improve nutrient uptake kinetics. In parallel, cellular silicon (Si) contents are elevated in iron‐limited diatoms relative to nitrogen (N) and carbon (C). Variations in degree of silicification and nutritional requirements of iron‐limited diatoms have been hypothesized to account for higher cellular Si and/or lower cellular N and C, respectively. However, in some diatoms, frustule silicification does not significantly change when cells are iron‐limited. Instead, changes in the Si‐containing valve surface area relative to volume within these diatoms is hypothesized to be responsible for the variations in the cellular Si : N and Si : C ratios. In particular, some examined iron‐limited pennate diatoms have reduced widths relative to their lengths (i.e. lower length‐normalized widths, LNW) compared to iron‐replete cells. In the pennate diatom Fragilariopsis kerguelensis, the mean LNWs of valves preserved in sediments throughout the Southern Ocean (a well‐characterized iron‐limited region) is positively correlated with satellite‐derived, climatological net primary productivity in the overlying waters. Because of the specific morphological changes in pennate diatom frustules in response to iron availability, the valve morphometerics (e.g. LNWs) can potentially be used as a diagnostic tool for iron‐limited diatom growth and relative changes in the Si : N (and Si : C) ratios in extant diatom assemblages as well as those preserved in the sediments.     

Does pigment composition reflect phytoplankton community structure in differing temperature and light conditions in a deep alpine lake? An approach using HPLC and delayed fluorescence techniques1

In vivo delayed fluorescence (DF) and HPLC/CHEMTAX pigment analyses were used to investigate seasonal and depth distributions of phytoplankton in a deep alpine mesotrophic lake, Mondsee (Austria). Using chl a equivalents, we determined significant relationships with both approaches. Community structure derived from pigment ratios of homogenous samples was compared with microscopic estimations using biovolume conversion factors. An advantage of the HPLC/CHEMTAX method was that it gave good discrimination among phytoplankton groups when based on a pigment ratio matrix derived from multiple regression analysis. When a single algal group was dominant, such as epilimnetic diatoms or hypolimnetic cyanobacteria in the deep chl maxima, HPLC/CHEMTAX results were significantly correlated with microscopic estimations (diatoms: r = 0.93; cyanobacteria: r = 0.94). Changes in the composition of photosynthetically active pigments were investigated with DF and benefited from excitation spectra that considered all light‐harvesting pigments, which made it possible to assess the enhancement of accessory photosynthetically active pigments relative to active chl a (chl a_DF672). Changes in similarity index, based on normalized DF spectra, confirmed compositional shifts observed by microscopy. At chosen wavelengths of DF spectra, 534 and 586 nm, we generally observed a significantly inverse relationship between normalized DF intensities and temperature and light along both seasonal and depth gradients. The relative increase in photosynthetically active pigments other than chl a_DF672 under low light and temperature was caused by an increasing dominance of diatoms and/or phycobilin‐rich cyanobacteria and Cryptophyta. DF spectra provided a more accurate picture of community pigments acclimated to light and temperature conditions than the β‐carotene:chl a ratio derived from HPLC.     

Molecular Phylogenetic Analysis of Archaea and Bacteria in Wind Cave,South Dakota

The diversity of bacteria and archaea was characterized from sediments collected from Wind Cave located in Wind Cave National Park in the Black Hills of South Dakota. Wind Cave is a limestone dissolution cave with strata that started forming over 300 million years ago, making it one of the oldest in the world. Previous work suggested that the cave was largely a detritus based system ultimately dependent upon allochthonous energy and carbon from photosynthesis of the overlying vegetation, and algae growing near lights along the tour routes. In this work, we used a molecular phylogenetic approach to characterize the microbial structure and infer a corresponding ecosystem function where appropriate. Four bacterial divisions and subdivisions were found in the culture collection, which represented 14 phylotypes, whereas 12 divisions and subdivisions were identified in the clonal analysis comprising 49 phylotypes. The predominant groups were the γ-Proteobacteria and Acidobacteria. Although a few of the clones resembled sequences from other cave and subterranean systems, no cave-specific bacterial community was evident in this work. Archaeal phylotypes (20 Crenarchaeota and 2 Euryarchaeota) were detected, with a large proportion of the Crenarchaeota resembling sequences from a South African gold mine. One archaeal cluster in particular appears to be specific to the subterranean environment. Most of the microbial sequences were not related to known chemolithoautotrophs, therefore we conclude that this particular community is likely detritus based where allochthonous energy and carbon are transported into the cave by infiltrating waters.     

Predicting Epipelic Diatom Exopolymer Concentrations in Intertidal Sediments from Sediment Chlorophyll a

Abstract In many intertidal cohesive—sediment habitats, epipelic diatoms are the dominant microphytobenthic organisms. In such sediments, concentrations of colloidal carbohydrate [including the exopolymeric substances (EPS) produced by diatoms during motility] are closely correlated with the biomass (chlorophyll a) of epipelic diatoms. A model describing this relationship (log (conc. coll. carbo. + 1) = 1.40 + 1.02(log (chl. a conc. + 1)) was derived from published data. It was validated against published and unpublished data from 6 different estuaries, and accounted for 64.6% of the variation in sediment colloidal carbohydrate concentrations. The model was valid for intertidal habitats with cohesive sediments where epipelic diatoms constituted >50% of the microphytobenthic assemblage. In sites with noncohesive sediments, or where the microphytobenthic assemblage was dominated by other algal groups, the model was not applicable. The mean percentage of EPS in colloidal carbohydrate extracts varied between 11 and 37% for axenic cultures of epipelic diatoms (with higher values obtained during stationary phase), and between 22.7% and 24.3% for natural sediments dominated by epipelic diatoms. Assuming an EPS percentage of 25% in colloidal extracts yielded an EPS chl. a ratio of 2.62:1. Maximum rates of EPS production in diatom cultures occurred at the beginning of stationary phase (1.6–5.09 μg EPS μg⁻¹ chl a d⁻¹), with Nitzschia sigma having a significantly (P < 0.05) higher rate of production than N. frustulum, Navicula perminuta and Surirella ovata. Similar rates of EPS production were measured in the field. The dynamics of EPS production and loss on mudflats is discussed, with reference to the model and these production rates. Received: 25 February 1997; Accepted: 23 May 1997     

Visual predation during springtime foraging of the North Atlantic right whale (Eubalaena glacialis)

To assess the role that vision plays in the ability of the North Atlantic right whale (Eubalaena glacialis) to detect its primary prey species, the calanoid copepod Calanus finmarchicus, we have compared the absorbance spectrum of the E. glacialis rod visual pigment, the transmittance spectra of C. finmarchicus carotenoid pigments, as well as the downwelling irradiance and horizontal radiance spectra collected during springtime at three locations in the western Gulf of Maine. The E. glacialis rod visual pigment absorbs light maximally at 493 nm, while microspectrophotometric measurements of the C. finmarchicus carotenoid pigments reveal transmission spectra with minima matching very well with the E. glacialis rod visual pigment absorbance spectra maximum. Springtime spectral downwelling irradiance and horizontal radiance values from the surface waters of Cape Cod Bay and at all depths in Great South Channel overlap the E. glacialis rod absorbance spectrum, allowing C. finmarchicus to appear as a high‐contrast dark silhouette against a bright background spacelight, thus facilitating visually guided contrast foraging. In contrast, spectral downwelling irradiance and horizontal radiance at depth in Cape Cod Bay, and all depths in Wilkinson Basin, do not overlap the E. glacialis rod absorbance spectrum, providing little if any useful light for contrast vision.     

Action spectra of microalgal photosynthesis and depth distribution of spectral scalar irradiance in a coastal marine sediment of Limfjorden, Denmark

Abstract The role of complementary spectral utilization of light for the zonation of different groups of oxygenic phototrophic organisms in sediments was studied. The marine sediment was covered by a dense population of diatoms with an underlying population of cyanobacteria. Action spectra for photosynthesis and spectral scalar irradiance, E ₀, were measured directly in the sediment at a spatial resolution of 0.1 mm by the use of oxygen and light microsensors. The action spectrum for the diatoms was similar to the attenuation spectrum of the scalar irradiance, K ₀, in the diatom layer with Chl. a and carotenoids being the major photosynthetic pigments. The action spectrum of the cyanobacteria showed photosynthesis maxima at the absorption regions of Chl. a and phycocyanin. The measured depth distribution of spectral scalar irradiance and the action spectra of diatoms and cyanobacteria were used to calculate the spectral quality for photosynthesis of the 400–700 nm light to which the two populations were exposed. This spectral quality was compared to that of the light incident on the sediment surface. Due to preferential extinction of wavelengths, at which their photosynthetically active pigments had maximal absorption, the relative light quality for diatoms was reduced to 85% of the quality of incident light at a similar total quantum flux. This effect was partly due to spectral alterations of light backscattered from the underlying sediment with cyanobacteria. The cyanobacteria at the bottom of the euphotic zone, in contrast, experienced a light spectrum which was favorably altered, to 107% in quality, due to absorption by the overlying diatoms. It was concluded that these changes in spectral light quality can be considered as only one of more factors explaining the zonation of the two phototrophic populations, and that total light intensity and the chemical microenvironment are probably more important factors.     

An improved method for estimating R-phycoerythrin and R-phycocyanin contents from crude aqueous extracts of <Emphasis Type="Italic">Porphyra</Emphasis> (Bangiales,Rhodophyta)

One frequently-cited method for determining phycoerythrin (PE) and phycocyanin (PC) contents from crude aqueous extracts of red seaweeds utilizes peaks and troughs of absorbance spectra. The trough absorbance values are used to establish a linear or logarithmic baseline attributable to background scatter of particulate cellular debris not removed by centrifugation. Pigment contents are calculated by subtracting baseline values from PE and PC absorbance peaks. The baseline correction is intended to make the method independent of centrifugation time and/or speed. However, when crude extracts of Porphyra were analyzed using this protocol, R-PE and R-PC estimates were significantly affected by centrifugation time, suggesting that the method was not reliable for the genus. The present study has shown that with sufficient centrifugation, background scatter in Porphyra extracts can be removed, the remaining spectrum representing the overlapping absorbance peaks of water-soluble pigments in the extract. Using fourth derivative analysis of Porphyra extract absorbance spectra, peaks corresponding to chlorophyll, R-PE, R-PC, and allophycocyanin (APC) were identified. Dilute solutions of purified R-PE, R-PC and chlorophyll were scanned separately to identify spectral overlaps and develop new equations for phycobilin quantification. The new equations were used to estimate R-PE and R-PC contents of Porphyra extracts and purified R-PE, R-PC and chlorophyll solutions were mixed according to concentrations corresponding to the sample estimates. Absorbances and fourth derivative spectra of the sample extract and purified pigment mixtures were compared and found to coincide. The newly derived equations are more accurate for determining R-PE and R-PC of Porphyra than previously published methods.     

Action spectra of microalgal photosynthesis and depth distribution of spectral scalar irradiance in a coastal marine sediment of Limfjorden, Denmark

The role of complementary spectral utilization of light for the zonation of different groups of oxygenic phototrophic organisms in sediments was studied. The marine sediment was covered by a dense population of diatoms with an underlying population of cyanobacteria. Action spectra for photosynthesis and spectral scalar irradiance, E₀, were measured directly in the sediment at a spatial resolution of 0.1 mm by the use of oxygen and light microsensors. The action spectrum for the diatoms was similar to the attenuation spectrum of the scalar irradiance, K₀, in the diatom layer with Chl.a. and carotenoids being the major photosynthetic pigments. The action spectrum of the cyanobacteria showed photosynthesis maxima at the absorption regions of Chl.a. and phycocyanin. The measured depth distribution of spectral scalar irradiance and the action spectra of diatoms and cyanobacteria were used to calculate the spectral quality for photosynthesis of the 400–700 nm light to which the two populations were exposed. This spectral quality was compared to that of the light incident on the sediment surface. Due to preferential extinction of wavelengths, at which their photosynthetically active pigments had maximal absorption, the relative light quality for diatoms was reduced to 85% of the quality of d incident light at a similar total quantum flux. This effect was partly due to spectral alterations of light backscattered from the underlying sediment with cyanobacteria. The cyanobacteria at the bottom of the euphotic zone, in contrast, experienced a light spectrum which was favorably altered, to 10% in quality, due to absorption by the overlying diatoms. It was concluded that these changes in spectral light quality can be considered as only one of more factors explaining the zonation of the two phototrophic populations, and that total light intensity and the chemical microenvironment are probably more important factors.     

On-line determination of pigment composition and biomass in cultures of microalgae

Summary Rhodomonas sp. was grown in a photo-bioreactor equipped with a measuring cell in a spectrophotometer as part of an external flow loop. The apparent absorbance from 400 to 800 nm of the cell suspension was recorded at predetermined intervals and stored in a computer. From the spectra, the biomass and the concentrations of the two pigments chlorophyll a and phycoerythrin were determined in nitrogen-limited batch cultures.     

Paleopigment evidence of competition between phytoplankton and a cyanobacterial algal mat in a meromictic lake near Toronto,Ontario Canada

Crawford Lake, a meromictic lake located near Toronto, Canada, was cored to determine if algal pigments preserved in its sediments would make it possible to infer past changes in lake productivity over the last five hundred years. From 1500 to 1910 A.D. the sediments display extremely high levels of oscillaxanthin and myxoxanthophyll while chlorophyll derivatives and total carotenoids were relatively low. As the lake became increasingly more eutrophic in the latter part of the twentieth century, this relationship reversed itself. Competition for light between the deep dwelling cyanobacteria in the algal mat on the lake's bottom (8–14 m) and phytoplankton in the overlying surface layers of the water column (5–7 m) was attributed to the observed reduction in oscillaxanthin and myxoxanthophyll as Crawford Lake eutrophied. Because the major cyanobacteria in Crawford Lake are benthic mat forming Lyngbya and Oscillatoria, and not phytoplankton, competition for light with the overlying phytoplankton is critical in determining the total quantity of oscillaxanthin and myxoxanthophyll preserved in the lake's profundal sediments. These findings have major implications for the use of cyanobacterial pigments as indicators of lake trophic status in lakes where benthic algal mats are present.     

Microscale vertical distribution of algal and bacterial plankton in Lake Vechten (The Netherlands)

Abstract Depth profiles of horizontal light-beam attenuation and in situ fluorescence recorded at 1 to 2 cm depth intervals in Lake Vechten contained pronounced maxima related to phytoplankton and bacterioplankton layers. This was confirmed by microscopy and HPLC analysis of pigments in discrete samples. Algae were sparse throughout the oxic water column compared with green and purple sulphur bacteria in the anoxic zone of the lake, but plankton layers comprising diatoms, green algae, euglenophytes and cryptomonads were observed. Maximum concentrations of phototrophic bacteria, with dominant representatives Synechococcus, Thiopedia, Chloronema and Chlorobium , occurred very close to one another and within the anoxic zone at depths of pigment maxima and major fluorescence peaks. Plankton layers affected downwelling irradiance in two ways: first, by markedly increasing overall light extinction in the vicinity of the layers, e.g. between 4 m and 8 m the minimum wavelength-specific extinction coefficient, which was at 580 nm ( k ₅₈₀) increased from 0.5 m⁻¹ to 1.5 m⁻¹; secondly, changes in the shape of extinction ( k ) spectra corresponded to in vivo absorbance characteristics of photosynthetic pigments of plankton in the layers. It is inferred that field microbial ecological studies may greatly benefit from simultaneous use of in situ optical techniques.     

GROWTH OF VITAMIN B12-REQUIRING MARINE DIATOMS IN MIXED LABORATORY CULTURES WITH VITAMIN B12-PRODUCING MARINE BACTERIA12

The vitamin B₁₂ requirement of several marine diatoms can be satisfied in B₁₂^?limited laboratory cultures by heterotrophic marine bacteria isolated from the same waters and from sediments. The bacteria can utilize diatom excretory products, or the remains of dead diatom cells, in the production of the vitamin. The growth of 12 B₁₂¹? requiring diatoms (7 genera) in mixed cultures with 14 different bacteria (without added B₁₂) was compared to the growth of those same diatoms in axenic cultures with excess added B₁₂. Diatom growth was generally rapid in the first few days, followed by sustained, slower growth. The diatom yields in mixed cultures ranged from 0.8 to 84% of the yields in axenic cultures with added B₁₂. In a detailed study of one mixed culture, increases in diatom densities were paralleled by increases in cell densities of the bacterium during the first few days of exponential diatom growth. During the period of slow diatom growth, when diatom densities oscillated but steadily increased, the decreases in diatom densities were associated with increased bacterial growth. This suggests that death of a fraction of the B₁₂-limited diatom population releases sufficient organic matter to stimulate growth of the bacteria and their subsequent excretion of B₁₂; this B₁₂ in turn stimulates further growth of the diatoms. Diatom-bacteria interactions leading to the production of B₁₂ may be important in maintaining viable populations of B₁₂-requiring diatoms in nutrient-poor waters during periods between blooms, when conditions are unfavorable for rapid growth.     

Estimation of chlorophyll content and daily primary production of the major algal groups by means of multiwavelength-excitation PAM chlorophyll fluorometry: performance and methodological limits

The performance and methodological limits of the Phyto-PAM chlorophyll fluorometer were investigated with laboratory grown algae cultures and natural phytoplankton from the rivers Saar and Saale. The Phyto-PAM is a 4-wavelength chlorophyll fluorometer with the functional combination of chlorophyll (Chl) estimation and assessment of photosynthetic activity, both differentiated into the main algal groups. The reliability of fluorescence-based Chl estimation strongly depends on the group specific calibration of the instrument and the resulting chlorophyll/fluorescence (Chl/F) ratios in reference algal cultures. A very high reliability of the Chl estimation was obtained in the case of constant Chl/F-ratios. Algae grown at different light intensities showed marked differences in Chl/F-ratios, reflecting differences in pigment composition and Chl a specific absorption (a*). When the Phyto-PAM was calibrated with laboratory grown diatoms, the Chl a in river grown diatoms was underestimated, due a lower content of accessory pigments and stronger pigment packaging. While this aspect presently limits the application of PAM fluorometry in limnology, this limitation may be overcome by future technical progress in the detection of dynamic changes in Chl/F-ratio via fluorescence-based measurements of the functional PS II absorption cross-section. Practically identical Chl/F-ratios were found for the diatom-dominated waters of the rivers␣Saar and Saale, suggesting that the same instrument calibration parameters may be applied for hydrographically similar surface waters. For this particular case, despite of the present methodological limitations, the potential of PAM fluorometry in limnology could be demonstrated. Light response curves were measured to estimate primary production with a spectrally resolved model in daily courses at two sampling sites. Fluorescence based primary production was closely correlated with measured oxygen evolution rates until midday. In the afternoon, at the water surface the fluorescence approach gave higher␣rates than the measured oxygen evolution. Possible explanations for the observed differences are discussed.     

Biogeochemical processes at the sediment–water interface, Bombah Broadwater, Myall Lakes

Myall Lakes has experienced algal blooms in recent years which threaten water quality. Biomarkers, benthic fluxes measured with chambers, and pore water metabolites were used to identify the nature and reactivity of organic matter (OM) in the sediments of Bombah Broadwater (BB), and the processes controlling sediment-nutrient release into the overlying waters. The OM in the sediments was principally from algal sources although terrestrial OM was found near the Myall River. Terrestrial faecal matter was identified in muddy sediments and was probably sourced via runoff from farm lands. The reactive OM which released nutrients into the overlying waters was from diatoms, dinoflagellates and probably cyanobacteria. Microcystis filaments were observed in surface sediments. OM degradation rates varied between 5.3 and 47.1 mmol m^?2 day^?1 (64–565 mg m^?2 day^?1), were highest in the muddy sediments and sulphate reduction rates accounted for 20–40% of the OM degraded. Diatoms, being heavy sink rapidly, and are an important vector to transport catchment N and P to sites of denitrification and P-trapping in the sediments. Denitrification rates (mean ～4 mmol N m^?2 day^?1), up to 7 mmol N m^?2 day^?1 (105 mg N m^?2 day^?1) were measured, and denitrification efficiencies were highest (mean = 86 ± 4%) in the sandy sediments (～20% of the area of BB), but lower in the muddy sediments (mean = 63 ± 15%). These differences probably result from higher OM loads and anaerobic respiration in muddy sediments. Most DIP (>70%) from OM degradation was not released into overlying waters but remained trapped in surface sediments. Biophysical (advective) processes were responsible for the measured metabolite (O₂, CO₂, DSi, DIN and DIP) fluxes across the sediment–water interface.