Marian L Yallop

Comparative structure, primary production and biogenic stabilization of cohesive and non-cohesive marine sediments inhabited by microphytobenthos

The microstructure, rate of primary production and erodibility of noncohesive and cohesive marine sediments colonized by autotrophic microbial assemblages were investigated. Microbial development on non-cohesive sediments (Island of Texel, The Netherlands) formed millimetre-thick stratified mats. A surface sheath of extracellular polymeric substances (EPS) was visible on these mats; diatoms dominated the surface 500 μm with an underlying layer of cyanobacteria, mainly Microcoleus chthonoplastes. In comparison, biofilms on cohesive sediment (Bristol Channel, U.K.) were relatively thin (100 μm) and unstratified. The microspatial distribution of the algal biomass was confirmed by measurements of oxygen evolution and rates of primary production using oxygen micro-electrodes. While gross primary production was greater in the non-cohesive sediments, peak rates of photosynthetic activity and algal biomass were similar for the two sites. The erodibility of colonized areas was measured and the mechanisms of biogenic stabilization were examined by low-temperature scanning electron microscopy (LTSEM). Cohesive sediment stabilization was purely by the secretion of EPS while both EPS binding and network formation by cyanobacteria were visualized within the non-cohesive sediments. Areas where mat and biofilm systems were visible were more resistant to sediment erosion than adjacent areas without biofilm development. Direct comparison between biogenic effects at the two sites must be treated with caution because of the different nature of the sediments but the index of biogenic stabilization for both sites is reported. The stability of the non-cohesive sediments was greatly increased by the presence of a microbial mat. The hydration state of the EPS matrix was considered to have a potential effect on the critical erosion threshold of the sediments.

Interrelationships between Rates of Microbial Production, Exopolymer Production, Microbial Biomass and Sediment Stability in Biofilms of Intertidal Sediments

A bstractThe upper few millimeters of intertidal sediment supports a varied biomass of microbial consortia and microphytobenthos. Many of these organisms release extracellular polymers into the surrounding sediment matrix that can result in sediment cohesion and the increased stability of the sediment. The relationship between the heterotrophic and autotrophic components of these biofilms is not well understood. A combination of mesocosm and field investigations were used to investigate the relationship between microbial production rate (algae and bacteria), the extracellular carbohydrates, biomass, and stability in conjunction with a variety of environmental factors. An inverse relationship was found between rates of algal production and sediment stability both in the field and in laboratory mesocosms, though the relationship was significant only in the field (P < 0.001). Stability of sediments increased with increasing bacterial production rate (P < 0.001). Positive correlations were found between sediment stability and a range of other variables, including algal biomass (P < 0.001), colloidal-S EPS (P < 0.001), colloidal-S carbohydrate (P < 0.01), colloidal-S EDTA (P < 0.01), and sediment water content (P < 0.001). Using the data acquired, a preliminary model was developed to predict changes in sediment stability. Chlorophyll a, water content, and colloidal-S EPS were found to be the most important predictors of stability in intact cores incubated under laboratory conditions. Differences observed in patterns of the surface (0–2 mm) distribution of colloidal-S carbohydrate and chlorophyll a when expressed on a dry weight or areal basis were attributed to effects of dewatering and concomitant changes in wet bulk density. The polymeric carbohydrate (colloidal-S EPS) component of the biofilms was not found to be a constant fraction of the colloidal-S carbohydrate extract, varying from 16 to 58%, and the percentage of polymer decreased logarithmically as chlorophyll a concentrations increased and the biofilms matured (P < 0.001). Changes in the relationships between these variables over the period of biofilm development and maturation highlight the difficulties in their use to predict sediment stability. Exopolymer concentrations were more closely correlated with algal biomass than with bacterial numbers. Rates of algal carbon fixation were considerably greater than those for bacteria, suggesting that the algae have a much greater potential for exopolymer production. It is suggested that the microphytobenthos secretions make a more important contribution to sediment stability.

Photophysiology and albedo-changing potential of the ice algal community on the surface of the Greenland ice sheet

Darkening of parts of the Greenland ice sheet surface during the summer months leads to reduced albedo and increased melting. Here we show that heavily pigmented, actively photosynthesising microalgae and cyanobacteria are present on the bare ice. We demonstrate the widespread abundance of green algae in the Zygnematophyceae on the ice sheet surface in Southwest Greenland. Photophysiological measurements (variable chlorophyll fluorescence) indicate that the ice algae likely use screening mechanisms to downregulate photosynthesis when exposed to high intensities of visible and ultraviolet radiation, rather than non-photochemical quenching or cell movement. Using imaging microspectrophotometry, we demonstrate that intact cells and filaments absorb light with characteristic spectral profiles across ultraviolet and visible wavelengths, whereas inorganic dust particles typical for these areas display little absorption. Our results indicate that the phototrophic community growing directly on the bare ice, through their photophysiology, most likely have an important role in changing albedo, and subsequently may impact melt rates on the ice sheet.

Microbial community structure and standing stocks in the NE Atlantic in June and July of 1996

The standing stocks of nanophytoplankton and picoplankton in the northeast Atlantic Ocean in June and July 1996 were quantified using flow cytometry and microscopy. Diatoms and dinoflagellates were analysed by microscopy and coccolithophores, other nanophytoplankton, picoeukaryotic phytoplankton, cyanobacteria (Synechococcus spp.), prochlorophytes (Prochlorococcus spp.) and heterotrophic bacteria by flow cytometry. The research was divided into three components: a lagrangian study of a nutrient replete cold-core eddy centred around 59° 12′N 20° 12′W; a transect close to the 20°W meridian from 59° 18′N to 37°N, which passed through contrasting water masses; and a lagrangian study in oligotrophic waters, centred around 36° 42′N 19° 12′W. The eddy was characterised by a bloom of the coccolithophore Coccolithus pelagicus whose standing stocks averaged 4.26 g C m−2 over the upper 50 m. C. pelagicus and other nanophytoplankton (excluding diatoms and dinoflagellates) dominated the standing stocks of the microbial community, averaging approx. 70% of the total microbial standing stocks of the groups quantified. The majority of the remaining biomass was accounted for by the picoeukaryotic phytoplankton and heterotrophic bacteria. The microbial community immediately outside the eddy was significantly different in both composition and standing stocks. There were no C. pelagicus outside the eddy and fewer nanophytoplankton, resulting in microbial standing stocks of approx. one-third that found in the eddy. The transect was characterised by a frontal region at approx. 52° 30′N. There was a general decrease in the standing stocks of all components of the microbial community from the start of the transect to the front. Just to the south of the front, nanophytoplankton, Synechococcus spp. and heterotrophic bacteria showed marked increases in standing stocks, especially the nanophytoplankton, which increased from 3.43 to 7.90 g C m−2. The nanophytoplankton dominated the microbial standing stocks throughout the transect, even in the oligotrophic waters where the integrated carbon biomass was 4.58 g C m−2, representing 69% of the total microbial standing stocks. During the lagrangian study around 37°N the picoplanktonic community was dominated by heterotrophic bacteria. However, heterotrophic bacteria standing stocks decreased with time, along with Synechococcus spp. and picoeukaryotic phytoplankton. Peak biomass for these three groups shifted deeper down in the water column with time. Prochlorococcus spp. were only present towards the end of the transect and at the oligotrophic site. At the oligotrophic site their standing stocks increased, unlike other groups, so that they became the dominant picophytoplanktonic group.

Recommendations for sampling littoral diatoms in lakes for ecological status assessments

This review summarises the existing literature and outlines the theoretical basis for using standard methods for sampling diatoms from rivers to sample littoral diatoms and other phytobenthos from standing waters. The European Unions Water Framework Directive has created a statutory obligation for EU Member States to monitor macrophytes and phytobenthos in lakes. Although there has been a considerable amount of work using phytobenthos (especially diatoms) to monitor river water quality in Europe, there are fewer studies on the use of phytobenthos for monitoring in lakes. European standard methods for sampling diatoms from rivers should be suitable, with only minor modifications, for sampling littoral diatoms from lakes and other standing water bodies. These recommendations should be applicable to all temperate regions.

Uncertainty in ecological status assessments of lakes and rivers using diatoms

The EU’s Water Framework Directive requires all surface water bodies to be classified according to their ecological status. As biological communities show both spatial and temporal heterogeneity, expressions of ecological status will, inevitably, have an element of uncertainty associated with them. A consequence of this environmental heterogeneity is that there is a risk that status inferred from one or more samples is different to the true status of that water body. In order to quantify the scale of temporal uncertainty associated with benthic diatoms, replicate samples were collected from sites across the ecological status gradient in lakes and rivers in the UK. Variability (expressed as standard deviation of temporal replicate samples from a single site) could be described using a polynomial function and this was then used to calculate the risk of placing a water body in the wrong ecological status class. This risk varied depending on the distance from the class boundaries and the number of replicates. Based on these data, we recommend that ecological status is determined from a number of samples collected from a site over a period of time.

Measuring rates of gross photosynthesis and net community production in cryoconite holes: a comparison of field methods

Abstract Photosynthesis by microbes on the surfaces of glaciers and ice sheets has the potential to fix carbon, alter the albedo of ice surfaces via the production of organic matter and so enhance ice melt. It could also be important for supplying labile organic matter and nutrients to in situ and downstream ecosystems. This study compares in situ 24 hour incubation methods for measuring rates of gross photosynthesis, respiration and net community production (NCP) in cryoconite holes on three Svalbard valley glaciers. Rates of gross photosynthesis and respiration measured by the ΔCO2 method were closely balanced, resulting in rates of NCP close to the detection limit (mean of –1.3 μg C g−1 d–1) consistent with previous measurements in Arctic cryoconite holes. This suggests that organic matter within cryoconite holes may be derived largely from allochthonous sources. The molar ratio of ΔO2 to ΔCO2 in incubations gave mean respiratory and photosynthetic quotients of 0.80 ± 0.17 (1 × SD) and 1.24 ± 0.20 (1 × SD), respectively. The 14C method typically underestimated rates of gross photosynthesis (ΔCO2 method) by more than one order of magnitude and measured a rate closer to NCP.

Assessment of ecological status in UK lakes using benthic diatoms

Abstract: The European Unions Water Framework Directive (WFD) requires that all water bodies in Europe achieve good ecological status (GES) by 2015. We developed an ecological classification tool for UK lakes based on benthic diatoms, a key component of the biological-quality element macrophytes and phytobenthos. A database of 1079 epilithic and epiphytic diatom samples and matching environmental data was assembled from 228 UK lakes. The data set was divided into 3 lake types: low, medium, and high alkalinity. A lake trophic diatom index (LTDI) was developed based on modification of the trophic diatom index (TDI) for rivers, and ecological quality ratios (EQRs) were generated for each lake type. The high/good status boundary was defined as the 25th percentile of EQRs of all reference sites (identified based on independent sedimentary-diatomassemblage data or catchment point-source and landuse data), whereas the good/moderate boundary was set at the point at which nutrient-sensitive and nutrient-tolerant taxa were present in equal relative abundance. The moderate/poor and poor/bad boundaries were defined by equal division of the remaining EQR gradient. Samples from reference sites were used to predict the expected LTDI value for each sample, and these values were compared with the classifications derived from the LTDI. For lakes identified as reference sites, 68% were classified as having high status and 32% as having good. The model predicted 81% of nonreference lakes to have good or worse status. The model was applied to 17 English lakes (10 low- and 7 medium-alkalinity) for which classification based on other WFD tools was available. The classifications based on LTDI gave the same status (within 1 class) as other biological elements for 11 of the 17 lakes (65%). Thus, the LTDI gives a reliable assessment of the condition of the littoral biofilm and is a key component of a WFD-compliant tool kit for classifying UK standing waters.

Functional group diversity increases with modularity in complex food webs

Biodiversity increases the ability of ecosystems to provide multiple functions. Most studies report a positive relationship between species richness and the number of ecosystem functions. However, it is not known whether the number of functional groups is related to the structure of the underlying species interaction network. Here we present food web data from 115 salt marsh islands and show that network structure is associated with the number of functional groups present. Functional group diversity is heterogeneously distributed across spatial scales, with some islands hosting more functional groups than others. Functional groups form modules within the community so that food webs with more modular architectures have more functional group diversity. Further, in communities with different interaction types, modularity can be seen as the multifunctional equivalent of trophic complementarity. Collectively, these findings reveal spatial heterogeneity in the number of functional groups that emerges from patterns in the structure of the food web.

Distribution patterns and biomass estimates of diatoms and autotrophic dinoflagellates in the NE Atlantic during June and July 1996

Qualitative and quantitative analyses of microphytoplankton communities were determined from samples collected in the northeast Atlantic Ocean in the early summer of 1996 during the PRIME Cruise of the RRS Discovery. A combination of light microscopy and scanning electron microscopy techniques was used to determine the species composition of two of the main groups of phytoplankton: Bacillariophyceae and Dinophyta. Two series of samples were collected; the first set of samples was collected between 18 and 29 June 1996 during a Lagrangian time-series study in the vicinity of 59°N 20°W tracking a mesoscale cold-core eddy; the second set of samples was collected between 4 and 10 July 1996 during a transect along the 20°W meridian from 59 to 37°N. A total of 155 samples were analysed over various depths down to 150 m, and 78 phytoplankton species were identified. Samples taken during the Lagrangian time-series study were dominated by diatom species, including Ephemera planamembranacea and Pseudo-nitzschia species, whilst the main representative of the microphytoplankton dinoflagellates was Ceratium fusus. On the transect, several Ceratium species were common, including C. furca C. fusus, and C. lineatum, and three other autotrophic dinoflagellates were frequent including Prorocentrum minimum, Oxytoxum scolopax and Gonyaulax polygramma. A number of diatoms dominated the profiles along the transect including Leptocylindrus mediterraneus, Thalassiosira oestrupii, and representatives of the genera Haslea and Pseudo-nitzschia. Standing stocks of both groups were low and typical of post-bloom carbon levels. Diatom biomass exceeded that of dinoflagellate biomass in the eddy although the reverse situation was seen in the more southerly stations along the transect. Maximum abundances of the dinoflagellate communities were situated in the surface waters within the mixed layer, while depth maxima of certain diatoms were noted at around 40 m below the depth of the mixed layer both in the Langrangian time-series study and along the transect. Microstratification and nutrient stress may have contributed to losses of diatoms within the mixed layer due to sedimentation. Hierarchical classification and ordination techniques were used to identify patterns in species assemblages. Four possible clusters of phytoplankton were identified, each associated with a particular suite of environmental variables.