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Tett, P., Carreira, C., Mills, D. K., van Leeuwen, S., Foden, J., Bresnan, E., and Gowen, R. J. 2008. Use of a Phytoplankton Community Index to assess the health of coastal waters. – ICES Journal of Marine Science, 65: 1475–1482. Monitoring of marine-ecosystem status and health requires indicators of community structure and function. As a structural indicator, we propose a Phytoplankton Community Index (PCI) based on the abundance of “life-forms” such as “pelagic diatoms” or “medium-sized autotrophic dinoflagellates”. To calculate the PCI, data showing seasonal variation in these abundances are plotted in “life-form space” of two or more dimensions. Data from a “type-specific reference condition” are then enclosed within a reference envelope. Comparison data are plotted into the same coordinate system, and the PCI is the proportion (between 0 and 1) of these new data that fall within the reference envelope. Results from initial applications of this method are shown for UK coastal waters in the northern North Sea (near Stonehaven), a Scottish fjord (Loch Creran), and the eastern Irish Sea (including Liverpool Bay). The Stonehaven data (1997–2005) were used to compare values obtained from weekly sampling with those from monthly sampling. A spatial comparison between more- and less-nutrient-enriched waters in the eastern Irish Sea (1991–2003) showed little difference in phytoplankton community structure. Loch Creran has experienced a large change in the “balance of organisms”, and hence a reduction in the PCI, between 1979–1981 and 2006/2007, associated with a decrease in chlorophyll but no apparent change in nutrients. These results are discussed in relation to the intended uses of the PCI as an index of biological quality for the Water Framework Directive (WFD) and an index of ecosystem health in the context of eutrophication. Although the method only measures change, it can also be used as an indicator of biological quality if the reference conditions are those defined for a WFD waterbody, and as an indicator of health if appropriately calibrated. Suggestions are made for further development.

Global change impacts on marine biogeochemistry will be partly mediated by heterotrophic bacteria. Besides ocean warming, future environmental changes have been suggested to affect the quantity and quality of organic matter available for bacterial growth. However, it is yet to be determined in what way warming and changing substrate conditions will impact marine heterotrophic bacteria activity. Using short-term (4 days) experiments conducted at three temperatures (−3°C, in situ, +3°C) we assessed the temperature dependence of bacterial cycling of marine surface water used as a control and three different dissolved organic carbon (DOC) substrates (glucose, seagrass, and mangrove) in tropical coastal waters of the Great Barrier Reef, Australia. Our study shows that DOC source had the largest effect on the measured bacterial response, but this response was amplified by increasing temperature. We specifically demonstrate that (1) extracellular enzymatic activity and DOC consumption increased with warming, (2) this enhanced DOC consumption did not result in increased biomass production, since the increases in respiration were larger than for bacterial growth with warming, and (3) different DOC bioavailability affected the magnitude of the microbial community response to warming. We suggest that in coastal tropical waters, the magnitude of heterotrophic bacterial productivity and enzyme activity response to warming will depend partly on the DOC source bioavailability.

The bioavailability of organic matter (OM) was assessed at three locations during the dry and wet seasons in the Great Barrier Reef (GBR), by measuring changes in particulate organic matter (POM) and dissolved organic matter (DOM) concentrations during laboratory incubations over 50 d. The sites did not show any difference in salinity and, therefore, observed changes could be related to factors such as disparities in the biological activity and/or the impact of sediment resuspension rather than to location. Our results demonstrate that the POM pool has a higher bioavailability than the DOM pool, with the C, N, and P‐containing compounds being more bioavailable than the C and N‐containing molecules, which in turn are more labile than compounds containing just C. The addition of labile POM to the DOM pool did not impact the bioavailability of DOM, suggesting that priming had no major impact on the degradation of this pool in these experiments. Furthermore, our results demonstrate that 94% and 75% of the bioavailable N and P are contained in the organic fraction delivering enough nutrients to sustain phytoplankton productivity in the GBR. Using the obtained degradation rate constants and an average water residence time of 2 weeks, we show that most bioavailable POM (> 96%) and DOM (> 83%) is degraded before reaching the outer shelf. Our results emphasize that OM is a key and mostly unaccounted part of the C, N, and P cycles in tropical coastal waters of the GBR.

c Viral abundances in benthic environments are the highest found in aquatic systems. Photosynthetic microbial mats represent benthic environments with high microbial activity and possibly high viral densities, yet viral abundances have not been examined in such systems. Existing extraction procedures typically used in benthic viral ecology were applied to the complex matrix of microbial mats but were found to inefficiently extract viruses. Here, we present a method for extraction and quantification of viruses from photosynthetic microbial mats using epifluorescence microscopy (EFM) and flow cytometry (FCM). A combination of EDTA addition, probe sonication, and enzyme treatment applied to a glutaraldehyde-fixed sample resulted in a substantially higher viral (5-to 33-fold) extraction efficiency and reduced background noise compared to previously published methods. Using this method, it was found that in general, intertidal photosynthetic microbial mats harbor very high viral abundances (2.8 ؋ 10 10 ؎ 0.3 ؋ 10 10 g ؊1 ) compared with benthic habitats (10 7 to 10 9 g ؊1 ). This procedure also showed 4.5-and 4-fold-increased efficacies of extraction of viruses and bacteria, respectively, from intertidal sediments, allowing a single method to be used for the microbial mat and underlying sediment. Photosynthetic microbial mats are vertically stratified benthic microbial communities that are found worldwide in environments ranging from hot springs to sea ice (e.g., see reference 1). The top layer of these mats is mostly composed of photoautotrophs (filamentous cyanobacteria and eukaryotic phytobenthos) that produce organic carbon, which is decomposed in a succession of layers of different heterotrophic prokaryotes reflecting concentration gradients in oxygen and other electron acceptors (e.g., see references 1 to 4). The intertwined filamentous cyanobacteria in the top layer and the excretion of exopolymric substances (EPS) make the microbial mats very stable and resistant to wind and wave erosion (5). Viruses are diverse, abundant, and ecologically important components of microbial communities, acting as major drivers of biodiversity and organic matter flux (e.g., see references 6 to 8). In sediments, viruses have been shown to affect prokaryote host mortality (9), spatial distribution (10), and biogeochemical cycling (11). However, while microbial mats have been intensively studied with regard to their biogeochemistry and biodiversity (e.g., see references 12 and 13), studies on the ecological role of viruses in these mats are, to our knowledge, lacking.One of the challenges of assessing the role of viruses in sediments and other surface-associated environments, such as photosynthetic mats, is the need for reliable quantitative measures to determine their abundance. Depending on the type of sediment (intertidal, coastal, or deep sediments) (14-16), different methods have been used to extract viruses and bacteria. In microbial mats, EPS bind microorganisms, viruses, and particles together in a complex matrix (17), mak...

A lytic virus that infects a European strain of the freshwater filamentous cyano-bacterium Cylindrospermopsis raciborskii was isolated from a lake in the Netherlands and partially characterised. With a genome size of 110 ± 15 kb, an icosahedral capsid of 65 ± 1 nm (n = 22) and a long non-contractile tail of 612 ± 31 nm (n = 15), this dsDNA cyanophage CrV appears to belong to the Siphoviridae family. CrV was highly host specific, not infecting other filamentous cyanobacteria species isolated from the same lake, nor 4 Australian strains of C. raciborskii. The latent period of this cyanophage was 20−24 h. Varying the irradiance affected cyanophage−host interactions: at low light (20 µmol quanta m −2 s −1) the latent period was 1.3 times longer compared with at mid light (90 µmol quanta m −2 s −1); burst size at mid light was 332 CrV per lysed host cell, at low light it was halved (48%) and at high light (250 µmol quanta m −2 s −1) the burst size was further reduced to only 14% of that of mid light. Temperature also affected the virus growth characteristics: the CrV latent period at high temperature (30°C) was reduced to just 11% (compared with a mid temperature of 22°C), but still the burst size increased to 541 CrV per lysed host cell; at low temperature (15°C) the latent period was prolonged 1.3-fold and the burst size was reduced to 43%. Our findings indicate that ecologically relevant environmental factors can affect the extent of viral lysis of C. raciborskii, advancing our understanding of the spread of this invasive cyanobacterium across Europe.

The short-term effects of inorganic N and P (nitrate, ammonium, phosphate) and organic C and N (glucose, amino acids) inputs, added separately as well as jointly, on phytoplankton and bacterioplankton community composition were studied in 6 microcosm experiments conducted in a eutrophic coastal embayment under contrasting hydrographic conditions. The responses of the different bacterioplankton and phytoplankton groups to the distinct nutrient inputs were highly variable among experiments, which was partially related to changes in the initial environmental conditions. Gammaproteobacteria and nanoflagellates were the most responsive groups to nutrient additions. Inorganic nutrients did not promote important changes in the microbial plankton community structure but did significantly reduce diatom diversity. In contrast, organic additions promoted changes mainly in bacterioplankton groups, whilst mixed additions provoked changes in both bacterial and phytoplankton groups. While nanoflagellates increased equally in abundance after inorganic and mixed additions (2.9-fold), dinoflagellates and diatoms increased their abundances more in the mixed treatment (2.3-fold and 2.2-fold, respectively) than in the inorganic treatment. Organic and mixed additions did not provoke changes in diatom or dinoflagellate diversity. The magnitude of response of Gammaproteobacteria largely explained changes in bulk bacterial biomass and activity, whereas changes in bulk phytoplankton biomass and primary production associated to nutrient enrichment were mostly explained by the response of diatoms and large picoeukaryotes. Our results demonstrate that the type of nitrogen inputs (inorganic and/or organic) strongly affects the microbial plankton community composition and functioning in this coastal ecosystem.