Understanding of the processes that control CO 2 concentrations in the aquatic environment has been hampered by the absence of a direct method to make continuous measurements over both short-and long-term time intervals. We describe an in situ method in which a non-dispersive infrared (NDIR) sensor is enclosed in a water impermeable, gas permeable polytetrafluoroethylene (PTFE) membrane and deployed in a freshwater environment. This allows measurements of CO 2 concentration to be made directly at a specific depth in the water column without the need for pumps or reagents. We demonstrate the potential of the method using examples from different aquatic environments characterized by a range of CO 2 concentrations (0Ð5-8Ð0 mg CO 2 -C l 1 , equivalent to ca 40-650 µmol CO 2 l 1 ). These comprise streams and ponds from tropical, temperate and boreal regions. Data derived from the sensor was compared with direct measurements of CO 2 concentrations using headspace analysis. Sensor performance following long-term (>6 months) field deployment conformed to manufacturers' specifications, with no drift detected. We conclude that the sensor-based method is a robust, accurate and responsive method, with a wide range of potential applications, particularly when combined with other in situ sensor-based measurements of related variables.

Biology, 19 (7). 2133-2148. 10.1111/gcb.12209 Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Accepted ArticleThis article is protected by copyright. All rights reserved.Corresponding Author: Kerry J Dinsmore, 0131 445 8583 kjdi@ceh.ac.uk AbstractThe aquatic pathway is increasingly being recognised as an important component of catchment carbon and greenhouse gas (GHG) budgets, particularly in peatland systems due to their large carbon store and strong hydrological connectivity. In this study we present a complete 5-year dataset of all aquatic carbon and GHG species (POC, DOC, DIC, CO 2 , CH 4 , N 2 O) from an ombrotrophic Scottish peatland. We show that short term variability in concentrations exists across all species and this is strongly linked to discharge. Seasonal cyclicity was only evident in DOC, CO 2 and CH 4 concentration; however temperature correlated with monthly means in all species except DIC. Whilst the temperature correlation with monthly DOC and POC concentrations appeared to be related to biological productivity in the terrestrial system, we suggest the temperature correlation with CO 2 and CH 4 was primarily due to in-stream temperature-dependent solubility. Interannual variability in total aquatic carbon concentration was strongly correlated with catchment GPP indicating a strong potential terrestrial aquatic linkage. DOC represented the largest aquatic carbon flux term (19.3 ± 4.59 g C m -2 yr -1 ), followed by CO 2 evasion (10.0 g C m -2 yr -1 ). Despite an estimated contribution to the total aquatic carbon flux of between 8 -48%, evasion estimates have the greatest uncertainty. Interannual variability in total aquatic carbon export was low in comparison with variability in terrestrial biosphere-atmosphere exchange, and could be explained primarily by temperature and precipitation. Our results therefore suggest that climatic change is likely to have a significant impact on annual carbon losses through the aquatic pathway, and as such aquatic exports are fundamental to the understanding of whole catchment responses to climate change.

Heterogeneity is a well-recognized feature of natural environments, and the spatial distribution and movement of individual species is primarily driven by resource requirements. In laboratory experiments designed to explore how different species drive ecosystem processes, such as nutrient release, habitat heterogeneity is often seen as something which must be rigorously controlled for. Most small experimental systems are therefore spatially homogeneous, and the link between environmental heterogeneity and its effects on the redistribution of individuals and species, and on ecosystem processes, has not been fully explored. In this paper, we used a mesocosm system to investigate the relationship between habitat composition, species movement and sediment nutrient release for each of four functionally contrasting species of marine benthic invertebrate macrofauna. For each species, various habitat configurations were generated by selectively enriching patches of sediment with macroalgae, a natural source of spatial variability in intertidal mudflats. We found that the direction and extent of faunal movement between patches differs with species identity, density and habitat composition. Combinations of these factors lead to concomitant changes in nutrient release, such that habitat composition effects are modified by species identity (in the case of NH4-N) and by species density (in the case of PO4-P). It is clear that failure to accommodate natural patterns of spatial heterogeneity in such studies may result in an incomplete understanding of system behaviour. This will be particularly important for future experiments designed to explore the effects of species richness on ecosystem processes, where the complex interactions reported here for single species may be compounded when species are brought together in multi-species combinations.

Spring snowmelt in the arctic and boreal regions represents the most significant event in the hydrological year. We measured concentrations and fluxes of different carbon species in 2 small contrasting (control v drained) forested peatland catchments in E. Finland between April and June 2008 and compared these to long-term annual fluxes. Measurements were made using a combination of continuous sensors (CO 2 , temperature, pH, discharge) and routine spot sampling (DOC, POC, DIC, CO 2 , CH 4 , N 2 O). The highest concentrations of CO 2 and CH 4 in streamwater were observed under low flow conditions before the spring flood event, reflecting accumulation and downstream release of gaseous C at the end of the winter period. Over the length of the study mean CH 4 concentrations were 109 higher in the drained site. The snowmelt event was associated with a dilution of DOC and CO 2 , with the drained catchment showing a much flashier hydrological response compared to the control site, and post-event, a slower recovery in DOC and CO 2 concentrations. Fluxes of all carbon species during the snowmelt event were significant and represented 37-45% of the annual flux. This highlights the challenge of quantifying aquatic C fluxes in areas with large temporal variability and suggests that inability to ''capture'' the spring snowmelt event may lead to under-estimation of C fluxes in northern regions.

Despite the complexity of natural systems, heterogeneity caused by the fragmentation of habitats has seldom been considered when investigating ecosystem processes. Empirical approaches that have included the influence of heterogeneity tend to be biased towards terrestrial habitats; yet marine systems offer opportunities by virtue of their relative ease of manipulation, rapid response times and the well-understood effects of macrofauna on sediment processes. Here, the influence of heterogeneity on microphytobenthic production in synthetic estuarine assemblages is examined. Heterogeneity was created by enriching patches of sediment with detrital algae (Enteromorpha intestinalis) to provide a source of allochthonous organic matter. A gradient of species density for four numerically dominant intertidal macrofauna (Hediste diversicolor, Hydrobia ulvae, Corophium volutator, Macoma balthica) was constructed, and microphytobenthic biomass at the sediment surface was measured. Statistical analysis using generalized least squares regression indicated that heterogeneity within our system was a significant driving factor that interacted with macrofaunal density and species identity. Microphytobenthic biomass was highest in enriched patches, suggesting that nutrients were obtained locally from the sediment-water interface and not from the water column. Our findings demonstrate that organic enrichment can cause the development of heterogeneity which influences infaunal bioturbation and consequent nutrient generation, a driver of microphytobenthic production.