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The effects of climate change on coastal biodiversity are a major concern because altered community compositions may change associated rates of ecosystem functioning and services. Whilst responses of single species or taxa have been studied extensively, it remains challenging to estimate responses to climate change across different levels of biological organisation. Studies that consider the effects of moderate realistic near‐future levels of ocean warming and acidification are needed to identify and quantify the gradual responses of species to change. Also, studies including different levels of biological complexity may reveal opportunities for amelioration or facilitation under changing environmental conditions. To test experimentally for independent and combined effects of predicted near‐future warming and acidification on key benthic species, we manipulated three levels of temperature (winter ambient, +0.8 and +2°C) and two levels of pCO2 (ambient at 450 ppm and elevated at 645 ppm) and quantified their effects on mussels and algae growing separately and together (to also test for inter‐specific interactions). Warming increased mussel clearance and mortality rates simultaneously, which meant that total biomass peaked at +0.8°C. Surprisingly, however, no effects of elevated pCO2 were identified on mussels or algae. Moreover, when kept together, mussels and algae had mutually positive effects on each other's performance (i.e. mussel survival and condition index, mussel and algal biomass and proxies for algal productivity including relative maximum electron transport rate [rETRmax], saturating light intensity [Ik] and maximum quantum yield [Fv/Fm]), independent of warming and acidification. Our results show that even moderate warming affected the functioning of key benthic species, and we identified a level of resistance to predicted ocean acidification. Importantly, we show that the presence of a second functional group enhanced the functioning of both groups (mussels and algae), independent of changing environmental conditions, which highlights the ecological and potential economic benefits of conserving biodiversity in marine ecosystems. [ABSTRACT FROM AUTHOR]

Our understanding of how increasing atmospheric CO2 and climate change influences the marine CO2 system and in turn ecosystems has increasingly focused on perturbations to carbonate chemistry variability. This variability can affect ocean-climate feedbacks and has been shown to influence marine ecosystems. The seasonal variability of the ocean CO2 system has already changed, with enhanced seasonal variations in the surface ocean pCO2 over recent decades and further amplification projected by models over the 21st century. Mesocosm studies and CO2 vent sites indicate that diurnal variability of the CO2 system, the amplitude of which in extreme events can exceed that of mean seasonal variability, is also likely to be altered by climate change. Here, we modified a global ocean biogeochemical model to resolve physically and biologically driven diurnal variability of the ocean CO2 system. Forcing the model with 3-h atmospheric outputs derived from an Earth system model, we explore how surface ocean diurnal variability responds to historical changes and project how it changes under two contrasting 21st-century emission scenarios. Compared to preindustrial values, the global mean diurnal amplitude of pCO2 increases by 4.8 μatm (+226%) in the high-emission scenario but only 1.2 μatm (+55%) in the high-mitigation scenario. The probability of extreme diurnal amplitudes of pCO2 and [H+] is also affected, with 30-to 60-fold increases relative to the preindustrial under high 21st-century emissions. The main driver of heightened pCO2 diurnal variability is the enhanced sensitivity of pCO2 to changes in temperature as the ocean absorbs atmospheric CO2. Our projections suggest that organisms in the future ocean will be exposed to enhanced diurnal variability in pCO2 and [H+], with likely increases in the associated metabolic cost that such variability imposes. [ABSTRACT FROM AUTHOR]

A major challenge for predicting future landscape carbon (C) balances is to understand how environmental changes affect the transfer of C from soils to surface waters. Here, we evaluated 14 yr (2006–2019) of stream dissolved inorganic carbon (DIC) concentration and export data for 14 nested boreal catchments that are subject to climatic changes, and compared long‐term patterns in DIC with patterns in dissolved organic carbon (DOC). Few streams displayed significant concentration or export trends in DIC at annual time scales. However, most streams showed decreasing DIC concentrations during spring flood over this 14‐yr period, and about half showed declines during summer. Although annual runoff has generally not changed during this period, an intra‐annual redistribution in runoff, with increases in spring flood discharge, explained much of the seasonal changes in DIC concentration. We observed negative DIC–discharge relationships in most streams, suggesting source limitation of DIC, whereas DOC mostly showed chemostatic behavior. The different trends and patterns observed for DIC vs. DOC underpin intra‐annual changes in the composition of the total C pool (i.e., the DIC/DOC ratio) and reflect fundamental differences in how these C forms are produced, stored in riparian soils, and mobilized by hydrological events. Collectively, our results highlight the sensitivity of riverine DIC to the intra‐annual distribution of runoff, but also important heterogeneity across the network that suggests local processes can also modify the mobilization of DIC in boreal landscapes. [ABSTRACT FROM AUTHOR]

Over the last few decades, there has been an increasing recognition for seagrasses' contribution to the functioning of nearshore ecosystems and climate change mitigation. Nevertheless, seagrass ecosystems have been deteriorating globally at an accelerating rate during recent decades. In 2017, research into the condition of eelgrass (Zostera marina) along the eastern coast of James Bay, Canada, was initiated in response to reports of eelgrass decline by the Cree First Nations of Eeyou Istchee. As part of this research, we compiled and analyzed two decades of eelgrass cover data and three decades of eelgrass monitoring data (biomass and density) to detect changes and assess possible environmental drivers. We detected a major decline in eelgrass condition between 1995 and 1999, which encompassed the entire east coast of James Bay. Surveys conducted in 2019 and 2020 indicated limited changes post‐decline, for example, low eelgrass cover (<25%), low aboveground biomass, smaller shoots than before 1995, and marginally low densities persisted at most sites. Overall, the synthesized datasets show a 40% loss of eelgrass meadows with >50% cover in eastern James Bay since 1995, representing the largest scale eelgrass decline documented in eastern Canada since the massive die‐off event that occurred in the 1930s along the North Atlantic coast. Using biomass data collected since 1982, but geographically limited to the sector of the coast near the regulated La Grande River, generalized additive modeling revealed eelgrass meadows are affected by local sea surface temperature, early ice breakup, and higher summer freshwater discharge. Our results caution against assuming subarctic seagrass ecosystems have avoided recent global declines or will benefit from ongoing climate warming. [ABSTRACT FROM AUTHOR]

Inland aquatic ecosystems play an important role in the global carbon cycle as they actively mineralize large amounts of terrestrial organic matter. However, there is not much evidence that this allochthonous organic matter affects the energy and matter flow through trophic chain via the microbial food web in tropical reservoirs. We hypothesize that the fresh terrestrial dissolved organic matter (DOM) input in aquatic ecosystems increases net heterotrophy via microbes. A field experiment was conducted in mesocosms, with two treatments: one received detritus input of freshly sampled terrestrial vegetation and the other treatment had no additions (control). The detritus input resulted in increased net heterotrophy and respiration rates after 2 days and increased primary production after 21 days. Moreover, it also changed the zooplankton community to the dominance of copepods, cyclopoids and rotifers, which could have stabilized bacterial abundance and increased bacterial respiration (BR). Our results suggest that fresh terrestrial organic matter input in aquatic systems experiencing wide water level fluctuations (e.g. due to changes in climatic patterns) affect metabolism through two main pathways: (i) initially increasing net heterotrophy via direct DOM bacterial mineralization and planktonic respiration, (ii) later stimulating primary production and net autotrophy due to the nutrients mineralization. [ABSTRACT FROM AUTHOR]

Stream networks transport and emit substantial volumes of carbon dioxide (CO2) into the atmosphere. We gathered open monitoring data from streams in three Scandinavian countries and estimated CO2 partial pressure (pCO2) at 2,298 sites. Most of the sites (87%) were supersaturated when averaged across the year with an overall mean pCO2 of 1,464 μatm (range: 17–15,646). Using remote sensing data, we modeled a realistic stream network including streams above ~2.5 m wide and calculated catchment averages of multiple variables associated with geomorphometry, stream network proximity, and land cover. We compared the ability of eight machine learning models to predict pCO2 and found that the Random Forest model achieved the highest accuracy, with a root‐mean‐square error of 0.22 (log10(pCO2)) and R2 of 0.66. Mean catchment elevation, slope, and permanent water cover were the most important predictor variables. We used the predictive model to create a high‐resolution (25‐m resolution) map with predicted stream pCO2 throughout the 268.807 km stream network in Denmark, Sweden, and Finland. Predicted pCO2 averaged 1,134 μatm (range: 154–8,174). We used surface runoff, air temperature, and stream channel slope to estimate gas transfer velocity and CO2 flux throughout the network. Mean stream CO2 fluxes ranged from 1.0 and 1.2 in Sweden and Finland, respectively, 3 to 3.2 g C m−2 day−1 in Denmark. Better‐performing models improve our ability to predict pCO2 in stream networks and reduce the uncertainty of upscaling estimates of carbon emissions from inland waters to countries and continents. Key Points: Carbon dioxide partial pressure in streams can be predicted from catchment characteristicsPrediction of carbon dioxide partial pressure can be improved using machine learningPrediction of carbon dioxide partial pressure, followed by estimation of fluxes, can be performed across large stream networks [ABSTRACT FROM AUTHOR]

SUMMARY Global changes in climatic conditions are expected to disrupt marine ecosystems. Ocean warming is one of many concerns, since more than 90% of the Earth's warming occurs in the oceans. Macrophyte‐dominated communities recently have become the focus of climate mitigation due to their high carbon sequestration rate. Therefore, there is an urgent need to understand the effects of environmental variables on the phenological response of photosynthesis in entire macrophyte communities (i.e., community production). We conducted 30 monitoring surveys from May 2015 to February 2017 and collected time‐series data of environmental variables in Zostera marina (3 m depth) and Sargassum siliquastrum (1 m depth) communities. The community production and respiration from two different macrophyte communities were calculated from dissolved oxygen time‐series. Analysis of the time‐series indicated strong diurnal frequencies for dissolved oxygen, light, and net ecosystem production, whereas weekly frequencies dominated for water temperature, chlorophyll a fluorescence, and current speed. Water temperature appeared to drive mean gross ecosystem production over the course of a year and light induced variations in the short‐term and were similar in both macrophyte communities. [ABSTRACT FROM AUTHOR]

High‐altitude tropical grasslands, known as "páramos," are characterized by high solar radiation, high precipitation, and low temperature. They also exhibit some of the highest ecosystem carbon stocks per unit area on Earth. Recent observations have shown that páramos may be a net source of CO2 to the atmosphere as a result of climate change; however, little is known about the source of this excess CO2 in these mountainous environments or which landscape components contribute the most CO2. We evaluated the spatial and temporal variability of surface CO2 fluxes to the atmosphere from adjacent terrestrial and aquatic environments in a high‐altitude catchment of Ecuador, based on a suite of field measurements performed during the wet season. Our findings revealed the importance of hydrologic dynamics in regulating the magnitude and likely fate of dissolved carbon in the stream. While headwater catchments are known to contribute disproportionately larger amounts of carbon to the atmosphere than their downstream counterparts, our study highlights the spatial heterogeneity of CO2 fluxes within and between aquatic and terrestrial landscape elements in headwater catchments of complex topography. Our findings revealed that CO2 evasion from stream surfaces was up to an order of magnitude greater than soil CO2 efflux from the adjacent terrestrial environment. Stream carbon flux to the atmosphere appeared to be transport limited (i.e., controlled by flow characteristics, turbulent flow, and water velocity) in the upper reaches of the stream, and source limited (i.e., controlled by CO2 and carbon availability) in the lower reaches of the stream. A 4‐m waterfall along the channel accounted for up to 35% of the total evasion observed along a 250‐m stream reach. These findings represent a first step in understanding ecosystem carbon cycling at the interface of terrestrial and aquatic ecosystems in high‐altitude, tropical, headwater catchments. Plain Language Summary: Mountain streams are a critical part of the global carbon cycle. Multiple studies have highlighted this importance in different regions around the globe, and some have shown that mountain streams are a significant source of carbon dioxide emissions. The Tropical Andes are unique because of proximity to the equator, high elevation, and high rainfall. We studied a high‐altitude, tropical grassland ecosystem known as a "páramo," which contains some of the highest carbon stocks on Earth. Our results demonstrate that hydrologic connectivity between wetlands and streams is critical for the movement of carbon downstream and also for the emissions of carbon to the atmosphere. Our study highlights that hydrologic dynamics play an important role in determining the amount and the fate of dissolved carbon within the stream as well as the spatial differences in carbon emissions across the aquatic‐terrestrial interface. These results represent a first step in understanding ecosystem‐level carbon cycling between wetland and stream ecosystems in high‐altitude, tropical, headwater catchments. Key Points: Dissolved CO2 from a high‐altitude peatland was carried into the stream and evaded to the atmosphere within the first ~150 m of the channelStream CO2 evasion was controlled mainly by geomorphic and hydrologic variablesAquatic CO2 fluxes were up to an order of magnitude greater than terrestrial CO2 fluxes in adjacent landscape positions [ABSTRACT FROM AUTHOR]

Urbanization growth may alter the hydrologic conditions and processes driving carbon concentrations in aquatic systems through local changes in land use. Here, we explore dissolved carbon concentrations (DIC and DOC) along urbanization gradient in Santa Catarina Island to evaluate potential increase of CO2 in streams. Additionally, we assessed chemical, physical, and biotic variables to evaluate direct and indirect effects of urbanization in watersheds. We defined 3 specific urbanization levels: high (> 15% urbanized area), medium (15–5% urbanized area), and low (< 5% urbanized area) urbanization. The results showed that local changes due to growth of urban areas into watersheds altered the carbon concentrations in streams. DOC and DIC showed high concentrations in higher urbanization levels. The watersheds with an urban building area above 5% showed pCO2 predominantly above the equilibrium with the atmosphere. These findings reveal that local modifications in land use may contribute to changes in global climate by altering the regional carbon balance in streams. [ABSTRACT FROM AUTHOR]

Heat fluxes at the lake surface play an integral part in determining the energy budget and thermal structure in lakes, including regulating how lakes respond to climate change. We explore patterns in turbulent heat fluxes, which vary across temporal and spatial scales, using in situ high‐frequency monitoring data from 45 globally distributed lakes. Our analysis demonstrates that some of the lakes studied follow a marked seasonal cycle in their turbulent surface fluxes and that turbulent heat loss is highest in larger lakes and those situated at low latitude. The Bowen ratio, which is the ratio of mean sensible to mean latent heat fluxes, is smaller at low latitudes and, in turn, the relative contribution of evaporative to total turbulent heat loss increases toward the tropics. Latent heat transfer ranged from ~ 60% to > 90% of total turbulent heat loss in the examined lakes. The Bowen ratio ranged from 0.04 to 0.69 and correlated significantly with latitude. The relative contributions to total turbulent heat loss therefore differ among lakes, and these contributions are influenced greatly by lake location. Our findings have implications for understanding the role of lakes in the climate system, effects on the lake water balance, and temperature‐dependent processes in lakes. [ABSTRACT FROM AUTHOR]

Alpine swamp meadows on the Tibetan Plateau, with the highest soil organic carbon content across the globe, are extremely vulnerable to climate change. To accurately and continually quantify the gross primary production (GPP) is critical for understanding the dynamics of carbon cycles from site-scale to global scale. Eddy covariance technique (EC) provides the best approach to measure the site-specific carbon flux, while satellite-based models can estimate GPP from local, small scale sites to regional and global scales. However, the suitability of most satellite-based models for alpine swamp meadow is unknown. Here we tested the performance of four widely-used models, the MOD17 algorithm (MOD), the vegetation photosynthesis model (VPM), the photosynthetic capacity model (PCM), and the alpine vegetation model (AVM), in providing GPP estimations for a typical alpine swamp meadow as compared to the GPP estimations provided by EC-derived GPP. Our results indicated that all these models provided good descriptions of the intra-annual GPP patterns ( R2>0.89, P<0.0001), but hardly agreed with the inter-annual GPP trends. VPM strongly underestimated the GPP of alpine swamp meadow, only accounting for 54.0% of GPP_EC. However, the other three satellite-based GPP models could serve as alternative tools for tower-based GPP observation. GPP estimated from AVM captured 94.5% of daily GPP_EC with the lowest average RMSE of 1.47 g C m-2. PCM slightly overestimated GPP by 12.0% while MODR slightly underestimated by 8.1% GPP compared to the daily GPP_EC. Our results suggested that GPP estimations for this alpine swamp meadow using AVM were superior to GPP estimations using the other relatively complex models. [ABSTRACT FROM AUTHOR]

'Blue Carbon', which is carbon captured by marine living organisms, has recently been highlighted as a new option for climate change mitigation initiatives. In particular, coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long-term sequestration of carbon. However, the direct contribution of Blue Carbon to the uptake of atmospheric CO2 through air-sea gas exchange remains unclear. We performed in situ measurements of carbon flows, including air-sea CO2 fluxes, dissolved inorganic carbon changes, net ecosystem production, and carbon burial rates in the boreal (Furen), temperate (Kurihama), and subtropical (Fukido) seagrass meadows of Japan from 2010 to 2013. In particular, the air-sea CO2 flux was measured using three methods: the bulk formula method, the floating chamber method, and the eddy covariance method. Our empirical results show that submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO2. This finding is contrary to the conventional perception that most near-shore ecosystems are sources of atmospheric CO2. The key factor determining whether or not coastal ecosystems directly decrease the concentration of atmospheric CO2 may be net ecosystem production. This study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO2. [ABSTRACT FROM AUTHOR]

Extreme events such as heat waves have increased in frequency and duration over the last decades. Under future climate scenarios, these discrete climatic events are expected to become even more recurrent and severe. Heat waves are particularly important on rocky intertidal shores, one of the most thermally variable and stressful habitats on the planet. Intertidal mussels, such as the blue mussel Mytilus edulis, are ecosystem engineers of global ecological and economic importance, that occasionally suffer mass mortalities. This study investigates the potential causes and consequences of a mass mortality event of M. edulis that occurred along the French coast of the eastern English Channel in summer 2018. We used an integrative, climatological and ecophysiological methodology based on three complementary approaches. We first showed that the observed mass mortality (representing 49 to 59% of the annual commercial value of local recreational and professional fisheries combined) occurred under relatively moderate heat wave conditions. This result indicates that M. edulis body temperature is controlled by non-climatic heat sources instead of climatic heat sources, as previously reported for intertidal gastropods. Using biomimetic loggers (i.e. 'robomussels'), we identified four periods of 5 to 6 consecutive days when M. edulis body temperatures consistently reached more than 30 °C, and occasionally more than 35 °C and even more than 40 °C. We subsequently reproduced these body temperature patterns in the laboratory to infer M. edulis thermal tolerance under conditions of repeated heat stress. We found that thermal tolerance consistently decreased with the number of successive daily exposures. These results are discussed in the context of an era of global change where heat events are expected to increase in intensity and frequency, especially in the eastern English Channel where the low frequency of commercially exploitable mussels already questions both their ecological and commercial sustainability. [ABSTRACT FROM AUTHOR]