Limnology and Oceanography Letters最新文献

Nearshore environments are typically supersaturated with the potent greenhouse gases methane and carbon dioxide, due to intense remineralization of the elevated supply of organic carbon in these systems. These environments are characterized by overlapping biogeochemical gradients and heterogeneous morphology, and the overall spatial variability in nearshore greenhouse gas concentrations remains unclear. We measured surface water partial pressures of carbon dioxide and methane synoptically with water quality parameters in the coastal Baltic Sea, covering two ice-free seasons. The high-frequency flow-through data revealed sites with recurring very high partial pressures of carbon dioxide and methane (i.e., hot spots) scattered around the 50 km × 40 km study area, exceeding overall partial pressure averages by 455 μatm (CH₄) and 2396 μatm (CO₂). High partial pressures were linked with elevated inputs of allochthonous and autochthonous organic matter, underpinning the major role of organic enrichment of coastal environments in global carbon cycling.

Vertical motion is an important driver of sunlight exposure in aquatic environments, shaping the growth and fate of materials and organisms. We derive a simple model accounting for turbulent depth fluctuations of particles to predict the depth that contributes the most sunlight exposure (effective depth) as well as the single depth that, if measured at one place over time, produces the same total sunlight exposure as a moving particle (functional depth). Field measurements of light and depth in rivers using neutrally buoyant drifters and buoys validate our model. Effective depth varied from 0.1 to 1.5 m below the water surface and was ~ 30% of the overall water depth on average. Functional depth varied from 0.67 to 2.3 m and was ~ 50% of the overall water depth on average. Functional and effective depth are physically based concepts incorporating turbulent motion, spatial variability, and water clarity offering new approaches to characterize light exposure in aquatic environments.

Urbanization drives multiple environmental changes that influence critical ecosystem processes. Factors such as salinization by deicing road salts, reduced water clarity (and greater light attenuation) from eutrophication and sediment loading, and warming constrain not only the biodiversity of ponds, but also their physical mixing (with consequences for oxygen availability and the provision of ecosystem services). Leveraging an extensive urban gradient in the Greater Toronto Area, we collected summertime depth profiles from 50 stormwater retention ponds to investigate their vertical stratification. We found that water columns were generally stratified but contrary to expectations, we found relatively minor roles of basin area and depth. Instead, we discovered an overwhelming effect of salinity along with significant impacts of temperature and water clarity on water density gradients. Findings extend our fundamental understanding of mixing regimes in small, shallow waterbodies and indicate increasing risks to pond functioning in a warmer and saltier future.

Species invasions can disrupt aquatic ecosystems by re-wiring food webs. A trophic cascade triggered by the invasion of the predatory zooplankter spiny water flea (Bythotrephes cederströmii) resulted in increased phytoplankton due to decreased zooplankton grazing. Here, we show that increased phytoplankton biomass led to an increase in lake anoxia. The temporal and spatial extent of anoxia experienced a step change increase coincident with the invasion, and anoxic factor increased by 11 d. Post-invasion, anoxia established more quickly following spring stratification, driven by an increase in phytoplankton biomass. A shift in spring phytoplankton phenology encompassed both abundance and community composition. Diatoms (Bacillaryophyta) drove the increase in spring phytoplankton biomass, but not all phytoplankton community members increased, shifting the community composition. We infer that increased phytoplankton biomass increased labile organic matter and drove hypolimnetic oxygen consumption. These results demonstrate how a species invasion can shift lake phenology and biogeochemistry.

We measured dissolved organic carbon (DOC) and total dissolved amino acid (TDAA) in seawater and sediment porewater of the Ulleung Basin in the East/Japan Sea. The DOC and TDAA concentrations were 1.1- and 1.4-fold higher in the euphotic zone, and 11- and 43-fold higher in sediment porewater, respectively, than those in the deep ocean. Consequently, in the deep ocean, TDAA and DOC input fluxes from porewater were 2- and 0.4-fold of those from the euphotic zone, respectively. This larger contribution of benthic flux for TDAA and its shorter residence time in the benthic boundary layer (BBL) (1.3 ± 0.9 yr) seem to result in steep TDAA increases in the BBL, although DOC concentrations remained relatively uniform throughout the entire deep ocean. AA-derived indices also show enhanced bioavailability of dissolved organic matter in the BBL. Benthic inputs seem to supply a significant amount of bioavailable TDAA to the deep ocean, fueling microbial activity.

Coastal mountain rivers export disproportionately high quantities of terrestrial organic carbon (OC) directly to the ocean, feeding microbial communities and altering coastal ecology. To better predict and mitigate the effects of wildfires on aquatic ecosystems and resources, we must evaluate the relationships between fire, hydrology, and carbon export, particularly in the fire-prone western United States. This study examined the spatiotemporal export of particulate and dissolved OC (POC and DOC, respectively) and particulate and dissolved black carbon (PBC and DBC, respectively) from five coastal mountain watersheds following the 2020 CZU Lightning Complex Fires (California, USA). Despite high variability in watershed burn extent (20–98%), annual POC, DOC, PBC, and DBC concentrations remained relatively stable among the different watersheds. Instead, they correlated significantly with watershed discharge. Our findings indicate that hydrology, rather than burn extent, is a primary driver of post-fire carbon export in coastal mountain watersheds.

Despite the perpetual darkness of the deep sea, contrasting the sunlit epipelagic waters, many deep-sea organisms exhibit rhythmic activities. To discern environmental cues that may serve as entrainment signals for deep-sea organisms, this study investigated the soundscape of the abyssal plain south of Minamitorishima Island. Our analysis revealed clear diel and seasonal patterns, primarily driven by evening fish choruses and marine mammal vocalizations. These evening choruses, discernible above the background noise, likely serve as a circadian time cue for organisms capable of perceiving them within the aphotic depths. In addition, the frequent detection of whistles and echolocation clicks suggests this region functions as a foraging ground for marine mammals. These acoustic cues might guide organisms with auditory capabilities toward habitats rich in sinking food debris and whale falls. By elucidating the ecological processes shaping abyssal soundscape dynamics, these findings open new directions for further exploration in deep-sea chronobiology.

Mixotrophic eukaryotes are important bacterivores in oligotrophic open oceans, but their significance as grazers in more nutrient-rich waters is less clear. Here, we investigated the bacterivory partition between mixotrophs and heterotrophs in a productive, estuary-influenced coastal region in the East China Sea. We found ubiquitous, actively feeding phytoplankton populations and taxa with mixotrophic potential by identifying ingestion of fluorescent prey surrogate and analyzing community 18S rRNA gene amplicons. Potential and active mixotrophs accounted for 10–63% of the total eukaryotic community and 17–69% of bacterivores observed, respectively, contributing 6–48% of estimated in situ bacterivory. The much higher mixotroph fitness outside of the turbid plume was potentially driven by increased light and decreased nutrient availability. Our results suggest that, although heterotrophs dominated overall in situ bacterivory, mixotrophs were abundant and important bacterivores in this low-latitude mesotrophic coastal region.

Sediment macrofauna play a vital role in sustaining aquatic food webs and biogeochemical cycles. Previous research demonstrated that bioturbation indirectly affects methane (CH₄) dynamics through mobilization of porewater and alteration of microbial processes in the surrounding sediment. However, little is known on the direct contribution of macrofauna holobionts (the assemblage of invertebrate host and associated microbiome) to biogeochemical fluxes. Here, we investigated how 19 taxa of macrofauna holobionts, from different estuarine habitats spanning 40° to 63° latitude, directly contribute to CH₄ fluxes. Deep burrowing infauna and deposit feeders were responsible for the highest CH₄ production, whereas epifauna and filter feeders promoted oxidative CH₄ consumption. Among the different environmental parameters, salinity was inversely correlated with CH₄ production by macrofauna holobionts, with the process suppressed at high salinity (≥ 33). This study provides empirical evidence on how functional traits and environmental factors influence sediment invertebrates' contribution to CH₄ fluxes.