Limnology and Oceanography Letters最新文献

The combination of iron limitation and microzooplankton grazing controls phytoplankton productivity and taxonomic composition in high-nutrient low-chlorophyll (HNLC) regions. While increased productivity and diatom contribution triggered by iron enrichment support this view, direct measurements of underpinning group-specific growth and grazing rates are scarce for the Southern Ocean. To assess these rates, we conducted dilution experiments coupled to high-performance liquid chromatography and flow-cytometry in sub-Antarctic waters on and off Campbell Plateau, southeast of Aotearoa-New Zealand. Off the plateau, growth and grazing were closely balanced for all groups despite a two-fold difference between slow- and fast-growing groups. On Campbell Plateau, where HNLC conditions were alleviated, the balance was disrupted, mainly by the preferential growth of diatoms and green algae, which was stimulated beyond grazing. Our results expand the recognized ability of diatoms to escape grazing control to picoplanktonic green algae that also avoid grazing and contribute significantly to phytoplankton productivity and biomass accumulation.

Terrestrial plants are sensitive indicators of global warming because their annual cycles of growth and senescence are changing as warming proceeds. Single celled algae are distinct life forms capable of population bursts in any season, so there is uncertainty about phytoplankton phenology as a comparable indicator of global warming. We analyzed 4+ decades of monthly chlorophyll a measurements at two sites in San Francisco Bay and found abrupt shifts during summer months leading to a 48-day advance in the annual pattern of chlorophyll-a accumulation at one site and a 36-day delay at the other. These large phenological changes were not associated with changing temperature, but they were associated with changes in top–down control by bivalve filter feeders as biological communities were restructured by (1) introduction of a non-native clam, and (2) a shift in atmospheric forcing of the NE Pacific. This study illustrates that changes in phytoplankton phenology are not necessarily responses to or indicators of global warming. However, they can be indicators of human disturbances and natural climate oscillations having effects large enough to mask the effect of climate warming.

Dinitrogen (N₂) fixation provides the major source of reactive nitrogen in the open ocean, sustaining biological productivity. The Indian Ocean (IO) covers 22% of the ocean surface, while it only represents 1% of the global diazotroph database. Hence, constraining the sources of nitrogen in the IO is crucial. Here, we compile three decades of N₂ fixation and diazotroph DNA data in the IO. Our analysis reveals basin-scale yearly rates between ~ 7 and 13 Tg N yr⁻¹. These rates are in the range of previous modeling-based estimates but may represent a lower bound estimate due to the lack of data in this basin. Diazotroph variability among sub-basins may suggest endemicity but needs to be taken with caution due to biased sampling toward certain seasons and uneven spatial coverage. We provide recommendations for a more accurate representation of the IO in the global nitrogen budget and our knowledge of diazotroph biogeography.

The Gulf of Carpentaria and Torres Strait in north-eastern Australia support globally significant seagrass ecosystems that underpin fishing and cultural heritage of the region. Reliable data on seagrass distribution are critical to understanding how these ecosystems are changing, while managing for resilience. Spatial data on seagrass have been collected since the early 1980s, but the early data were poorly curated. Some was not publicly available, and some already lost. We validated and synthesized historical seagrass spatial data to create a publicly available database. We include a site layer of 48,612 geolocated data points including information on seagrass presence/absence, sediment, collection date, and data custodian. We include a polygon layer with 641 individual seagrass meadows. Thirteen seagrass species are identified in depths ranging from intertidal to 38 m below mean sea level. Our synthesis includes scientific survey data from 1983 to 2022 and provides an important evidence base for marine resource management.

Pyrosomes are colonial tunicates that form gelatinous tubes and occasionally produce bioluminescent swarms. The rapid “bloom-bust” dynamics of pyrosomes have the potential to outcompete other zooplankton, restructure marine food webs, enhance carbon export, and interfere with human activities. Pyrosomes have been recorded for at least two centuries, yet much remains unknown about their physiology, bloom mechanisms, and ecosystem impacts. Recent bloom reports prompt the question of whether frequencies or densities are increasing, and whether potential population changes are caused by ocean change. This review clarifies current information on pyrosome biology and ecology and explores apparent contradictions in habitat preferences and diel vertical migration (DVM). We posit that the dominant species, Pyrosoma atlanticum, may preferentially inhabit waters below 18°C and alter its DVM activity to maintain overall body temperature near this threshold. Expanding our knowledge of pyrosomes is needed to facilitate their inclusion in ecosystem models and forecasts of future population distributions.

Ponds are globally abundant and important to the global carbon cycle. Although ponds have large greenhouse gas emissions, they also sequester carbon in their sediments. Here, we studied organic carbon (OC) burial rates in 22 temperate experimental ponds with negligible watersheds, where carbon sequestration derives solely from autochthonous primary production. The ponds were built identically in 1964 and have since experienced different management strategies, allowing us to test how management actions influence burial rates. On average, the ponds accumulated 67.1 g OC m⁻² yr⁻¹ (range 38.3–113.6 g OC m⁻² yr⁻¹)—about double the global average burial rate for lakes and similar to global averages for wetlands. Carbon burial rates were higher in ponds with macrophytes, fish, and higher N : P loads. We contend that the global carbon sink for inland waters may be substantially underestimated due to the undervaluation of OC burial in natural and artificial ponds.

Megaripples are current-generated seafloor bedforms of well-sorted sand or gravel and wavelengths over 1 m. In this aquatic eddy covariance study, we measured large rates of benthic primary production and respiration for a shallow-water sandy megaripple field exposed to strong tidally driven currents and intense sunlight. Current and light were the main short-term drivers of a highly dynamic oxygen exchange. Daytime oxygen release as high as 300 mmol m⁻² d⁻¹ and nighttime oxygen uptake up to −100 mmol m⁻² d⁻¹ were likely sustained by current-driven transport of oxygen, nutrients, and organic matter (fuel) into and out of the sand and superimposed by rapid internal cycling. Seasonal differences in temperature (45%) and light (69%) between April and September were the main long-term drivers of substantially greater rates of gross primary production and respiration in September. The megaripples functioned as an intense metabolic hotspot with carbon cycling rates larger than those of most near-shore sediments.

Stream thermal regimes are being altered by climate change with consequences for aquatic organisms. Most documented long-term changes in stream temperature are from large rivers. We know less about water temperature trends for small headwater streams, especially those found in northern landscapes that contain small lakes. We analyzed 36 yr of stream temperature observations from a long-term watershed study in Ontario, Canada, to understand how headwater streams are responding to climate variability. We found that groundwater-fed (GWF) and lake-fed (LF) streams exhibit contrasting responses, as GWF streams warmed in the spring (0.19–0.60°C per decade) and LF streams warmed in the fall (0.39–0.72°C per decade). Both stream types exhibited weak temperature trends in summer and winter. These results highlight that a stream network perspective that includes headwater streams and small lakes, and accounts for seasonal changes in thermal regimes, is important for understanding aquatic ecosystem response to climate variability.

Originally developed for terrestrial science and applications, the US Geological Survey Landsat surface reflectance (SR) archive spanning ~ 40 yr of observations has been increasingly utilized in large-scale water-quality studies. These products, however, have not been rigorously validated using in situ measured reflectance. This letter quantifies and demonstrates the quality of the SR products by harnessing a sizeable global dataset (N = 1100). We found that the Landsat 8/9 SR in the green and red bands marginally meet the targeted accuracy requirements (30%), whereas the uncertainties in the blue and coastal-aerosol bands ranged from 48% to 110%. We further observed > +25% biases in the visible bands of Landsat 5/7 SR, which can introduce an apparent downward trend when applied in time-series analyses combined with Landsat 8/9. Users must exercise caution when using this archive for trend analyses, and progress in atmospheric correction is required to foster advanced applications of the Landsat archive for aquatic science.