Limnology and Oceanography最新文献

The nutrient filter function is an important ecosystem service of seagrass meadows that mitigates the consequences of coastal eutrophication. In northeast Hainan in China, large seagrass areas were lost due to chronic eutrophication induced by untreated pond aquaculture effluents. However, in adjacent areas, seagrasses could survive due to seasonal exposure (i.e., not chronic) to eutrophication only. In a way, the conditions in these areas represent a transitional environment which allows investigating the effect of eutrophication on seagrass performance and their nitrogen uptake capacity. We tested how a 4-week in situ nutrient enrichment affected inorganic nitrogen uptake rates of a multispecies seagrass meadow in eutrophic and non-eutrophic seasons, in light and in darkness. All species maintained nitrogen uptake in the dark and preferred ammonium over nitrate. In the eutrophic season, the seagrass leaf biomass and growth were lower resulting in a lower nitrogen filter capacity. Among the species present, Cymodocea rotundata and Cymodocea serrulata covered 48% and 45%, respectively, of their daily nitrogen demand for leaf growth through leaf uptake from the water column, while it was only 30% for Thalassia hemprichii, the last remaining species in meadows degraded by eutrophication, as deduced from previous studies. It indicates that a multispecies seagrass meadow has a higher nitrogen filter capacity than a monospecific T. hemprichii meadow. By reducing seagrass diversity and, hence, the nitrogen filter function, eutrophication triggers a self-reinforcing process. Once the nitrogen filtering capacity of a seagrass meadow is exhausted, further eutrophication and seagrass loss are expected.

The important role mangrove forests play in sequestering organic carbon is well known, yet rates of organic carbon accumulation in macro-tidal mangrove ecosystems are poorly resolved. Here we use ²¹⁰Pb dating to present a 125-yr record of carbon, nutrient and trace metal accumulation in sediments from an Amazon macro-tidal mangrove forest. We find that the rate of organic carbon accumulation ranged from 23.7 to 74.7 g m⁻² yr⁻¹ (average 38 ± 13.5 g m⁻² yr⁻¹), significantly lower than global averages for mangrove forests. These low rates may be associated with sediment grain-size and sediment–water interface processes that drive organic matter oxidation and reduce carbon stocks in these highly dynamic macro-tidal forests. Total nitrogen accumulation ranged from 1.4 to 5.1 g m⁻² yr⁻¹ (average 2.7 ± 0.9 g m⁻² yr⁻¹) and phosphorus from 1.5 to 8.4 g m⁻² yr⁻¹(average 4.3 ± 1.9 g m⁻² yr⁻¹). Trace metal accumulation rates (As, Pb, Cr, Cu, Mn, Ni, Zn, Hg, Bo, V, Co, Mo, S, and Ba) were also lower than other tropical mangrove forests globally, but trace metal in more recent sediments for Mn, As, Cu, and Hg were elevated, likely reflecting human footprint in the region since early the 20^th century. The ability to accurately quantify carbon accumulation rates in mangrove ecosystems is critical for climate change mitigation strategies and the implementation of carbon offset schemes globally.

As an important destination for upstream materials and element accumulation, lake sediments hold a multitude of contextual information about climatic changes and anthropogenic disturbances. However, understanding the lake sedimentation state to link catchment pressures and biogenic regime shifts remains challenging. To address this, research was conducted on the lake sedimentation state, including the dynamics, catchment drivers, effects on biogenic regimes, and responses to typical historical events using nine sediment cores from Dongting Lake, China, a typical global priority ecoregion. Three transitions of lake sedimentation state were distinguished over the century (1937, 1968, and 1993), whereby hydrologic dynamics and land-use changes in the watershed were the direct drivers with relative contributions of 16.30% and 14.56%, respectively. Lake sedimentation state and organic matter inputs not only preceded sediment biogenic elements at the shift time but also exhibited nonlinear trigger effects on biogenic element contents (R² = 0.47, p < 0.01), which promoted an increase in sediment biogenic element burial rates. Rate of change analysis indicated that intensive human activities altered the relationship between the sedimentation state and biogenic regime shift, thus revealing the response to anthropogenic events in the catchment. Pathways quantified by the partial least squares path model established the link between watershed attributes and lake biogenic properties via the lake sedimentation state. Our findings revealed a cascading linkage among catchment eco-surroundings, lake sedimentation states, and biogenic regime shifts. The research further provided insights into driver-response relationships in lake-catchment systems.

Ocean acidification (OA) has considerably changed the metabolism and structure of plankton communities in the ocean. Evaluation of the response of the marine bacterioplankton community to OA is critical for understanding the future direction of bacterioplankton-mediated biogeochemical processes in the ocean. Understanding the diversity of functional genes is important for linking the microbial community to ecological and biogeochemical processes. However, the influence of OA on the functional diversity of bacterioplankton remains unclear. Using high-throughput functional gene microarray technology (GeoChip 4), we investigated the functional gene structure and diversity of bacterioplankton under three different pCO₂ levels (control: 175 μatm, medium: 675 μatm, and high: 1085 μatm) in a large Arctic Ocean mesocosm experiment. We observed a higher evenness of microbial functional genes under elevated pCO₂ compared with under low pCO₂. OA induced a more stable community as evaluated by decreased dissimilarity of functional gene structure with increased pCO₂. Molecular ecological networks under elevated pCO₂ became more complex and stable, supporting the central ecological tenet that complexity begets stability. In particular, increased average abundances were found under elevated pCO₂ for many genes involved in key metabolic processes, including carbon degradation, methane oxidization, nitrogen fixation, dissimilatory nitrite/nitrate reduction, and sulfide reduction processes. Altogether, these results indicate a significant influence of OA on the metabolism potential of bacterioplankton in the Arctic Ocean. Consequently, our study suggests that biogeochemical cycling mediated by these microbes may be altered by the OA in the future.

Reservoirs are important sites for nitrogen processing, especially those located in agricultural and urban watersheds. Nitrogen inputs promote N₂O production and emission, but the microbial pathways controlling N₂O have been seldom studied in reservoir water columns. We determined N₂O concentration in the water column of 12 reservoirs during the summer stratification and winter mixing. We explored the potential microbial sources and sinks of N₂O by quantifying key genes involved in ammonia oxidation (bacterial and archaeal amoA) and denitrification (nirS and nosZ). Dissolved N₂O varied up to three orders of magnitude (4.7–2441.2 nmol L⁻¹) across systems, from undersaturated to supersaturated values (37%–24,174%) depending on reservoirs and depths. N₂O concentration depended on nitrogen and oxygen availabilities, with the lowest and highest N₂O values at suboxic conditions. Ammonia-oxidizing archaea dominated over ammonia-oxidizing bacteria but were not related to the dissolved N₂O. In contrast, the abundance of the nirS gene was significantly related to N₂O concentration, and three orders of magnitude higher than amoA abundance. Denitrifying bacteria appeared consistently in the water column of all reservoirs. The nirS and nosZ genes appeared in oxic and suboxic waters, but they were more abundant in suboxic waters. The nitrate concentration, and nirS and nosZ relative abundances explained the dissolved N₂O. Besides, nirS abundance was related positively with total phosphorus and cumulative chlorophyll a, a proxy for fresh organic matter. Therefore, P inputs, not just N inputs, promoted N₂O production by denitrification in the water column of reservoirs.