Regime shifts, trends, and variability of lake productivity at a global scale

The transition from macrophyte-dominated to algae-dominated lake systems enhances arsenic release from sediments

Declining macrophytes in eutrophic lakes are altering material cycling in sediments. However, the transformation of arsenic (As) in response to these changes remains poorly understood. In this study, high-resolution dialysis was used to measure dissolved As in sediments from macrophyte-dominated (MD) and algae-dominated (AD) zones across different seasons. The relationship between sedimentary As fractionation and environmental variations was analyzed, and the As transformation process was explored. Results showed that the shift from macrophyte- to algae-dominated zones enhanced As release in sediments. Dissolved As in pore water of AD peaked at 120.36 μg/L in summer, exhibiting the highest release intensity, while MD showed a notable As release profile in spring (34.92 μg/L). In spring, decomposition and acidification of macrophyte residues, along with organic matter (OM) complexation, promoted the release of adsorbed As in MD. In contrast, reduction and dissolution of iron (Fe) oxides, along with competition for adsorption sites by dissolved phosphorus (P), drove As release in AD during summer. The high humification and low redox potential in MD sediments in summer promoted As-S co-precipitation, leading to As sequestration instead of release, this contrasts with the common view that warmer temperatures favor As release from sediments. The conversion from macrophytes to algae in eutrophic lakes may exacerbate the risk of As release, warranting further investigation.

Abrupt changes in algal biomass of thousands of US lakes are related to climate and are more likely in low-disturbance watersheds

Climate change is predicted to intensify lake algal blooms globally and result in regime shifts. However, observed increases in algal biomass do not consistently correlate with air temperature or precipitation, and evidence is lacking for a causal effect of climate or the nonlinear dynamics needed to demonstrate regime shifts. We modeled the causal effects of climate on annual lake chlorophyll (a measure of algal biomass) over 34 y for 24,452 lakes across broad ecoclimatic zones of the United States and evaluated the potential for regime shifts. We found that algal biomass was causally related to climate in 34% of lakes. In these cases, 71% exhibited abrupt but mostly temporary shifts as opposed to persistent changes, 13% had the potential for regime shifts. Climate was causally related to algal biomass in lakes experiencing all levels of human disturbance, but with different likelihood. Climate causality was most likely to be observed in lakes with minimal human disturbance and cooler summer temperatures that have increased over the 34 y studied. Climate causality was variable in lakes with low to moderate human disturbance, and least likely in lakes with high human disturbance, which may mask climate causality. Our results explain some of the previously observed heterogeneous climate responses of lake algal biomass globally and they can be used to predict future climate effects on lakes.

Carbon sinks associated with biological carbon pump in karst surface waters: Progress, challenges, and prospects

The biological carbon pump (BCP) associated with aquatic photosynthesis in karst surface waters converts dissolved inorganic carbon (DIC) into organic carbon. In the context of global climate change, BCP could be an important carbon sink mechanism, ultimately regulating atmospheric carbon dioxide (CO2) and mitigating climate change. Because of the high DIC and pH, and low dissolved CO2 [CO2 (aq)], the hydrochemical characteristics of karst surface water bodies cause C limitation in BCP efficiency. The effect of CO2 fertilization on water bodies can promote autochthonous production, thereby creating carbon sinks in such water bodies. The significant sink-enhancement potential of BCP in karst surface water bodies has attracted widespread attention. The stability of the autochthonous organic carbon (AOC) produced by BCP in karst aquatic ecosystems is key to the formation of long-term carbon sinks by carbonate weathering. In this review, we summarize recent progress in the carbonate weathering of carbon sinks in karst surface waters with coupled BCPs. Furthermore, we elucidated the possibility of using CO2 (aq) fertilization to achieve carbon sinks and its mechanism of action. On this basis, we propose three processes and mechanisms that could affect AOC stability and outline the challenge of accurately estimating carbonate weathering carbon sinks associated with BCP in karst surface waters. Our comprehensive analyses facilitated the identification of the role of karst surface aquatic ecosystems in the global carbon cycle by providing a reference and scientific basis.

Heterotrophic denitrification enhancement via effective organic matter degradation driven by suitable iron dosage in sediment

The control of internal pollution was important throughout the restoration of the lake, especially the removal of sediment internal nitrogen. Experiments involving incubation were conducted in this study to investigate the effects of iron remediation on nitrogen in both water and sediment. Adding iron with varying dosage had different effects on the nutrients content and other properties of water and sediment in remediation. The higher the addition dosage of iron, the more iron ions were released into the interstitial and overlying water. The effect of 5% and 10% iron dosage on the interstitial and overlying water were more obvious, which can significantly increase the pH and decrease the ORP of the sediment, and significantly increase the TN and NH4+-N contents in overlying water. Nevertheless, higher iron addition dosage decreased relative abundance of the genera related to denitrification (Thiobacillus) and DNRA (Bacillus). The relative abundance of Anaerolineae was increased with the iron addition dosage, promoted the reduction of organic matter and iron cycle in sediment. The iron addition dosage of 2% had less effect on the overlying water quality, and promoted the nitrogen removal process by changing the abundance of microorganisms related to the sediment nitrogen cycle. This study provides essential information for internal pollution control of lakes and serves as a valuable reference for developing eutrophication management framework.

Unveiling the global dynamics of dissolved organic carbon in aquatic ecosystems: Climatic and anthropogenic impact, and future predictions

Dissolved organic carbon (DOC) and its biodegradability (BDOC%) in aquatic ecosystems significantly impact the global carbon cycle, varying greatly across rivers, lakes, and estuaries due to environmental and anthropogenic factors. However, a thorough understanding of these variations is still lacking. This study investigated the interactions between climate, hydrology, physiography, soil, land cover, and human activity on DOC dynamics in rivers, lakes, and estuaries. Utilizing a robust dataset comprising 744 global data points for DOC concentrations (0.18-29.33 mg/L) and 341 samples for BDOC% (0.44 %-81.12 %), spanning a wide range of geographic and climatic gradients across six continents, machine learning techniques were employed to elucidate the relationships between DOC and BDOC% and environmental and anthropogenic factors and to develop predictive models for global DOC and BDOC storage. Results showed that climate primarily affected DOC and BDOC% levels, with other factors varying by ecosystem type. In rivers, soil and human activity had positive influences, while in lakes, hydrology had a positive effect and human activity had a negative one. In estuaries, soil positively impacted the levels of DOC and BDOC%, whereas human activity had a negative effect. Furthermore, we created separate random forest models for DOC and BDOC% based on different factors in each aquatic ecosystem (R2 = 0.50-0.89), and applied to data of environmental and anthropogenic factors worldwide, predicting DOC and BDOC storage for 181 countries. Notably, large countries like Canada, Russia, the United States (U.S.), Brazil, and China accounted for 76.07 % and 51.56 % of the total global DOC and BDOC storage, respectively. Storage prediction models under future climate scenarios indicated significant impacts in Europe under the high fossil fuel use scenario. Thus, prioritizing high-storage, climate-vulnerable areas is essential for effective climate change strategies, aiding in the protection of aquatic ecosystems, maintaining the global carbon balance, and promoting sustainable development.

Continuous Abrupt Vegetation Shifts in the Global Terrestrial Ecosystem

Previous studies have primarily focused on single abrupt shifts; however, the actual ecosystem will experience continuous abrupt shifts (CAS), including different directions shifts (DDS) and same direction shifts (SDS). The patterns and drivers of these CAS remain unclear. We examined the patterns of the DDS and SDS by two vegetation datasets and then tested climate drivers comprising atmospheric temperature (MAT), atmospheric precipitation (MAP), soil temperature (ST) and soil water content (SW); finally, hysteresis effects were examined with reference to principal drivers. The results demonstrate that the DDS and SDS varied across climatic regions. The ST, SW, MAT and MAP were the primary drivers of the DDS, while the MAT and MAP were the primary drivers of the SDS. Furthermore, the presence of hysteresis effects was validated via the DDS. This study presents the widespread occurrence of the CAS and the divergent roles of climate change on the DDS and SDS globally.

Relationship between phosphorus stoichiometric homeostasis and deepwater adaptability of submerged macrophytes in Erhai Lake, China: Insights from allometric plasticity

The state transition theory suggests that the decline of submerged macrophytes in shallow lakes is closely associated with reduced stoichiometric homeostasis, particularly phosphorus homeostasis (HP). The degradation typically progresses from deeper to shallower regions, indicating a potential positive correlation between the deepwater adaptability (DA) and HP values of submerged macrophytes. Here, we investigated the distribution pattern of submerged macrophytes across different water depths of Erhai Lake to test this hypothesis. The results revealed a significant positive correlation between the DA and HP values of submerged macrophytes. Allometric analysis indicated that the morphological plasticity of submerged macrophytes was linked to their HP. Species with higher HP values, like Potamogeton maackianus, had robust plasticity strategies, particularly "real plasticity", that enabled them to cope with deeper water stress. In contrast, species with lower HP values (Ceratophyllum demersum and Hydrilla verticillata) experienced nutrient declines, which hindered their adaptation. Additionally, species with higher HP values exhibited closer connections within the plant traits-environment network, indicating that their morphological plasticity adjustments allow better adaptation to the environmental changes caused by increasing water depth. These results confirm the relationship between DA and HP in submerged macrophytes and explain the mechanisms underlying the correlation, thus expanding regime shift theory.

Effects of carbon limitation and carbon fertilization on karst lake-reservoir productivity

Nitrogen and phosphorus are universally recognized as limiting elements in the eutrophication processes affecting the majority of the world's lakes, reservoirs, and coastal ecosystems. However, despite extensive research spanning several decades, critical questions in eutrophication science remain unanswered. For example, there is still much to understand about the interactions between carbon limitation and ecosystem stability, and the availability of carbon components adds significant complexity to aquatic resource management. Mounting evidence suggests that aqueous CO2 could be a limiting factor, influencing the structure and succession of aquatic plant communities, especially in karstic lake and reservoir ecosystems. Moreover, the fertilization effect of aqueous CO2 has the potential to enhance carbon sequestration and phosphorus removal. Therefore, it is important to address these uncertainties to achieve multiple positive outcomes, including improved water quality and increased carbon sinks in karst lakes and reservoirs.

Time-series-analysis-based detection of critical transitions in real-world non-autonomous systems

Real-world non-autonomous systems are open, out-of-equilibrium systems that evolve in and are driven by temporally varying environments. Such systems can show multiple timescale and transient dynamics together with transitions to very different and, at times, even disastrous dynamical regimes. Since such critical transitions disrupt the systems' intended or desired functionality, it is crucial to understand the underlying mechanisms, to identify precursors of such transitions, and to reliably detect them in time series of suitable system observables to enable forecasts. This review critically assesses the various steps of investigation involved in time-series-analysis-based detection of critical transitions in real-world non-autonomous systems: from the data recording to evaluating the reliability of offline and online detections. It will highlight pros and cons to stimulate further developments, which would be necessary to advance understanding and forecasting nonlinear behavior such as critical transitions in complex systems.

Leveraging Bayesian network to reveal the importance of water level in a shallow lake ecosystem: A study based on Paleo-diatom and fish community

Lake ecological processes and nutrient patterns are increasingly affected by water level variation around the world. Still, the long-term effects of water level change on lake ecosystems and their implications for suitable lake level management have rarely been studied. Here, we studied the ecosystem dynamics of a mesotrophic lake located in the cold and arid region of northern China based on long-term paleo-diatom and fishery records. Utilizing a novel Copula-Bayesian Network model, possible hydrological-driven ecosystem evolution was discussed. Results show that increased nutrient concentration caused by the first water level drop in the early 1980s incurred a transition of sedimental diatoms towards pollution-resistant species, and the following water level rise in the mid-1980s brought about considerable external loading, which attributed to eutrophication and caused the miniaturization of fishery structure. In the 21st century, a continuous water level plummet further reduced the sediment diatom biomass and the fish biomass by altering nutrient concentration. However, with the implementation of the water diversion project in 2011, oligotrophic species increased, and the ecosystem developed for the better. From the perspective of water quality protection requirements and the ecological well-being of Lake Hulun, the appropriate water level should be around 542.42-544.15 m. In summary, our study highlights the coupling effect of water level and water quality on Lake Hulun ecosystem and gives shed to lake water level operation and management under future climate change and human activities.

Reduced precipitation can induce ecosystem regime shifts in lakes by increasing internal nutrient recycling

Eutrophication is a main threat to continental aquatic ecosystems. Prevention and amelioration actions have been taken under the assumption of a stable climate, which needs reconsideration. Here, we show that reduced precipitation can bring a lake ecosystem to a more productive regime even with a decline in nutrient external load. By analyzing time series of several decades in the largest lake of the Iberian Peninsula, we found autocorrelated changes in the variance of state variables (i.e., chlorophyll and oxygen) indicative of a transient situation towards a new ecosystem regime. Indeed, exceptional planktonic diatom blooms have occurred during the last few years, and the sediment record shows a shift in phytoplankton composition and an increase in nutrient retention. Reduced precipitation almost doubled the water residence time in the lake, enhancing the relevance of internal processes. This study demonstrates that ecological quality targets for aquatic ecosystems must be tailored to the changing climatic conditions for appropriate stewardship.

Anthropogenic Eutrophication Drives Major Food Web Changes in Mwanza Gulf, Lake Victoria

Discerning ecosystem change and food web dynamics underlying anthropogenic eutrophication and the introduction of non-native species is necessary for ensuring the long-term sustainability of fisheries and lake biodiversity. Previous studies of eutrophication in Lake Victoria, eastern Africa, have focused on the loss of endemic fish biodiversity over the past several decades, but changes in the plankton communities over this same time remain unclear. To fill this gap, we examined sediment cores from a eutrophic embayment, Mwanza Gulf, to determine the timing and magnitude of changes in the phytoplankton and zooplankton assemblages over the past century. Biogeochemical proxies indicate nutrient enrichment began around ~ 1920 CE and led to rapid increases in primary production, and our analysis of photosynthetic pigments revealed three zones: pre-eutrophication (prior to 1920 CE), onset of eutrophication with increases in all pigments (1920-1990 CE), and sustained eutrophication with cyanobacterial dominance (1990 CE-present). Cladoceran remains indicate an abrupt decline in biomass in ~ 1960 CE, in response to the cumulative effects of eutrophication and lake-level rise, preceding the collapse of haplochromine cichlids in the 1980s. Alona and Chydorus, typically benthic littoral taxa, have remained at relatively low abundances since the 1960s, whereas the abundance of Bosmina, typically a planktonic taxon, increased in the 1990s concurrently with the biomass recovery of haplochromine cichlid fishes. Overall, our results demonstrate substantial changes over the past century in the biomass structure and taxonomic composition of Mwanza Gulf phytoplankton and zooplankton communities, providing a historical food web perspective that can help understand the recent changes and inform future resource management decisions in the Lake Victoria ecosystem.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10021-024-00908-x.

Using Knowledge-Guided Machine Learning To Assess Patterns of Areal Change in Waterbodies across the Contiguous United States

Lake and reservoir surface areas are an important proxy for freshwater availability. Advancements in machine learning (ML) techniques and increased accessibility of remote sensing data products have enabled the analysis of waterbody surface area dynamics on broad spatial scales. However, interpreting the ML results remains a challenge. While ML provides important tools for identifying patterns, the resultant models do not include mechanisms. Thus, the "black-box" nature of ML techniques often lacks ecological meaning. Using ML, we characterized temporal patterns in lake and reservoir surface area change from 1984 to 2016 for 103,930 waterbodies in the contiguous United States. We then employed knowledge-guided machine learning (KGML) to classify all waterbodies into seven ecologically interpretable groups representing distinct patterns of surface area change over time. Many waterbodies were classified as having "no change" (43%), whereas the remaining 57% of waterbodies fell into other groups representing both linear and nonlinear patterns. This analysis demonstrates the potential of KGML not only for identifying ecologically relevant patterns of change across time but also for unraveling complex processes that underpin those changes.

National-scale remotely sensed lake trophic state from 1984 through 2020

Lake trophic state is a key ecosystem property that integrates a lake's physical, chemical, and biological processes. Despite the importance of trophic state as a gauge of lake water quality, standardized and machine-readable observations are uncommon. Remote sensing presents an opportunity to detect and analyze lake trophic state with reproducible, robust methods across time and space. We used Landsat surface reflectance data to create the first compendium of annual lake trophic state for 55,662 lakes of at least 10 ha in area throughout the contiguous United States from 1984 through 2020. The dataset was constructed with FAIR data principles (Findable, Accessible, Interoperable, and Reproducible) in mind, where data are publicly available, relational keys from parent datasets are retained, and all data wrangling and modeling routines are scripted for future reuse. Together, this resource offers critical data to address basic and applied research questions about lake water quality at a suite of spatial and temporal scales.

Remotely sensing potential climate change tipping points across scales

Potential climate tipping points pose a growing risk for societies, and policy is calling for improved anticipation of them. Satellite remote sensing can play a unique role in identifying and anticipating tipping phenomena across scales. Where satellite records are too short for temporal early warning of tipping points, complementary spatial indicators can leverage the exceptional spatial-temporal coverage of remotely sensed data to detect changing resilience of vulnerable systems. Combining Earth observation with Earth system models can improve process-based understanding of tipping points, their interactions, and potential tipping cascades. Such fine-resolution sensing can support climate tipping point risk management across scales.

The influence of Amazon River connectivity to littoral meanders on long-term carbon accumulation: A case study of Lake Yahuarcaca

The objective of this investigation is to evaluate the recent changes in the accumulation of organic matter and carbon on the Yahuarcaca lake system, by means of a multiproxy paleolimnological study. The methodology based on lithological descriptions of 210Pb/137Cs-dated cores allowed us to infer the centennial sedimentation processes and carbon accumulation rates. Sedimentary facies, grain size, magnetic susceptibility, loss on ignition, carbonate, chlorophyll derivatives, stable isotopes of δ13C/δ15N, and carbon accumulation rate were analyzed. LANDSAT and photographic record of satellite images were used to reconstruct the historical geomorphological evolution of the Lake. Sediment cores yielded basal ages of 1827 and 1828 Common Era, representing the formation of lakes as a consequence of the Amazon meandering process. Two main paleolimnological stages were identified, with a boundary transition set at 1980-1984 Common Era, attributed to the geomorphological closure and complete lake separation from the Amazon and the onset of full lentic conditions. This inference was mainly based on both sharp increases in the sedimentation rate from 0.2 to >1 cm yr-1 and carbon accumulation that increased seven-fold (from 2 to 14 g m-2 yr-1) from 1980 to 1984 Common Era. The flood-pulse and connection to the Amazon defined the magnitude of organic inputs, where areas more distant/isolated from the river showed higher accumulation of carbon from autochthonous production, with an average of 8.9 % and 1.10 g m-2 yr-1 (carbon accumulation rate). Those areas closer and connected to the river were strongly related to the interannual hydrological variability, with a lower mean carbon content (5.9 %) and 0.73 g m-2 yr-1 (carbon accumulation rate). We concluded that carbon burial was highest within the most distant spot from the Amazon River because of the weaker connection to the river itself and the more stable lentic conditions for net sedimentation.

Persistent response of the bottom free-living bacteria to typhoon events in a subtropical reservoir

Typhoon-induced perturbations can result in long-lasting effects on aquatic communities in subtropical lakes or reservoirs. However, the responses of bacterial communities and their related nutrient cycling to episodic typhoon events throughout the water column in deep waters remain largely unknown. Here, we conducted a four-year field study to reveal the depth-specific responses of both free-living (FL) and particle-attached (PA) bacteria to typhoon events in a subtropical deep reservoir from 2015 to 2018. By comparing the depth-specific responses of FL and PA bacteria, we found that typhoon-induced inputs of organic matter and microorganisms significantly increased FL bacterial diversity and changed FL bacterial community composition in bottom waters perhaps through the density current or undercurrent. Typhoon events had a more persistent effect on FL than PA bacterial communities, especially in bottom waters of the reservoir. Free-living bacteria were more associated with nutrient cycling in bottom waters than particle-attached bacteria. These findings provide deep understanding of how FL and PA bacteria respond to typhoon events at community level in subtropical deep reservoir and thus help us to improve reservoir management in a rapidly changing world.

Tipping points of nitrogen use efficiency in freshwater phytoplankton along trophic state gradient

Eutrophication and harmful algal blooms have severe effects on water quality and biodiversity in lakes and reservoirs. Ecological regime shifts of phytoplankton blooms are generally thought to be driven by the rapidly rising nutrient use efficiency of bloom-forming species over short periods, and often exhibit nonlinear dynamics. Regime shifts of trophic state, eutrophication, stratification, and clear or turbid waters are well-studied topics in aquatic ecology. However, information on the prevalence of regime shifts in relationships between trophic states and phytoplankton resource transfer efficiencies in ecosystems is still lacking. Here, we provided a first insight into regime shifts in nitrogen use efficiency of phytoplankton along the trophic state gradient. We explored the regime shifts of phytoplankton resource use efficiency and detected the tipping points by combining four temporal or spatial datasets from tropical to temperate zones in Asia and Europe. We first observed significant abrupt transitions (abruptness > 1) in phytoplankton nitrogen use efficiency along the trophic state gradient. The tipping point values were lower in subtropical/tropical waterbodies (mesotrophic states; TSIc: around 50) than those in temperate zones (eutrophic states; TSIc: 60-70). The regime shifts significantly reduced the primary production transfer efficiency via zooplankton (from 0.15 ± 0.03 to 0.03 ± 0.01; mean ± standard error) in the aquatic food web. Nitrogen-fixing filamentous cyanobacteria can drive eutrophication under mesotrophic state. Our findings imply that the time-window of opportunity for harmful algae prevention and control in lakes and reservoirs is earlier in subtropical/tropical regions.

Progress and opportunities in advancing near-term forecasting of freshwater quality

Near-term freshwater forecasts, defined as sub-daily to decadal future predictions of a freshwater variable with quantified uncertainty, are urgently needed to improve water quality management as freshwater ecosystems exhibit greater variability due to global change. Shifting baselines in freshwater ecosystems due to land use and climate change prevent managers from relying on historical averages for predicting future conditions, necessitating near-term forecasts to mitigate freshwater risks to human health and safety (e.g., flash floods, harmful algal blooms) and ecosystem services (e.g., water-related recreation and tourism). To assess the current state of freshwater forecasting and identify opportunities for future progress, we synthesized freshwater forecasting papers published in the past 5 years. We found that freshwater forecasting is currently dominated by near-term forecasts of water quantity and that near-term water quality forecasts are fewer in number and in the early stages of development (i.e., non-operational) despite their potential as important preemptive decision support tools. We contend that more freshwater quality forecasts are critically needed and that near-term water quality forecasting is poised to make substantial advances based on examples of recent progress in forecasting methodology, workflows, and end-user engagement. For example, current water quality forecasting systems can predict water temperature, dissolved oxygen, and algal bloom/toxin events 5 days ahead with reasonable accuracy. Continued progress in freshwater quality forecasting will be greatly accelerated by adapting tools and approaches from freshwater quantity forecasting (e.g., machine learning modeling methods). In addition, future development of effective operational freshwater quality forecasts will require substantive engagement of end users throughout the forecast process, funding, and training opportunities. Looking ahead, near-term forecasting provides a hopeful future for freshwater management in the face of increased variability and risk due to global change, and we encourage the freshwater scientific community to incorporate forecasting approaches in water quality research and management.

Assessing temporal variability of lake turbidity and trophic state of European lakes using open data repositories

Water turbidity is one of the more important water quality parameters that is strictly linked with the productivity of the lake and is commonly used as an indicator of the trophic state. However, limited field data availability across wide geographic gradients may hinder the conduction of large scale longitudinal studies. In this study, time series of lake turbidity and trophic state index (TSI) between 2002 and 2012 were obtained from the Copernicus Lake Water products to create a large longitudinal dataset of lake variables for 22 European lakes. The dataset was combined with estimates of nutrient concentrations and surface water temperature obtained from the Hydrological Predictions for the Environment (HYPE) and ERA5-Land data repositories, that were used as environmental predictors. Hence, the validity of the lake water quality parameters was tested by a) exploring their spatial and temporal variability and b) identifying associations with the environmental predictors. For this purpose, seasonal Mann-Kendall tests were applied to find significant inter-annual trends of turbidity and TSI for each lake, and generalized additive models (GAMs) were employed to identify the main parameters that shape their temporal dynamics. Although we did not find significant inter-annual changes, our findings highlighted the strong influence of seasonality and surface water temperature in defining the temporal variability patterns in most of the lakes. In addition, the importance of nutrients varied among lakes as several lakes exhibited narrow nutrient gradients reflecting relatively stable nutrient conditions during the examined period. Other lake intrinsic factors, such as local climate and biotic interactions, are important drivers of shaping turbidity and nutrient dynamics. This study highlighted the usefulness of combining lake data from large repositories in conducting large scale spatial studies as a valuable asset for future lake research and management purposes.

A Critical Gap in Seagrass Protection: Impact of Anthropogenic Off-Shore Nutrient Discharges on Deep Posidonia oceanica Meadows

In the Mediterranean, anthropogenic pressures (specifically those involving nutrient loads) have been progressively moved to deeper off-shore areas to meet current policies dealing with the protection of marine biodiversity (e.g., European Directives). However, conservation efforts devoted to protecting Posidonia oceanica and other vulnerable marine habitats against anthropogenic pressures have dedicated very little attention to the deepest areas of these habitats. We studied the remote influence of off-shore nutrient discharge on the physiology and structure of deep P. oceanica meadows located nearest to an urban sewage outfall (WW; 1 km) and an aquaculture facility (FF; 2.5 km). Light reduction and elevated external nutrient availability (as indicated by high δ¹⁵N, total N and P content and N uptake rates of seagrass tissues) were consistent with physiological responses to light and nutrient stress. This was particularly evident in the sites located up to 2.5 km from the WW source, where carbon budget imbalances and structural alterations were more evident. These results provide evidence that anthropogenic nutrient inputs can surpass critical thresholds for the species, even in off-shore waters at distances within the km scale. Therefore, the critical distances between this priority habitat and nutrient discharge points have been underestimated and should be corrected to achieve a good conservation status.