Global historical trends and drivers of submerged aquatic vegetation quantities in lakes.

Submerged aquatic vegetation (SAV) in lake littoral zones is an inland water wetland type that provides numerous essential ecosystem services, such as supplying food and habitat for fauna, regulating nutrient fluxes, stabilizing sediments, and maintaining a clear water state. However, little is known on how inland SAV quantities are changing globally in response to human activities, where loss threatens the provisioning of these ecosystem services. In this study, we generate a comprehensive global synthesis of trends in SAV quantities using time series (>10 years) in lakes and identify their main drivers. We compiled trends across methods and metrics, integrating both observational and paleolimnological approaches as well as diverse measures of SAV quantities, including areal extent, density, or abundance classes. The compilation revealed that knowledge on SAV is mostly derived from temperate regions, with major gaps in tropical, boreal, and mountainous lake-rich regions. Similar to other wetland types, we found that 41% of SAV times series are largely decreasing mostly due to land use change and resulting eutrophication. SAV is, however, increasing in 28% of cases, primarily since the 1980s. We show that trends and drivers of SAV quantities vary regionally, with increases in Europe explained mainly by management, decreases in Asia due to eutrophication and land use change, and variable trends in North America consistent with invasive species arrival. By providing a quantitative portrait of trends in SAV quantities worldwide, we identify knowledge gaps and future SAV research priorities. By considering the drivers of different trends, we also offer insight to future lake management related to climate, positive restoration actions, and change in community structure on SAV quantities.

La végétation aquatique submergée (VAS) dans la zone littorale des lacs fait partie des milieux humides des eaux continentales et soutient plusieurs services écologiques, tels que fournir un habitat pour la faune, réguler les flux de nutriments et stabiliser les sédiments. Cependant, comment les changements des quantités de VAS varient mondialement en réponse aux activités humaines demeurent peu connu, alors que leur perte menace le maintien de ces services écologiques. Dans cette étude, nous avons généré une synthèse globale des séries temporelles des quantités de VAS dans les lacs et avons identifié leurs tendances et leurs facteurs explicatifs. Nous avons compilé les tendances à travers les méthodes et les métriques, intégrant à la fois les approches observationnelles et paléolimnologiques ainsi que des mesures diverses de quantité de VAS, telles que la superficie de couverture, la densité et les classes d'abondance. La compilation a révélé que les connaissances sur la VAS proviennent surtout des régions tempérées, avec peu d'information dans les régions boréales, tropicales et montagneuses riches en lacs. Comme pour les autres types de milieux humides, nous avons trouvé que la VAS est largement en déclin, tel que reporté dans 41% des séries temporelles principalement à cause des changements d'utilisation du territoire et de l'eutrophisation. La VAS est cependant en augmentation dans 28 % des cas, surtout depuis les années 1980. Nous montrons que les tendances de la VAS et les facteurs explicatifs varient par région. En Europe, les augmentations sont expliquées principalement par la gestion, en Asie, le déclin est fonction de l'eutrophisation et des changements d'utilisation du territoire, alors qu'en Amérique du Nord les tendances variables sont associées à l'arrivée de nouvelles espèces. En fournissant un portrait quantitatif des tendances de VAS à travers le monde, nous indiquons des lacunes dans les connaissances et les futures priorités de recherche. En se penchant sur les facteurs explicatifs, nous offrons des informations sur l'influence du climat, des actions de restauration positives et du changement de la structure des communautés sur la VAS qui pourront informer les gestionnaires des lacs.

Relative influence of watershed and geomorphic features on nutrient and carbon fluxes in a pristine and moderately urbanized stream.

Streams are important sites of elemental transformations due to the relatively high contact rates between flowing water and biogeochemically reactive sediments. Increased urbanization typically results in higher nutrient and carbon (C) inputs to streams from their watersheds and increased flow rates due to modification in channel form, reducing within stream net retention and increasing downstream exports. However, less is known on how moderate urbanization might influence the joint processing of C, nitrogen (N), and phosphorus (P) in streams or the relative influence of changes in watershed and stream features on their fluxes. In this study, we performed mass-balances of different C, N, and P species in multiple reaches with contrasting land use land cover and geomorphic features (pools, riffles, runs) to determine the effects of geomorphology versus human influence on elemental fluxes in a pristine and a semi-urban stream. N was the most responsive of all elements, where nitrate concentrations were 3.5-fold higher in the peri-urban stream. Dissolved organic carbon was only slightly higher in the peri-urban site whereas total P not significantly different between streams. In terms of fluxes, nitrate behaved differently between the streams with net retention occurring in the majority of the reaches of the pristine site, whereas net export was observed in all of the reaches of the semi-urban one. We found a decrease in nitrate concentrations with an increase in excess deuterium of the water (d-excess), an indicator of how overall water retention capacity of the watershed favored N loss. Within the stream, the presence of pools, and reduced channel slope, which also increase water retention time, again favored N loss. Overall, nitrate was the most sensitive nutrient to slight urbanization, where higher export to stream was influenced by land use, but where geomorphic features were more important in driving retention capacity.

Annual nitrification dynamics in a seasonally ice-covered lake.

We investigated the variability in ammonia oxidation (AO) rates and the presence of ammonia-oxidizing archaea and bacteria (AOB and AOA) over an annual cycle in the water column of a small, seasonnally ice covered, temperate shield lake. AO, the first step of nitrification, was measured in situ using 15N-labelled ammonium (NH4+) at 1% and 10% of photosynthetic active radiation during day and at the same depths during night. AO was active across seasons and light levels, ranging from undetectable to 333 nmol L-1 d-1 with peak activity in winter under ice cover. NH4+ concentration was the single most important positive predictor of AO rates. High NH4+ concentrations and reduced chlorophyll a concentrations under ice, which favoured AO, were coherent with high nitrate concentrations and super saturation in nitrous oxide. When targeting the ammonia monooxygenase (amoA) gene in samples from the photic zone, we found AOA to be omnipresent throughout the year while AOB were observed predominantly during winter. Our results demonstrate that AO is an ongoing process in sunlit surface waters of temperate lakes and at all seasons with pronounced nitrification activity observed during winter under ice. The combination of high NH4+ concentrations due to fall overturn, reduced light availability that limited phytoplankton competition, and the presence of AOB together with AOA apparently favoured these elevated rates under ice. We suggest that lake ice could be a control point for nitrification in oligotrophic temperate shield lakes, characterized as a moment and place that exerts disproportionate influence on the biogeochemical behaviour of ecosystems.