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Norsminde Fjord has received extensive geoarchaeological investigation, hosting one of the classic Stone Age shell midden sites in Denmark, and one of the best examples of the widespread oyster decline at the Mesolithic–Neolithic transition. Here, intra-shell δ18O (and δ13C) analyses from the common periwinkle Littorina littorea (L.) are used to infer inter-annual environmental changes at the Mesolithic–Neolithic transition (four from each period). This study utilises a modern δ18O L. littorea-salinity training set previously developed for the Limfjord, Denmark to quantify winter salinity. δ18O values range between +1.6% and +4.0% in the late Mesolithic and ‒6.3% to +2.0% in the early Neolithic. Using maximum δ18O values, winter salinity at the known temperature of growth cessation in L. littorea (i.e. +3.7 ± 1°C) for the first annual cycle of each shell ranges between 25.5 and 26.8 psu (standard deviation (SD): 0.56) for the late Mesolithic, with an average salinity of 26.1 psu. Early-Neolithic shells range between 19.4 and 28.2 psu (SD: 4.59) with an average salinity of 23.7 psu. No statistically significant change in salinity occurs between the late Mesolithic and early Neolithic. This result supports recent diatom/mollusc-based inferences that salinity was not the sole cause of the oyster decline, although some evidence is presented here for more variable seasonal salinity conditions in the early Neolithic, which (along sedimentary change and temperature deterioration) might have increased stress on oyster populations in some years. It is recommended here that for robust palaeoenvironmental inferences, where possible, multiple specimens should be used from the same time period in conjunction with multiproxy data.

The large‐scale shifts in the salinity of the Baltic Sea over the Holocene are well understood and have been comprehensively documented using sedimentary proxy records. More recent work has focused on understanding how past salinity fluctuations have affected other ecological parameters (e.g. primary productivity, nutrient content) of the Baltic basin, and salinity changes over key events and over short time scales are still not well understood. The International Ocean Drilling Program Expedition 347 cored the Baltic basin in order to collect basin‐wide environmental records through a glacial–interglacial cycle. Site M0059 is located in the Little Belt between the Baltic Sea and the Atlantic Ocean. A composite splice section from Site M0059 was analysed at a decadal resolution to study changes in salinity, nutrient conditions and other surface water column parameters based on changes in diatom assemblages and on quantitative diatom‐based salinity inferences. A mesotrophic slightly brackish assemblage is seen in the lowermost analysed depths, corresponding to 7800–7500 cal. a BP. An increase in salinity and nutrient content of the water column leads into a meso‐eutrophic brackish phase. The observed salinity increase is rapid, lasting from 7500 to 7150 cal. a BP. Subsequently, the Little Belt becomes oligotrophic and is dominated by tychopelagic diatoms from c. 7100 to c. 3900 cal. a BP. This interval contains some of the highest salinities observed followed by diatom assemblages similar to those of the Northern Atlantic Ocean, composed primarily of cosmopolitan open ocean marine diatoms. A return to tychopelagic productivity is seen from 3850 to 980 cal. a BP. Anthropogenic eutrophication is detected in the last 300 years of the record, which intensifies in the uppermost sediments. These results represent the first decadally resolved record in the region and provide new insight into the transition to a brackish basin and subsequent ecological development.

Sediment trapping is a widely accepted technique in lake studies for analyzing seasonal limnological events and can provide insight into ecological succession as well as the seasonal dynamics of organic and inorganic fluxes. More recently, organic flux measurement from traps has been especially important in estimating whole‐lake C sequestration as a basis for regional and global upscaling of C budgets across lake types. However, in‐trap mineralization or dissolution of components of collected sediment (seston) has not been systematically examined, and thus a reliable correction factor for in‐trap losses of various sedimentary fluxes (especially those involving organic carbon) is still unknown. This experimental study assesses the loss of algal biomass representative of a 6‐month carousel‐type (closed) sediment trap deployment in a deep, eutrophic lake under cold (~ 5°C) and anoxic ambient conditions typical of the hypolimnion in stratifying, temperate lakes. Results show a loss of organic matter (OM) at a consistent rate over 180 d, reducing the fraction of initial OM content by approximately a third after 180 d of deployment (linear regression of OM fraction loss = −0.001864t ). The significance of these findings is demonstrated by application to published trap data; at Rostherne Mere, UK, which implies that annual OM fluxes are underestimated by 18.2% on average (range 13.7–23.2%). This highlights the far‐reaching implications for lake sediment trap methodology and our understanding of seston taphonomy, suggesting a mineralization correction factor for OM should be applied to traps deployed for longer than 1 week. With loss correction factored in, this study supports the reliability of longer‐term (i.e., ~ 6 months) sediment trap deployment.

1. Nutrient availability and climate have substantial effects on the structure and function of lakes. Predicted changes to climate (particularly temperature) over the 21st century are expected to adjust physical lake functions, changing thermal and nutrient use processes. Both increasing anthropogenic nutrient inputs and net reductions following remediation will also drive ecological change. Therefore, there is an increasing necessity to disentangle the effects of nutrient and temperature change on lakes to understand how they might act in additive and antagonistic ways. 2. This study quantified internal and external nutrient loads at Rostherne Mere, U.K., a deep (zmax = 30 m), monomictic eutrophic lake (average annual total phosphorus >100 μg/L) that has a long, stable period of stratification (c. 8.5 months). A lake biophysical model (PROTECH) was used to assess the effect of changes in these loads and climate change on lake productivity in a factorial modelling experiment. 3. During the summer, phosphorus released from the sediment is largely restricted to the hypolimnion and phytoplankton production is supported by the external load. On overturn, phosphorus at depth is distributed throughout the water column with the elevated concentration persisting to support algal productivity in the following spring. 4. Consequently, the model showed that internal nutrient loading was the main driver of current and future changes in the concentration of phosphorus (responsible for up to 86% P reduction), phytoplankton chlorophyll a and cyanobacterial blooms. However, although the external phosphorus load had a relatively small influence on annual mean phosphorus concentration, it had a statistically significant effect on chlorophyll a concentration, because it supported algal production during summer stratification. 5. Climate had minimal direct impact, but a substantial indirect impact by altering the timing, depth and length of lake stratification (c. 14 days longer by 210

River impoundment constitutes one of the most important anthropogenic impacts on the World’s rivers. An increasing number of studies have tried to quantify the effects of river impoundment on riverine ecosystems over the past two decades, often focusing on the effects of individual large reservoirs. This study is one of the first to use a large-scale, multi-year diatom dataset from a routine biomonitoring network to analyse sample sites downstream of a large number of water supply reservoirs (n = 77) and to compare them with paired unregulated control sites. We analysed benthic diatom assemblage structure and a set of derived indices, including ecological guilds, in tandem with multiple spatio-temporal variables to disclose patterns of ecological responses to reservoirs beyond the site-specific scale. Diatom assemblage structure at sites downstream of water supply reservoirs was significantly different to control sites, with the effect being most evident at the regional scale. We found that regional influences were important drivers of differences in assemblage structure at the national scale, although this effect was weaker at downstream sites, indicating the homogenising effect of river impoundment on diatom assemblages. Sites downstream of reservoirs typically exhibited a higher taxonomic richness, with the strongest increases found within the motile guild. In addition, Trophic Diatom Index (TDI) values were typically higher at downstream sites. Water quality gradients appeared to be an important driver of diatom assemblages, but the influence of other abiotic factors could not be ruled out and should be investigated further. Our results demonstrate the value of diatom assemblage data from national-scale biomonitoring networks to detect the effects of water supply reservoirs on instream communities at large spatial scales. This information may assist water resource managers with the future implementation of mitigation measures such as setting environmental flow t