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The melting of the Greenland Ice Sheet is accelerating, with glaciers shifting from marine to land termination and potential consequences for fjord ecosystems downstream. Monthly samples in 2016 in two fjords in southwest Greenland show that subglacial discharge from marine-terminating glaciers sustains high phytoplankton productivity that is dominated by diatoms and grazed by larger mesozooplankton throughout summer. In contrast, melting of land-terminating glaciers results in a fjord ecosystem dominated by bacteria, picophytoplankton and smaller zooplankton, which has only one-third of the annual productivity and half the CO2uptake compared to the fjord downstream from marine-terminating glaciers.

Despite great advances, experiments concerning the response of ecosystems to climate change still face considerable challenges, including the high complexity of climate change in terms of environmental variables, constraints in the number and amplitude of climate treatment levels, and the limited scope of responses and interactions covered. Drawing on the expertise of researchers from a variety of disciplines, this Perspective outlines how computational and technological advances can help in designing experiments that can contribute to overcoming these challenges, and also outlines a first application of such an experimental design.

Tidal marsh vegetation is increasingly valued for its role in ecosystem‐based coastal protection due to its wave dissipating capacity. As the efficiency of wave dissipation is known to depend on specific vegetation properties, we quantified how these morphological, biochemical, and biomechanical properties of tidal marsh vegetation are, in turn, affected by wave exposure. This was achieved by field measurements at two locations, with contrasting wave exposure, in the brackish part of the Scheldt Estuary (SW Netherlands), where Scirpus maritimusis the dominant pioneer species. Our results show that shoots from more wave‐exposed conditions developed significantly shorter and thicker stems than the ones growing in more sheltered conditions. Furthermore, we show that the more exposed shoots are more flexible whereas the shoots growing in more sheltered conditions are stiffer. This may indicate plasticity in response to wave exposure following a stress‐avoidance strategy. Increasing stiffness was shown to be related to enhanced biogenic silica and lignin contents of the shoot tissue. These properties might affect the wave‐attenuating capacity of the marsh as stiff plants are known to mitigate waves more effectively than flexible ones. However, we also found higher shoot densities on the exposed site, which may partly explain why higher relative wave attenuation rates were found on the exposed site, despite the presence of more flexible individual shoots. This study highlights that the efficiency of wave attenuation by tidal marsh vegetation ultimately depends on mutual interactions between waves and plasticity in morphological, biochemical, and biomechanical plant properties.

We investigated long‐term trends (1965–2002) in dissolved inorganic nutrients in the tidal part of the Scheldt estuary (Belgium, The Netherlands). Annually averaged concentrations of dissolved silicate (DSi), dissolved inorganic nitrogen (DIN), and phosphate (DIP) increased significantly until the mid‐1970s, after which they declined linearly at rates of 0.6, 2.9, and 0.3 µmol L‐yr‐, respectively. This co‐occurred with a deterioration followed by a restoration of water column oxic conditions. Because of the differences in the reduction rate of DSi (1.2% yr‐), DIN (1.7% yr‐), and DIP (5.4% yr21), the N: P and Si : P ratios more than doubled from 1980 to 2002. The Si :N ratio varied from 0.2 to 0.4 and was positively correlated with river discharge. The part downstream from the confluence of the main rivers was a net sink for DSi during the entire period but evolved from a net sink to a net source for DIP, while the reverse was true for DIN. This differential behavior of the estuary with respect to DIN and DIP strongly buffered the altered loadings to the upper estuary. The input of oxygen‐consuming substances at the head of the estuary triggered a sequence of oxidation reactions. In the early 1970s, high loadings of ammonium and organic matter caused oxygen depletion and intense water‐column denitrification in the upstream part and intense nitrification downstream, with a nitrate maximum succeeding a nitrite peak. With oxic conditions improving and the input of ammonium decreasing, water‐column denitrification declined, the nitrification front migrated upstream, and the estuary evolved from a net producer of nitrite to a net consumer. Now, at the beginning of the 21st century, nitrate behaves almost conservatively over the entire estuary.

Many estuaries exhibit seasonality in the estuary‐scale distribution of suspended particulate matter (SPM). This SPM distribution depends on various factors, including freshwater discharge, salinity intrusion, erodibility, and the ability of cohesive SPM to flocculate into larger aggregates. Various authors indicate that biotic factors, such as the presence of algae and their excretion of sticky transparent exopolymer particles (TEP), affect the flocculation and erosion processes. Consequently, seasonality in these biotic factors may play a role in the observed seasonality in SPM. Whereas the impact of abiotic factors on seasonality in SPM is well studied, the relative contribution of biotically induced seasonality is largely unknown. In this study, we employ two approaches to assess the aggregated importance of biotically induced seasonality in flocculation and erosion on seasonality in SPM in the Scheldt estuary. In the first approach, we focus on seasonality of in situ observations in the Scheldt estuary of turbidity, floc size, Chlorophyll‐a, and TEP, showing that the abiotic parameters show seasonality, while seasonality in TEP is ambiguous. The second approach concerns a reverse engineering method to calibrate biotically affected parameters of a coupled sediment transport‐flocculation model to turbidity observations, allowing us to compare the modeled SPM concentrations to the observations. Driven by seasonality in freshwater discharge, the model captures the observed seasonality in SPM without requiring biotically induced seasonality in flocculation and erosion, which is supported by the absence of seasonality in TEP. Estuaries often show regions in which the water is more turbid due to a local elevation of suspended sediment. This local elevation regularly displays seasonality, which is partially driven by seasonality in the erosion properties of the sediment bed and the ability of suspended sediment to flocculate into larger flocs. Two crucial factors impacting the erosion and flocculation processes are the presence of algae and the corresponding excreted sticky substances, which are reported to stabilize the sediment bed but can also glue the suspended sediment particles together. In the present study, we study the aggregated importance of such biotic factors to seasonality in suspended sediment concentrations, applied to the Scheldt estuary. We first focus on seasonality of field observations of suspended sediment concentration, floc size, algal abundance, and the sticky substances excreted by the algae, all showing seasonality except the sticky substances. Next, we use a model that simulates the erosion, flocculation, and accumulation of suspended sediment to quantify a potential seasonality in the impact of biotic factors on flocculation and erosion. Surprisingly, we do not require seasonality in the impact of biotic factors on flocculation and erosion to explain the observed seasonality in sediment dynamics, which is supported by the absence of seasonality in the sticky substances excreted by the algae. New observations over various seasons in the Scheldt estuary show strong seasonality in floc size and Chlorophyll‐a (Chl‐a) but not in transparent exopolymer particlesWe develop a robust method to detect the importance of seasonal variation in biota on sediment dynamics using an idealized numerical modelIn the Scheldt estuary, seasonal variations in biota are not necessary to explain seasonal variations in sediment dynamics New observations over various seasons in the Scheldt estuary show strong seasonality in floc size and Chlorophyll‐a (Chl‐a) but not in transparent exopolymer particles We develop a robust method to detect the importance of seasonal variation in biota on sediment dynamics using an idealized numerical model In the Scheldt estuary, seasonal variations in biota are not necessary to explain seasonal variations in sediment dynamics

We monitored an intertidal mussel (Mytilus edulis L.) population between June 1981 and June 1982 in the Eastern Scheldt estuary (S.W. Netherlands). Density and biomass of the population remained relatively constant over the study period. The shell length growth was described by a Gompertz growth curve. The parameters of this equation were estimated from a log-log-modified Ford-Walford plot of the growth-ring data. The slope of the relationship between animal weight and shell length is season-dependent, mainly due to the spawning cycle in larger mussels.