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The main strait (Great Belt) connecting the North Sea and the Baltic Sea constitutes a quasi-stationary front and exposes phytoplankton to various degrees of water column mixing. Here, we examine phytoplankton community distributions (using the cell abundance of 4 readily identifiable diatoms) and estimate new production along the strait during early spring. Vertical turbulent mixing was ~10 times greater at stations in the strait compared to stations outside the strait. New production in the strait was on average ~50 mg C m-2 d-1, i.e. 8% of the average total primary production and could explain the increase in chlorophyll observed along the strait. An NMDS-analysis of phytoplankton community composition showed significant spatial groupings. However, variation of species abundances could not be explained by the general transport, where the abundance of the largest species decreased during the passage of the strait. A relatively small species (Guinardia delicatula) showed an increasing dominance in and above the subsurface chlorophyll maximum along the strait, and the bottom layer was also correspondingly dominated by a relatively small species (Skeletonema marinoi). This phytoplankton composition could be explained by photosynthetic traits associated with more efficient light usage of small cells together with increased nutrient supply in the strait.

In this paper, we discuss the first setup of a hydrodynamic model for the fjord-type estuary Kangerlussuaq, located in West Greenland. Having such a high-fidelity numerical model is important because it allows us to fill in the temporal and spatial gaps left by in situ data and it allows us to examine the response of the fjord to changes in ice sheet runoff. The numerical model is calibrated against in situ data, and a one-year simulation was performed to study the seasonal variability in the physical oceanographic environment and the fjord's response to changing meltwater runoff. The fjord consists of two distinct parts: a deep inner part that is 80 km long with weak currents and a shallow part that covers the outer 100 km of the fjord connected to the ocean. The outer part has very fast currents (∼1.3 m/s), which we suggest prevents winter sea ice formation. The dominant currents in the fjord are oriented parallel to the long axis of the fjord and are driven by tides and (during summer) freshwater inflow from meltwater-fed rivers. Furthermore, mixing processes are characterized by strong tidal mixing and bathymetric restrictions, and the deep-lying water mass is subject to renewal primarily in wintertime and is almost dynamically decoupled from the open ocean during summertime. Finally, a sensitivity study on the changing meltwater runoff was performed, showing that increasing freshwater runoff considerably strengthens stratification in the upper 100 m of the water column in the inner part of the fjord.

Observations from 2007 to 2010 of runoff, sediment and solute delivery from a segment of the Greenland Ice Sheet (GrIS) and the proglacial landscape draining into the fjord at Kangerlussuaq are presented. The observations include at least three jökulhlaups and extreme recordings from 2010. The average runoff from the 9743 km2 catchment was 377 mm (min. 267 mm and max. 565 mm) and the sediment delivery was 744 t km–2 y–1 with (min. 493 and max. 1221t km–2 y–1). Solute transport was only 0.4 % of the total load (sediment load + dissolved load). These new values are a factor two higher than values previously published for 2007 and 2008. The average effective erosion from the catchment was 0.28 mm (min. 0.18 and max. 0.45 mm). The erosion is larger than indicated from most other locations along the GrIS, but in the same order of magnitude as erosion in other glaciated areas at the same latitude, e.g. Norway. The sandur in the proglacial area acts as a sediment sink for a lot of the sediments from the GrIS.

Author Posting. © Cambridge University Press, 2004. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 515 (2004): 415-443, doi:10.1017/S0022112004000576., The investigation involves the hydraulic behaviour of a dense layer of fluid flowing over an obstacle and subject to entrainment of mass and momentum from a dynamically inactive (but possibly moving) overlying fluid. An approach based on the use of reduced gravity, shallow-water theory with a cross-interface entrainment velocity is compared with numerical simulations based on a model with continuously varying stratification and velocity. The locations of critical flow (hydraulic control) in the continuous model are estimated by observing the direction of propagation of small-amplitude long-wave disturbances introduced into the flow field. Although some of the trends predicted by the shallow-water model are observed in the continuous model, the agreement between the interface profiles and the position of critical flow is quantitatively poor. A reformulation of the equations governing the continuous flow suggests that the reduced gravity model systematically underestimates inertia and overestimates buoyancy. These differences are quantified by shape coefficients that measure the vertical non-uniformities of the density and horizontal velocity that arise, in part, by incomplete mixing of entrained mass and momentum over the lower-layer depth. Under conditions of self-similarity (as in Wood's similarity solution) the shape coefficients are constant and the formulation determines a new criterion for and location of critical flow. This location generally lies upstream of the critical section predicted by the reduced-gravity model. Self-similarity is not observed in the numerically generated flow, but the observed critical section continues to lie upstream of the location predicted by the reduced gravity model. The factors influencing this result are explored., M. H. N. would like to thank the Danish Natural Science Research Council for financial support. L. P. and K. H. were supported by the Office of Naval Research under grant N00014-1-01-0167 and by the National Science Foundation under grant OCE-0132903.