The distribution, composition and activity of phytoplankton, and accompanying changes in specific activities of bacterioplankton and copepods, were related to variahons in the vertical structure of the water column along a transect through the Skagerrak in May 1987. The Skagerrak is charactenzed by a doming of the pycnocline, with a deep mixed layer along the periphery and a very shallow pycnochne in central parts Average phytoplankton size increased with the depth of the upper mixed layer, and the central stratified area was characterized by small flagellates while large and chainforming diatoms dominated along the periphery. In situ fecundities of 2 copepod species (Ternora long~cornis and Acarha c l a u s~) increased linearly with the concentration of phytoplankton (chl a ) > 8 pm and were, therefore, highest along the Skagerrak margin. Specific growth rates of bactenoplankton appeared to depend on particle surface area rather than particle volume or chla, and showed a distributional pattern that was nearly the inverse of the distribution of copepod activity. That is, peak bacterial growth rates occurred in central, stratified parts and lower rates were found along the margin with a deep mixed layer. Thus a 'microbial loop' type of food web seemed to be evolving in the central, strongly stratified parts of the Skagerrak, while a shorter 'classical' type of food web appeared to dominate along the margin. The relation between food web structure and verucal mixing processes observed on oceanwide scales, e.g. between oligotrophic ocean gyres and the major upwelling systems, thus seems to recur on much smaller honzontal scales.

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The combined effect of pelagic respiration and stratification resulted in the development of an oxygen minimum layer in early April 1988 in the Kattegat, Denmark. Strong wind mlxing in midMarch created a cold and homogeneous surface layer above a saltier and warmer bottom layer This was followed by a large outflow of low salinity Baltic Proper water, which covered the higher salinity Kattegat water and created a surface layer trapping the cold former surface water a s a n intermediate layer for several weeks. Wind forcing was insufficient to mix the intermediate layer with the water column above. In this cold layer, 1 or 2 subsurface chlorophyll a maxima were observed, and the composition of the plankton community changed during the spring period towards heterotrophy, culminating with a maximum of 2200 ind. I-' of rotifers in the pycnocline a t the end of April. In the bottom part of the cold layer a distinct layer exhibiting an oxygen minlmuln developed. In early April the layer was observed 10 km off the coastline and covered approximately 500 km2 The upper 10 m of the water column was at the same time supersaturated, with a maximum of 152% 0, (554 pM 02), presumably due to spring bloom photosynthesis. The oxygen saturation In the bottom water was 70% 02, whereas an oxygen minimum was recorded below the chlorophyll a peak between the surface and bottom layers. Here, the oxygen saturation was 32% O2 (120 pM 0 2 ) with a plankton respiration of 14 pM O? d -l , yielding a n oxygen turnover of 8 d. Where the pycnocline intersected the bottom, maxima of phytoplankton biomass (33 pg chlorophyll a I-'), plankton respiration rates (16 pM 0, d -l ) and bacterial net production rates (3.6 FM C d -' ) occurred. Including the sediment oxygen uptake, a n oxygen turnover of approximately 3 d was estimated in this zone, and only 2 "h of full oxygen saturation (6 FM 0, concentration) was found. Microzooplankton and flagellates accounted for approximately ' / 3 of the total oxygen consumption. The vertical position of the pycnocline changed during April and May, which resulted in fish mortality when fish trapped in the bottom nets were exposed to the oxygen minimum layer. At the end of May, the oxygen minimum layer was no longer distinguishable from the Kattegat bottom water.

1. The longstanding debate in conservation biology on the importance of single large or several small (SLOSS) habitats for preserving biodiversity remains highly relevant, given the ongoing degradation and loss of natural habitats world-wide.Restoration efforts are often constrained by limited resources, and insights from SLOSS studies therefore have important implications if restoration efforts can be optimized by manipulating the spatial configuration of restored habitats. Yet, the relevance of SLOSS for habitat restoration remains largely unexplored.2. Here, we report the effects of spatial reef configuration on early colonization of marine organisms after restoring boulder reef habitats. Reefs were restored in single large (SL) and several small (SS) designs in the western Baltic Sea, where century-long boulder extraction has severely degraded large reef areas and likely exacerbated regional declines in commercially important gadoids (Gadidae spp.).We sampled the field sites using remote underwater video systems in a beforeafter control-impact (BACI) design and obtained probabilistic inferences on restoration and SLOSS effects from Bayesian hierarchical models.3. Probabilities of a positive restoration effect were high (>95%) for gadoids, labrids and demersal gobies, moderate (60%-75%) for species richness and sand gobies, and low (<5%) for flatfish abundance. Notably, gadoid abundance increased 60-fold and 129-fold on average at SL and SS respectively. The species composition at restored reefs deviated from control sites, mainly driven by large-bodied piscivores.4. Spatial reef configuration had the strongest effect on small-bodied mesopredators, including gobies, which were more abundant at SS and driving distinct species assemblages between the reef designs. In addition to providing suitable conditions for reef species, results suggest that SS can also benefit soft-bottom taxa, possibly through a dispersed predator-mediated effect relative to SL. Synthesis and applications.This study demonstrates that boulder reef restoration can strongly promote the abundance of exploited gadoids (e.g. Atlantic cod) and is therefore a promising management tool to support top-down controls by | 2937