Pelagic nutrient and energy transfer during spring in the open and coastal Skagerrak

Abstract

In May 1987 multidisciplinary investigations focusing on diel variations were performed at 4 horizontally stratified (pycnocline at 5 to 12 m) stations in the open Skagerrak (North Sea). Nutrients were lower above the pycnocline than below. Phytoplankton was numerically dominated by flagellates and monads indicating a regenerating plankton community, which was confirmed by the finding that about 80% of the nitrogen uptake in surface waters was as NH-4+ and urea, and about 20% was NO-3-. Percentage of 'new' primary production (based on NO-3- uptake) was similar to the percentage sedimentation rate (in C) of primary production (ca 20%). Growth of bacteria and grazing on bacteria were systematically dependent on time of day above, in and below the pycnocline. Abundance of bacteria and nonflagellates was not regularly dependent on time of day, but systematically elevated in the pycnocline, as was chlorophyll a. Zooplankton grazing in the surface water was highest at night and early morning. Benthic investigations indicated heterogeneity in the area. For comparison, samples of hydrography, nutrients, phytoplankton abundance and growth, and sedimentation were investigated at the same time at a coastal station with horizontal stratification and slighty lower salinity. Here 'new' primary production was estimated to about 50% of total production, based on percentage sedimentation. Nitrogen seemed in general to be in deficit for primary production relative to phosphorus in surface waters both in the open and coastal Skagerrak, but exceptions could occur. That nutrients were in surplus just below the shallow pycnocline shows, however, that those primary producers and bacteria which could utilize this reservoir had access to unlimited nutrient resources at that time. It was estimated that bacteria and flagellates made up gt 50% of total pelagic respiration and consumed slightly more than the net primary production.