Spatial variability of the plankton trophic interaction in the
North Sea: a new feature after the early 1970s
Marcos Llope
Traditionally, marine ecosystem structure was thought to be
controlled by its resources (bottom-up). In recent years, a number
of studies have highlighted the importance of predation in its
regulation (top-down). Evidence is accumulating that the type of
trophic forcing varies temporally and spatially, and an integrated
view – considering the interplay of both types of control – is
emerging. At the base of marine food webs, plankton is an essential
link in the functioning of the whole marine ecosystem so that the
prevailing type of control regulating their interaction has
implications for the transfer of energy across trophic levels and
eventually for the fisheries.
The North Sea is one of the most exploited areas in the world
and at the same time one of the most intensively sampled marine
systems, also thanks to the establishment of the Continuous
Plankton Recorder (CPR) Survey in the late 1940s. The CPR
systematically samples the plankton in space and time and therefore
provides a unique dataset for studying spatial dynamics over the
last decades in the North Sea marine ecosystem.
Using the plankton records collected by the CPR, this study
investigates over both temporal and spatial scales the variation of
the trophic relationships occurred since the late 1950s within the
North Sea plankton community. Our results show that until the
early-1970s a bottom-up control characterized the base of the food
web across the whole North Sea, with diatoms having a positive and
homogeneous effect on zooplankton filter-feeders. Afterwards,
different regional trophic dynamics were observed. In particular a
negative relationship between total phytoplankton and zooplankton
was detected off the west coast of Norway and the Skagerrak as
opposed to a positive one in the southern reaches. Our results
suggest that after the early 1970s diatoms remained the main food
source for zooplankton filter-feeders east of Orkney–Shetland and
off Scotland, while in the east, from the Norwegian Trench o the
German Bight, filter-feeders were mainly sustained by
dinoflagellates.
It seems probable that the observed relaxation of the bottom-up
force and the increased spatial heterogeneity are two consequences
of changes in the planktonic food web being driven by the
hydroclimatic changes, in particular the warming of the North Sea,
which would have occurred before the late 1980s regime shift. From
our present understanding, it is difficult to speculate on whether
this new configuration of the North Sea implies a loss of
resilience. Examples from other marine systems suggest that the
probability of shifting from one stable state to another is greater
in top-down regulated ecosystems. In terms of management, the
probability of trophic cascades due to overfishing is also higher
in these systems. Alternatively, bottom-up regulation would favour
the upward propagation of climate effects. None of these seem,
however, to fully explain the present North Sea trophodynamics.