Significance: Whether environmental conditions, harvesting, or predation pressure primarily regulate an ecosystem is still a question of much debate in marine ecology. Using a wealth of historical records, we describe how climate and fishing interact in a complex marine ecosystem. Through an integrative evidence-based approach, we demonstrate that indirect effects are key to understanding the system. Planktivorous forage fish provide an important role in the system, linking bottom-up and top-down processes such that fishing can indirectly impact the plankton and environmental effects can cascade up to impact demersal fish and predatory seabirds. Cascading trophic interactions can be mediated by opposing bottom-up and top-down forces; this combination has the potential to avert regime wide shifts in community structure and functioning.

Phytoplankton primary production is at the base of the marine food web; changes in primary production have direct or indirect effects on higher trophic levels, from zooplankton organisms to marine mammals and seabirds. Here, we present a new time‐series on gross primary production in the North Sea, from 1988 to 2013, estimated using in situ measurements of chlorophyll and underwater light. This shows that recent decades have seen a significant decline in primary production in the North Sea. Moreover, primary production differs in magnitude between six hydrodynamic regions within the North Sea. Sea surface warming and reduced riverine nutrient inputs are found to be likely contributors to the declining levels of primary production. In turn, significant correlations are found between observed changes in primary production and the dynamics of higher trophic levels including (small) copepods and a standardized index of fish recruitment, averaged over seven stocks of high commercial significance in the North Sea. Given positive (bottom‐up) associations between primary production, zooplankton abundance and fish stock recruitment, this study provides strong evidence that if the decline in primary production continues, knock‐on effects upon the productivity of fisheries are to be expected unless these fisheries are managed effectively and cautiously.

Shannon, Lynne ... et. al.-- Contribution to the Theme Section ‘Trophodynamics in marine ecology’.-- 26 pages, 4 figures, 8 tables, supplementary material in http://dx.doi.org/10.3354/meps10821© Inter-Research and Fisheries and Oceans Canada 2014 Trophic level (TL)-based indicators have been widely used to examine fishing impacts in aquatic ecosystems and the induced biodiversity changes. However, much debate has ensued regarding discrepancies and challenges arising from the use of landings data from commercial fisheries to calculate TL indicators. Subsequent studies have started to examine survey-based and model-based indicators. In this paper, we undertake an extensive evaluation of a variety of TL indicators across 9 well-studied marine ecosystems by making use of model- as well as surveyand catch-based TL indicators. Using detailed regional information and data on fishing history, fishing intensity, and environmental conditions, we evaluate how well TL indicators are capturing fishing effects at the community level of marine ecosystems. Our results highlight that the differences observed between TL indicator values and trends is dependent on the data source and the TL cut-off point used in the calculations and is not attributable to an intrinsic problem with TLbased indicators. All 3 data sources provide useful information about the structural changes in the ecosystem as a result of fishing, but our results indicate that only model-based indicators represent fishing impacts at the whole ecosystem levelL.J.S. was supported through the South African Research Chair Initiative, funded through the South African Department of Science and Technology (DST) and administered by the South African National Research Foundation (NRF). M.C. was supported by the European Commission through the Marie Curie CIG grant to BIOWEB and the Spanish Research Program Ramon y Cajal. Y.J.S. and M.T. were supported by the French project EMIBIOS (FRB, contract no. APP-SCEN-2010-II). L.J.S. and Y.S. were also funded by the European collaborative project MEECE – Marine Ecosystem Evolution in a Changing Environment – (FP7, contract n°212085). A.B. was supported by Fisheries and Oceans Canada SPERA project ‘Ecosystem indicators for ecosystem monitoring at different scales’ (www.dfo-mpo.gc.ca/science/ecosystem/ projects/detail-eng.asp?pid=21). K.K. was supported by Conservation International and the ‘Sea Around Us’ project, a collaboration between The University of British Columbia and The Pew Charitable Trusts. C.P.L. and C.P. were supported by the EU FP7 project DEVOTES (grant agreement no. 308392), www.devotes-project.eu. C.P.L. was also supported by Defra M1228. This is a contribution to the IndiSeas Working Group, endorsed by IOC-UNESCO (www.iocunesco.org) and the European Network of Excellence Euroceans (www.eur-oceans.eu).Peer Reviewe