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Increasing picocyanobacteria success in shelf waters contributes to long‐term food web degradation

Increasing picocyanobacteria success in shelf waters contributes to long‐term food web degradation
Increasing picocyanobacteria success in shelf waters contributes to long‐term food web degradation
Continental margins are disproportionally important for global primary production, fisheries and CO2 uptake. However, across the Northeast Atlantic shelves, there has been an ongoing summertime decline of key biota—large diatoms, dinoflagellates and copepods—that traditionally fuel higher tropic levels such as fish, sea birds and marine mammals. Here, we combine multiple time series with in situ process studies to link these declines to summer nutrient stress and increasing proportions of picophytoplankton that can comprise up to 90% of the combined pico‐ and nanophytoplankton biomass in coastal areas. Among the pico‐fraction, it is the cyanobacterium Synechococcus that flourishes when iron and nitrogen resupply to surface waters are diminished. Our field data show how traits beyond small size give Synechococcus a competitive edge over pico‐ and nanoeukaryotes. Key is their ability to grow at low irradiances near the nutricline, which is aided by their superior light‐harvesting system and high affinity to iron. However, minute size and lack of essential biomolecules (e.g. omega‐3 polyunsaturated fatty acids and sterols) render Synechococcus poor primary producers to sustain shelf sea food webs efficiently. The combination of earlier spring blooms and lower summer food quantity and quality creates an increasing period of suboptimal feeding conditions for zooplankton at a time of year when their metabolic demand is highest. We suggest that this nutrition‐related mismatch has contributed to the widespread, ~50% decline in summer copepod abundance we observe over the last 60 years. With Synechococcus clades being prominent from the tropics to the Arctic and their abundances increasing worldwide, our study informs projections of future food web dynamics in coastal and shelf areas where droughts and stratification lead to increasing nutrient starvation of surface waters.
Synechococcus, Western Channel Observatory, climate change, copepods, food quality, iron, nitrate, picoeukaryotes, stratification, time series
1354-1013
5574-5587
Schmidt, Katrin
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Birchill, Antony
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Atkinson,
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Brewin,
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Clark,
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Hickman, Anna
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Johns,
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Lohan, Maeve
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Milne, Angela
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Pardo, Silvia
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Polimene,
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Smyth, Timothy
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Tarran, Glen
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Widdicombe, Claire E.
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Woodward, E. Malcolm S.
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Ussher, S.
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Schmidt, Katrin
76b8ecdd-d125-45b9-93c8-0df1644a6f00
Birchill, Antony
b2541497-080a-4b7b-a245-3f4135ec1168
Atkinson,
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Brewin,
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Clark,
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Hickman, Anna
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Johns,
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Lohan, Maeve
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Milne, Angela
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Pardo, Silvia
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Polimene,
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Smyth, Timothy
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Tarran, Glen
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Widdicombe, Claire E.
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Woodward, E. Malcolm S.
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Ussher, S.
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Schmidt, Katrin, Birchill, Antony, Atkinson, , Brewin, , Clark, , Hickman, Anna, Johns, , Lohan, Maeve, Milne, Angela, Pardo, Silvia, Polimene, , Smyth, Timothy, Tarran, Glen, Widdicombe, Claire E., Woodward, E. Malcolm S. and Ussher, S. (2020) Increasing picocyanobacteria success in shelf waters contributes to long‐term food web degradation. Global Change Biology, 26 (10), 5574-5587. (doi:10.1111/gcb.15161).

Record type: Article

Abstract

Continental margins are disproportionally important for global primary production, fisheries and CO2 uptake. However, across the Northeast Atlantic shelves, there has been an ongoing summertime decline of key biota—large diatoms, dinoflagellates and copepods—that traditionally fuel higher tropic levels such as fish, sea birds and marine mammals. Here, we combine multiple time series with in situ process studies to link these declines to summer nutrient stress and increasing proportions of picophytoplankton that can comprise up to 90% of the combined pico‐ and nanophytoplankton biomass in coastal areas. Among the pico‐fraction, it is the cyanobacterium Synechococcus that flourishes when iron and nitrogen resupply to surface waters are diminished. Our field data show how traits beyond small size give Synechococcus a competitive edge over pico‐ and nanoeukaryotes. Key is their ability to grow at low irradiances near the nutricline, which is aided by their superior light‐harvesting system and high affinity to iron. However, minute size and lack of essential biomolecules (e.g. omega‐3 polyunsaturated fatty acids and sterols) render Synechococcus poor primary producers to sustain shelf sea food webs efficiently. The combination of earlier spring blooms and lower summer food quantity and quality creates an increasing period of suboptimal feeding conditions for zooplankton at a time of year when their metabolic demand is highest. We suggest that this nutrition‐related mismatch has contributed to the widespread, ~50% decline in summer copepod abundance we observe over the last 60 years. With Synechococcus clades being prominent from the tropics to the Arctic and their abundances increasing worldwide, our study informs projections of future food web dynamics in coastal and shelf areas where droughts and stratification lead to increasing nutrient starvation of surface waters.

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Accepted/In Press date: 24 April 2020
e-pub ahead of print date: 7 June 2020
Published date: 8 June 2020
Keywords: Synechococcus, Western Channel Observatory, climate change, copepods, food quality, iron, nitrate, picoeukaryotes, stratification, time series

Identifiers

Local EPrints ID: 442303
URI: http://eprints.soton.ac.uk/id/eprint/442303
ISSN: 1354-1013
PURE UUID: 404465dc-962d-40e9-9c17-5e5357638509
ORCID for Anna Hickman: ORCID iD orcid.org/0000-0002-2774-3934
ORCID for Maeve Lohan: ORCID iD orcid.org/0000-0002-5340-3108

Catalogue record

Date deposited: 13 Jul 2020 16:30
Last modified: 26 Nov 2021 03:00

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Contributors

Author: Katrin Schmidt
Author: Antony Birchill
Author: Atkinson
Author: Brewin
Author: Clark
Author: Anna Hickman ORCID iD
Author: Johns
Author: Maeve Lohan ORCID iD
Author: Angela Milne
Author: Silvia Pardo
Author: Polimene
Author: Timothy Smyth
Author: Glen Tarran
Author: Claire E. Widdicombe
Author: E. Malcolm S. Woodward
Author: S. Ussher

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