Top-down release of mesopredatory fish is a weaker structuring driver of temperate rocky shore communities than bottom-up nutrient enrichment

Abstract

Ecological impacts of top-down trophic cascades in combination with bottom-up nutrient enrichment are increasingly being reported. Such effects may be triggered by decline in the abundance and size of piscivore fish leading to a release of smallersized mesopredatory fish that are capable of reducing mesograzers and their buffering herbivorous effects, thus intensifying eutrophication symptoms. Hitherto, such mesopredator release has not been studied in controlled manner in macroalgaldominated rocky shore communities. This study utilised twelve littoral mesocosms in southeastern Norway to investigate the impact of increased nutrient levels and increased abundance of the mesopredatory goldsinny wrasse (Ctenolabrus rupestris) over 6 months, from spring (April) to autumn (October). The study mainly revealed typical eutrophication responses in both July and in October with nutrient enrichment leading to more ephemeral algae and less perennial Fucacean and red macroalgae. Significant responses to the addition of mesopredatory fish alone or with nutrients in combination were clearly fewer in July and almost non-existent in October. Mesopredatory fish reduced the number of some mesograzers, but not others, and some herbivores even benefitted. Few joint effects occurred between fish and nutrients and the anticipated negative consequences for perennial macrophytes were largely absent; the possible reasons for these observations are discussed in depth. Curiously, the presence of mesopredatory fish seems to favour some canopy-forming macrophytes. This last finding warrants further investigation as the labrid fishery in coastal waters of southern Norway and western Sweden is increasing and a too heavy reduction of mesopredators may have its own unforeseen ecosystem implications.

Disappearing Blue Mussels – Can Mesopredators Be Blamed?

“…The crabs preferred the small mussels, and they started to eat the barnacles first after most mussels were consumed. The low interest in barnacles among the wrasses are in accordance with gut content analysis by Kraufvelin et al (2020), revealing small blue mussels to be their preferred prey organisms.…”

“…These mesocosms are supplied with a continuous flow of 4 m 3 h −1 of seawater from 1 m depth in the fjord and they are equipped with wave machines and tidal regulation. For further description of the mesocosms, see Bokn et al (2003) and Kraufvelin et al (2006aKraufvelin et al ( ,b, 2010Kraufvelin et al ( , 2020. In 2019, the project CRABFISH started in early June and ended in mid-October, being set up for testing predatory top-down effects by the goldsinny wrasse (C. rupestris) and the green crab (C. maenas) on the entire seaweed ecosystem.…”

Mechanisms behind bottom-up effects: eutrophication increases fecundity by shortening the interspawning interval in stickleback

“…This gap in our knowledge is due to the intricate effects of eutrophication on species—it influences species not only directly but also indirectly through ecological interactions and feedbacks among species ( Candolin, Bertell & Kallio, 2018 ; Hoover & Tylianakis, 2012 ; Wootton, 1994 ). In addition, other human-induced disturbances can modify the effects of eutrophication, such as climate change ( Buma, 2015 ; Jackson et al, 2016 ; Russell et al, 2009 ), coastal construction ( Kraufvelin et al, 2010 ) and mesopredator release ( Kraufvelin, Christie & Gitmark, 2020 ; Ö et al, 2016 ). Thus, to unravel the effects of eutrophication on species, the underlying mechanisms and pathways need to be identified and delineated.…”

Peer Review #1 of "Mechanisms behind bottom-up effects: eutrophication increases fecundity by shortening the interspawning interval in stickleback (v0.1)"

“…This gap in our knowledge is due to the intricate effects of eutrophication on species -it influences species not only directly but also indirectly through ecological interactions and feedbacks among species (Candolin et al 2018;Hoover & Tylianakis 2012;Wootton 1994). In addition, other human-induced disturbances can modify the effects of eutrophication, such as climate change (Buma 2015;Jackson et al 2016;Russell et al 2009), coastal construction (Kraufvelin et al 2010) and mesopredator release (Kraufvelin et al 2020;Östman et al 2016). Thus, to unravel the effects of eutrophication on species, the underlying mechanisms and pathways need to be identified and delineated.…”