Declines in predatory fish promote bloom‐forming macroalgae

Abstract

In the Baltic Sea, increased dominance of ephemeral and bloom-forming algae is presently attributed to increased nutrient loads. Simultaneously, coastal predatory fish are in strong decline. Using field data from nine areas covering a 700-km coastline, we examined whether formation of macroalgal blooms could be linked to the composition of the fish community. We then tested whether predator or nutrient availability could explain the field patterns in two small-scale field experiments, by comparing joint effects on algal net production from nutrient enrichment with agricultural fertilizer and exclusion of larger predatory fish with cages. We also manipulated the presence of invertebrate grazers. The abundance of piscivorous fish had a strong negative correlation with the large-scale distribution of bloom-forming macroalgae. Areas with depleted top-predator communities displayed massive increases in their prey, small-bodied fish, and high covers of ephemeral algae. Combining the results from the two experiments showed that excluding larger piscivorous fish: (1) increased the abundance of small-bodied predatory fish; (2) changed the size distribution of the dominating grazers, decreasing the smaller gastropod scrapers; and (3) increased the net production of ephemeral macroalgae. Effects of removing top predators and nutrient enrichment were similar and additive, together increasing the abundance of ephemeral algae many times. Predator effects depended on invertebrate grazers; in the absence of invertebrates there were no significant effects of predator exclusion on algal production. Our results provide strong support for regional declines of larger predatory fish in the Baltic Sea promoting algal production by decreasing invertebrate grazer control. This highlights the importance of trophic interactions for ecosystem responses to eutrophication. The view emerges that to achieve management goals for water quality we need to consider the interplay between top-down and bottom-up processes in future ecosystem management of marine resources.

Effects of nutrients and zooplankton on an estuary's phytoplankton: inferences from a synthesis of 30 years of data

“…In some cases algal biomass appears to be as, or more, sensitive to topdown pressure within the food web as algae are to resource availability (Heffernan et al 2010, Perga et al 2010. The importance of top-down effects on processes that have historically been considered symptoms of eutrophication has recently been examined in experimental research (Eriksson et al 2009, Baden et al 2010) and time series analysis (Baden et al 2012). Our study took advantage of an ecosystem with the unique, long-term data on multiple trophic levels to explore a particular scenario of resource availability versus fish: the potential trophic cascade between anadromous fish and phytoplankton.…”

Regime shifts and resilience in China’s coastal ecosystems

“…The positive correlation between jellyfish and chlorophyll a concentration in the Yellow Sea suggest a cascading effect resulting from the jellyfish grazing on zooplankton that in turn reduce the grazing of zooplankton on phytoplankton (Dong et al 2012). This has been observed in the Baltic Sea where research revealed that declines in predatory fish promoted blooms of macroalgae with cascading effects down the food web (Eriksson et al 2009). Additionally, macroalgal blooms could strengthen hypoxic conditions which in turn further degrade the coastal ecosystems (Smetacek and Zingone 2013).…”

Cascading predator control interacts with productivity to determine the trophic level of biomass accumulation in a benthic food web

“…Accordingly, field studies support a positive relationship between primary production and trophic cascades in some systems (e.g., Sinclair et al 2000;Elmhagen and Rushton 2007;Moksnes et al 2008;Eriksson et al 2009;Sieben et al 2011), whereas a recent set of meta-analyses suggests that, in general, it is the traits of consumer species, and not productivity, that determine the prevalence of trophic cascades in predator removal experiments (Borer et al 2005(Borer et al , 2006. However, models that incorporate short-term population dynamics and allow for dispersal suggest that the an alternating positive relationship between trophic levels and primary productivity should also appear in environments where consumers immigrate and emigrate actively between patches depending on resource availability (Wootton and Power 1993;Oksanen et al 1995;Nisbet et al 1997).…”