Groundfish overfishing, diatom decline, and the marine silica cycle: Lessons from Saanich Inlet, Canada, and the Baltic Sea cod crash

Abstract

[1] In this study, we link groundfish activity to the marine silica cycle and suggest that the drastic mid-1980s crash of the Baltic Sea cod (Gadus morhua) population triggered a cascade of events leading to decrease in dissolved silica (DSi) and diatom abundance in the water. We suggest that this seemingly unrelated sequence of events was caused by a marked decline in sediment resuspension associated with reduced groundfish activity resulting from the cod crash. In a study in Saanich Inlet, British Columbia, Canada, we discovered that, by resuspending bottom sediments, groundfish triple DSi fluxes from the sediments and reduce silica accumulation therein. Using these findings and the available oceanographic and environmental data from the Baltic Sea, we estimate that overfishing and recruitment failure of Baltic cod reduced by 20% the DSi supply from bottom sediments to the surface water leading to a decline in the diatom population in the Baltic Sea. The major importance of the marginal ocean in the marine silica cycle and the associated high population density of groundfish suggest that groundfish play a major role in the silica cycle. We postulate that dwindling groundfish populations caused by anthropogenic perturbations, e.g., overfishing and bottom water anoxia, may cause shifts in marine phytoplankton communities.

The effect of top‐predator presence and phenotype on aquatic microbial communities

“…They can alter prey population structure and resource availability through trophic cascades (Frank, Petrie, Choi, & Leggett, 2005) or modify rates of prey movement, grazing, activity, dispersal, and colonization (Reynolds & Bruno, 2013;Schmitz et al, 2004). Such top predator-mediated effects can also influence primary producers and bacteria, and may, ultimately, alter biogeochemical cycles, ecosystem processes and abiotic conditions (Chapin et al, 2000;Hooper et al, 2005;Katz et al, 2009;McIntyre, Jones, Flecker, & Vanni, 2007).…”

Resuspension by fish facilitates the transport and redistribution of coastal sediments

“…In an intermittently anoxic basin such as Saanich Inlet, this groundfish movement would alter the areas of biologically reworked and resuspended seafloor. Such shifts may have basin-scale implication for mineralization and sequestration of organic matter (Yahel et al 2008) and dissolution of biogenic silica (Katz et al 2009). Oxygen-driven shifts in groundfish distributions may also affect sediment features, such as annual lamination and thus interpretation of sediment stratigraphy.…”

“…In contrast, biological resuspension at mid-depth in the inlet is extensive. Video recordings and backscatter time series show that groundfish generate about 100 resuspension events m 22 d 21 , resuspending , 1.3 liters bulk (about 1 kg dry) sediment m 22 d 21 (Yahel et al 2008;Katz et al 2009). …”

“…Dissolved oxygen data were calibrated using discrete water samples collected by GoFlo bottles and analyzed using computerized Winkler titration as described in Yahel et al (2008) Fish exclusion experiment-During July 2005 we used the ROV to deploy two cylindrical exclusion cages (excluders) in Saanich Inlet at 92-m depth (see study area in Fig. 1) where flatfish were abundant (Katz et al 2009). The excluders (1-m 2 diameter, 15-cm height with 2.5-cm plastic mesh) prevented fish from reaching the underlying seafloor and reworking the sediment.…”

“…We assume that groundfish activity alone resuspends 1 kg sediment m 22 d 21 (Katz et al 2009) and that this activity occurs uniformly along the oxygenated margins down to the depth where oxygen concentrations are too low to support it (, 130 m; Fig. 2).…”