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. 2017 Feb;183(2):587-595.
doi: 10.1007/s00442-016-3779-y. Epub 2016 Dec 3.

Brownification increases winter mortality in fish

Per Hedström  1 , David Bystedt  2 , Jan Karlsson  2 , Folmer Bokma  2 , Pär Byström  2
Affiliations

Brownification increases winter mortality in fish

Per Hedström et al. Oecologia. 2017 Feb.

Abstract

In northern climates, winter is a bottleneck for many organisms. Low light and resource availability constrains individual foraging rates, potentially leading to starvation and increased mortality. Increasing input of humic substances to aquatic ecosystems causes brownification of water and hence a further decrease of light availability, which may lead to further decreased foraging rates and starvation mortality during winter. To test this hypothesis, we measured the effects of experimentally increased humic water input on consumption and survival of young-of-the-year three-spined stickleback (Gasterosteus aculeatus) over winter in large outdoor enclosures. Population densities were estimated in autumn, and the following spring and food availability and consumption were monitored over winter. As hypothesized, mortality was higher under humic (76%) as compared to ambient conditions (64%). In addition, body condition and ingested prey biomass were lower under humic conditions, even though resource availability was not lower under humic conditions. Light conditions were significantly poorer under humic conditions. This suggests that increased mortality and decreased body condition and ingested prey biomass were not due to decreased resource availability but due to decreased search efficiency in this visual feeding consumer. Increased future brownification of aquatic systems may, therefore, negatively affect both recruitment and densities of fish.

Keywords: Brownification; Feeding efficiency; Light limitation; Metabolism; Winter mortality.

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Conflict of interest statement

Compliance with ethical standardsEthical approvalSampling methods, collection of experimental fish, method of sacrifices and design of experiment in this study comply with the current laws of Sweden and were approved by the local ethics committee of the Swedish National Board for Laboratory Animals in Umeå. (CFN, license no. A-20-14 to Pär Byström).

Figures

Fig. 1
Fig. 1
a Average DOC levels over time in ambient and humic enclosures, open circles are ambient treatment, closed circles are humic treatment, b light availability over time in the water column in ambient and humic enclosures and incoming surface light, triangles are incoming daylight, open circles are ambient treatment, closed circles are humic treatment. c Average temperature (left y-axis) and oxygen concentration (right y-axis) over time in ambient and humic enclosures, open circles are temperature in ambient treatment, closed circles are temperature in humic treatment, open squares are oxygen concentration in ambient treatment, closed squares are average oxygen concentration in humic treatment. Error bars denote ±1 SE
Fig. 2
Fig. 2
Average dry mass of a Zooplankton, b Chironomids, and c Total macro invertebrates over time in humic and ambient enclosures. d Average total prey biomass (dry weight per wet weight of fish) in stickleback stomachs and e body condition of sticklebacks over time in ambient and humic enclosures. Open circles are ambient treatment, closed circles are humic treatment, error bars denote ±1 SE
Fig. 3
Fig. 3
Diets (proportion of total biomass) of sticklebacks in ambient and humic enclosures over winter. From below: relative contribution of chironomidae (dark grey) zooplankton (light grey), other specimen (white) in diet
Fig. 4
Fig. 4
Average mortality rates of sticklebacks over winter in ambient and humic enclosures. Error bars denote ±1 SE
Fig. 5
Fig. 5
Average size distribution of sticklebacks in autumn (open bars) and the following spring (closed bars) for a ambient and b humic enclosures
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