Season affects strength and direction of the interactive impacts of ocean warming and biotic stress in a coastal seaweed ecosystem

Abstract

The plea for using more "realistic," community-level, investigations to assess the ecological impacts of global change has recently intensified. Such experiments are typically more complex, longer, more expensive, and harder to interpret than simple organism-level benchtop experiments. Are they worth the extra effort? Using outdoor mesocosms, we investigated the effects of ocean warming (OW) and acidification (OA), their combination (OAW), and their natural fluctuations on coastal communities of the western Baltic Sea during all four seasons. These communities are dominated by the perennial and canopy-forming macrophyte Fucus vesiculosus-an important ecosystem engineer Baltic-wide. We, additionally, assessed the direct response of organisms to temperature and pH in benchtop experiments, and examined how well organism-level responses can predict community-level responses to the dominant driver, OW. OW affected the mesocosm communities substantially stronger than acidification. OW provoked structural and functional shifts in the community that differed in strength and direction among seasons. The organism-level response to OW matched well the community-level response of a given species only under warm and cold thermal stress, that is, in summer and winter. In other seasons, shifts in biotic interactions masked the direct OW effects. The combination of direct OW effects and OW-driven shifts of biotic interactions is likely to jeopardize the future of the habitat-forming macroalga F. vesiculosus in the Baltic Sea. Furthermore, we conclude that seasonal mesocosm experiments are essential for our understanding of global change impact because they take into account the important fluctuations of abiotic and biotic pressures.This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.Additional Supporting Information may be found in the online version of this article.

Warming, but Not Acidification, Restructures Epibacterial Communities of the Baltic Macroalga Fucus vesiculosus With Seasonal Variability

“…The benthocosm experiments showed, that the macroalgal host F. vesiculosus suffered from warming in summer but was unaffected in the other seasons (also compare related F. vesiculosus tissue data of Graiff et al, 2015), whereas its biotic stressors (grazers and epiphytes) generally benefited from warming, except in summer when the grazers suffered but the epiphytes benefited (Graiff et al, 2015;Werner et al, 2016;Wahl et al, 2019). The bacterial communities on F. vesiculosus and in the surrounding seawater were affected by warming in all seasons but autumn.…”

“…During April 2013 to March 2014 four consecutive seasonal experiments were performed as described in detail in Wahl et al (2015a , 2019) and the “Materials and Methods” section, to monitor changes in the bacterial communities associated with F. vesiculosus in response to acidification and warming. During the experiments the monthly mean ambient seawater temperature increased from 8.2 ± 1.4 to 16.1 ± 1.3°C in the spring experiment ( Supplementary Figure S2 ; see also Graiff et al, 2015 ; Wahl et al, 2015a ).…”

“…Within 1 year, four consecutive seasonal experiments were performed during April 2013 to March 2014 as described by Wahl et al (2015a , 2019) for 10–11 weeks: spring (04/04, 05/02, 05/30, and 06/19/2013, 11 weeks), summer (07/04, 08/01, 08/29, and 09/17/2013, 11 weeks), autumn (10/10, 11/07, 12/05, and 12/17/2013, 10 weeks) and winter (01/16, 02/13, 03/13, and 04/01/2014, 11 weeks). For each experiment, stones bearing at least one F. vesiculosus individual were collected along the Kiel Fjord (at 54°27′14.951′′N 10° 11′ 51.886′′E) at a depth of ∼1 m (for a detailed description of the sampling site see Graiff et al, 2015 ).…”

Consistency of aquatic enclosed experiments: The importance of scale and ecological complexity

“…Although a great number of studies has been conducted to determine the responses of single‐species or communities to global warming and other anthropogenic impacts using mesocosm and laboratory experiments (e.g. Casties et al., 2019; Madeira et al., 2018; Pansch et al., 2018; Wahl et al., 2020), it still remains unclear how the type of the experiment (i.e. scale and ecological complexity) affects the outcome and to what extent the two types of experiments are comparable.…”

Model simulation of seasonal growth of Fucus vesiculosus in its benthic community

“…Throughout the seasons, Fucus is exposed to highly variable environmental conditions, especially annual and seasonal fluctuations in pH (7.4–8.5) and temperature (<0 to 20/25°C). However, Fucus performance (metabolism, growth, reproduction) and, thus, competitiveness vary dramatically along this range of environmental fluctuation (Wahl et al ). Furthermore, Torn et al () report on alarming shifts from the perennial Fucus community to a predominance of annual filamentous algae.…”