Abstract
Abstract The littoral zone of lakes is used as spawning, shelter, or feeding habitat for many fish species and hence is of key importance for overall lake functioning. Despite this, hardly any studies exist examining the long‐term dynamics and response of the littoral fish community, composed mostly of juvenile fish, to environmental change. Here, we study the response of total catch per unit effort (CPUE) and individual species CPUE of such a community to 17 years of oligotrophication and examine whether the species responses can be characterised as synchronous or asynchronous. We analyse a data set of beach seine catches carried out during morning and twilight, late spring and late summer at three sites in large and deep Lake Constance from 1997 to 2014. Generalised additive mixed models were used to explore changes in CPUE of the overall community and of the most frequently occurring species, and Kendall's W test was applied to examine whether the dynamics of fish species were synchronous or asynchronous. Species‐specific and total CPUE strongly differed between morning and twilight and between spring and summer indicating an important role of behavioural and life cycle adaptations of species for CPUE. In addition, also the CPUE of some species seeking shelter behind larger stones was lower at sites without these. Total CPUE did not decline suggesting the overall abundance of littoral fish was resilient to declining nutrients. In contrast, fish community composition changed strongly during the study period due to increases in some species (dace, loach, perch) and decreases in others (bream, burbot, chub, ruffe), indicating response diversity of fish to oligotrophication. The type of community dynamics was scale‐dependent, whereby significantly synchronous dynamics according to Kendall's W were observed when taking seasonal variability into account. In contrast, significantly asynchronous species dynamics were observed when only the low‐frequency variability of species dynamics was considered separately for spring and summer time series. Resilience of littoral fish total CPUE to oligotrophication might have important consequences for ecosystem dynamics and ecosystem services beyond the littoral zone. As small fish often impose strong predation pressure on zooplankton, their resilience might sustain a high top‐down control on zooplankton resulting in a further reduction of zooplankton biomass. This could contribute to delayed food web responses and reduced growth of fish with oligotrophication.
Highlights
In many lakes in Europe and Northern America, the ban of phosphorus in detergents and massive investment in lake restoration create cultural re-oligotrophication (Jeppesen et al, 2005)
We study the response of total catch per unit effort (CPUE) and individual species CPUE of such a community to 17 years of oligotrophication and examine whether the species responses can be characterised as synchronous or asynchronous
The lack of response of total CPUE might, in principle, have methodological reasons: Firstly, total phosphorus concentrations in Lake Constance started to decline in the early 1980s, while responses of the pelagic food web were observed with some delay, Several ecological processes possibly contributed to the resilience of total CPUE to oligotrophication: littoral productivity might not—or less strongly—have declined relative to pelagic productivity due to the release of phosphorus from littoral sediments (Güde, Seidel, Teiber, & Weyhmüller, 2000), allochthonous input of organic C from the shore region (Güde & Straile, 2016), and/or improved light conditions for benthic producers due to the decline of phytoplankton (Vadeboncoeur, Vander Zanden, & Lodge, 2002)
Summary
In many lakes in Europe and Northern America, the ban of phosphorus in detergents and massive investment in lake restoration create cultural re-oligotrophication (Jeppesen et al, 2005). The fish community in the littoral is highly diverse (Brosse, Grossman, & Lek, 2007; Fischer & Eckmann, 1997b), and highly dynamic (Fischer & Eckmann, 1997a) as these resources are used by some species only temporally, for example for a specific time of the year and/or a specific time during ontogenetic growth (Werner & Gilliam, 1984) as a result of ontogenetic habitat shifts (Hofmann & Fischer, 2001). Despite its diversity and importance, we are not aware of any long-term study (but see Brosse et al, 2007 for within-season studies) focussing on the abundance and biomass dynamics of the juvenile and small fish community in the shallow littoral zone. We expect that the CPUE of most frequent species decreases, i.e. species show synchronous dynamics in response to oligotrophication
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