The littoral zone

Abstract

The littoral zone of Lake Kinneret is characterized by high diversity of both abiotic (sand, stone) and biotic (vegetation) substrates, forming habitats of varying complexity that change markedly on both spatial and temporal scales. Most Kinneret fish and macro-invertebrates use littoral habitats for at least part of their life cycle. Many use littoral resources (substrate for attachment, refuge, food) for reproduction and growth. Higher species diversity and fish abundance and biomass are associated with higher degree of habitat complexity. Water level fluctuations (WLFs) influence the availability of littoral resources, with impacts not only on the littoral fauna and flora but indirectly also on the pelagic populations. Consecutive years of drought and a negative water balance for Lake Kinneret result in low lake levels and exposure of continuously increasing shore areas on which shore vegetation develops. The shore vegetation is inundated in high-rainfall winters, presenting a window of opportunity for the littoral zone biota that takes advantage of the temporarily augmented vegetative resources (e.g., enhanced reproduction of cichlids; refuge for young of the year of many fish species). Upon drawdown of the lake water levels, the proportion of rocky littoral declines, limiting the biota that dwell in this habitat. Correspondingly, littoral biota flourish on years of high water levels when rocky habitats are abundant. For example, fluctuation of population size of the endemic cyprinid Mirogrex (syn: Acanthobrama) terraesanctae, a major planktivore, was directly influenced by the availability of freshly inundated rocky substrate used for spawning. In a cascading effect, the pelagic zooplankton populations were impacted by the resulting fluctuations in Mirogrex population size and in their predation pressure. The case of Lake Kinneret underscores the importance of WLFs that can act as a major environmental factor influencing littoral habitat structure and resources, and resulting populations dynamic. Intensification of these fluctuations beyond their natural amplitude is likely to markedly impact the lake ecosystem structure and function.