Research papersThe neritic benthopelagic fauna: Which factors explain nocturnal distribution better?

- The benthopelagic fauna of the Northeast Black Sea is inventoried (76 taxa).
- Nocturnal dynamics of benthopelagic fauna is primarily controlled by and sea-floor biotope type/depth.
- Temperature and residual solar illumination are additional proximate cues for the nocturnal distribution.
- Significance of environmental factors may change with depth and sea-floor biotope type.

Nocturnal sampling of mesoplankton above different biotopes at depths of 10, 20, and 40 m, with a temporal resolution of 10 min (98 hauls) was made and 76 taxa were identified. Six environmental factors were tested as the proximate cues for the distribution of individual taxa and integral community parameters: type of sea-floor biotope/depth (SFB), temperature (T), time after sunset (TAS), residual solar illumination, (assessed as time to midnight, TTM), moon phase (MP), and moon altitude (MA). Canonical Correspondence Analyses (CCAs), Bray-Curtis analyses (BCAs), and one-way ANOSIM and SIMPER tests were used to assess the impact of the environmental factors on the distribution of individual taxa, integral community parameters, and community structure. Seventy six taxa were recorded, with 17 taxa, mostly decapod larvae, contributing >1% of the total amount of sampled individuals. Four decapod species comprised over 50% of the total benthopelagic abundance. Decapod larvae migrated as benthopelagic fauna and aggregated over sites where adults live; this behaviour was observed both in zoea and in megalopa stages (ANOVA and SIMPER tests). SFB, T, and TTM are the best proximate cues for the nocturnal distribution of the neritic benthopelagic fauna. Significance of environmental factors may change with depth and sea-floor biotope type: T and TTM are equally important above all explored depths; impact of TAS decreased and contribution of MP and MA increased above greater depths. Further studies of the mesoplanktonic samples, which yield greatly richer material than macroplanktonic samples, may favour this analysis.