Seawater temperature, productivity and marine fish metabolism in the Messinian eastern Mediterranean

Reconstructing paleoceanographic conditions for the entire water column remains challenging, due to the lack of proxies for seawater parameters below surface waters, which could be used to validate models. Fish otoliths and ostracods are used here to obtain biogeochemical proxy data of sea surface/bottom temperature and productivity, as well as the biological response of marine fishes to paleoenvironmental change. Our study area is located in the eastern Mediterranean Sea (Heraklion basin, Crete Island). During the Messinian age (specifically between 7.2 and 6.5 Ma), the Mediterranean–Atlantic connection was restricted leading to a strongly stratified water column. We study the sea surface and bottom conditions under these conditions.

Stable oxygen isotopes on ostracod valves (Bairdoppilata sp.) reflect the combined effect of bottom-water temperature and salinity changes. For fishes however, this depends on each species lifestyle. We analyzed two very common species: 1) Bregmaceros albyi, a surface-water pelagic species, and 2) Lesueurigobius friesii, a demersal fish dwelling on the sea bottom. Our hypothesis was that the stable oxygen isotopic ratios on B. albyi otoliths would reflect surface-water conditions, whereas those on L. friesii would correspond to bottom-water conditions. Furthermore, we obtained δ¹³C values for the same ostracod and otolith specimens. Stable carbon isotopic ratios in invertebrate shells indicate biological productivity, since carbon fractionation takes place as a single-step process during biomineralization. However, fish otolith aragonite mineralization is more complicated, involving more than one fractionation steps, and carbon is obtained from seawater, but also from diet. Therefore,  otolith δ¹³C is considered a proxy of the fish’s field metabolic rate, reflecting its ability to continue to grow and reproduce despite environmental change.

Our results show that B. albyi δ¹⁸O values correlate well with those derived from planktonic foraminifera shells, whereas L. friesii δ¹⁸O is in agreement with ostracod values, thereby confirming our hypothesis. Moreover, ostracod and foraminifera δ¹³C follow the same decreasing pattern. However, otolith δ¹³C remains stable, even after 6.8 Ma, when high-amplitude salinity variations prevail. This suggests that fish maintained their capacity to grow and reproduce despite significant changes in seawater conditions at least until 6.5 Ma. However, whether this reflects their resilience to these environmental changes or an adaptation mechanism such as reducing their growth rate or shifting their trophic preferences remains a mystery.