Recent benthic foraminifers as indicators of the sedimentary dynamics of the Tina Mayor and Tina Menor estuaries (S Bay of Biscay, N Spain)

Abstract

Recent benthic foraminifers from two estuaries in the North Spanish coastal region (Tina Mayor and Tina Menor) were analysed with the aim of characterising the ecology and dynamics of the different estuarine sub-environments. A total of 35 benthic foraminifer species were taxonomically determined in the Tina Mayor estuary, 22 of which contained living individuals during the sampling. For Tina Menor, there were 51 species, 30 of them biocenotic. The most abundant species in these estuaries were Ammonia tepida, Cribroelphidium williamsoni, Jadammina macrescens, Lobatula lobatula, Miliammina fusca, Rosalina globularis and Trochammina inflata. In Tina Menor, Haynesina germanica was also abundant. In the foraminifer assemblages, live (biocenotic) and dead (thanatocenotic) specimens were differentiated and processed statistically by a multivariate cluster-Q type analysis and detrended correspondence ordination analysis (DCA). Groupings of biocenotic assemblages were obtained for both estuaries indicating an ecologic separation of marine mouth, low and high marsh and zones with strong salinity oscillations in these estuaries. Thanatocenotic groupings, nevertheless, showed a different organisation of that of the ecologic groups, which were indicative of a transport of individuals from the mouth to the inner/mid estuary locations, along the main and/or secondary channels of these estuaries. Local differences between Tina Mayor and Tina Menor can be explained by the different physiographies of the estuary as a consequence of the different geological/sedimentological histories of both estuaries. Benthic foraminifer assemblages are distributed in the estuary following three zones: an outer estuary having marine characteristics with L. lobatula and R. globularis, a mid estuary, including a subtidal mudflat with variable salinity (A. tepida, C. williamsoni) and a marsh (J. macrescens, M. fusca and T. inflata), which is different in both estuaries.Tina Mayor is filled with more sediment, and it has a reduced mudflat and a large high marsh located above the MHW (Mean High Waters), where T. inflata is the species marker of the intervals between the MHW and MHWST (Mean High Waters Spring Tides) levels. In Tina Menor, estuary mudflats crossed with brackish secondary channels (with H. germanica) below MHWNT (Mean High Waters Neap Tides) are more developed, though the environments of low marsh with scarce vegetation (with M. fusca) are dominant in intervals between MHWNT and MHW. The relative abundance of M. fusca, particularly in the mid estuary, differentiates Tina Mayor and Tina Menor from other estuaries of Cantabria. Biocenotic and thanatocenotic assemblages show a general similarity in both estuaries (Sanders's index, >70%). Samples with low similarity are evidence of the effect of post-mortem transport mostly produced by suspension. The percentage of exotic specimens is always less than 35%, supporting the mesotidal characteristic of both estuaries. The present results confirm the value of benthic foraminifers as markers of estuarine sub-environments, supporting their value for detecting changes in past estuarine environments.