Mesoscale distribution of Oikopleura and Fritillaria (Appendicularia) in the Southern Gulf of Mexico: spatial segregation

The mesoscale spatial distribution of Oikopleura and Fritillaria in the southern Gulf of Mexico was analyzed to know the existence of segregation between them. Samples were taken on 97 stations in the 50 m upper layer. Temperature, salinity and turbidity were measured. The spatial segregation index 'D' was applied to Oikopleura and Fritillaria densities and its significance was tested with Monte Carlo method. Regression Tree (RT) analyses were performed to identify the main environmental factors affecting the distribution of genera. The D index value was 0.67, considered as a high value. RT analyses showed the water turbidity was the main factor affecting the spatial distribution of both genera. Fritillaria was found in turbid waters off the main fluvio-lagoon systems and Oikopleura was most abundant in the up welling waters over Yucatan Shelf. The segregation distribution of appendicularians in the study area appear to be strongly influenced by up welling and associated currents, and turbidity generated by continental water discharges. It could be assumed that food webs are an important associated factor. We hypothesize that a broad diet and higher filtration efficiency for larger particles (microfitoplankton) allows Oikopleura species to exploit the production in up welling areas, whereas Fritillaria species can not. It may explain its spatial segregation of these genera.