URDAIBAI 35 Time Series Inner Bay of Biscay Latitude:   43.3990     Longitude:   -2.6930 Associated Investigators: Fernando Villate (Ecology/Plankton) , Ibon Uriarte (Ecology/Plankton) ,   Arantza Iriarte (Ecology/Plankton)   Related Time Series: [ BILBAO 35 ]   [ URDAIBAI 35 ]   The Zooplankton Ecology Group of the University of the Basque Country initiated in 1997 a monitoring programme in the estuaries of Bilbao and Urdaibai (the latter also called estuary of Mundaka). Sampling is carried out monthly at the salinity sites of 35-34, 33, 30 and 26 (the last one only in Urdaibai) to obtain vertical profiles of temperature, salinity and dissolved oxygen, and to take samples for Chlorophyll a determination and zooplankton analysis. Water samples are obtained with oceanographic bottles and zooplankton samples by horizontal hauls with 200 µm plankton-nets, both at mid depth below the halocline. The estuaries of Bilbao (43 23 N 3 W) and Urdaibai (43 22 N, 2 43 W) are two shallow meso-macrotidal systems located on the Basque coast (Bay of Biscay), at middle latitudes of the northeastern Atlantic. The estuary of Bilbao is an urban system constituted by a narrow (50 to 150 m), ca. 15 km long, 2 to 9 m deep man-made channel and a wider (ca. 3.8 km) and deeper (10 to 25 m) embayment in the lower end. Most of the estuary shows noticeable water-column stratification and a permanent salt wedge is found in the bottom layer, where salinities are usually =30. The water column is partially mixed at the outer estuary, partially mixed to stratified at the intermediate zone and stratified at the inner estuary. The estuary of Urdaibai is also a short (12.5 km) system, but shallower (mean depth of 3 m) than the estuary of Bilbao. The central channel is bordered by salt marshes at its upper (15 m wide) and middle reaches and by relatively extensive intertidal flats (mainly sandy) and sandy beaches at its lower reaches (ca. 1 km wide). The watershed area is relatively small in relation to the estuarine basin, and river inputs are usually low in relation to the tidal prism. As a consequence, most of the estuary is marine-dominated, with high-salinity waters in the outer half and a stronger, decreasing axial gradient of salinity towards the head. In the marine region of these estuaries, the seasonal cycle of temperature shows generally minima (12-13 C) in January-February and maxima (21-22 C) in July-August. Interannual variability of temperature from 1997 to 2010 shows that the 2003-2006 period is warmest than the precedent and the later ones. The seasonal pattern of Chlorophyll a differs between estuaries. In Bilbao it shows a unimodal pattern with summer (July-August) maxima whereas in Urdaibai the seasonal pattern is bimodal, with a major peak in early spring (March-April) and a secondary peak in late summer-early autumn (August-September). The interannual variability of Chlorophyll a during the period 1997-2008 does not show clear trends in the Bilbao and Urdaibai estuaries. The seasonal patterns of abundance for total mesozooplankton and several high taxonomic level categories also differ between Bilbao and Urdaibai. In Bilbao, total mesozooplankton, copepods, appendicularians, gastropod larvae and bivalve larvae are more abundant during the spring-summer period, and hydromeduses, siphonophores and cirriped larvae in summer. Cladocerans are more abundant in spring. In Urdaibai, however, as in the case of Chlorophyll a, bimodal patterns of abundance with spring maxima and secondary peaks in late summer-early autumn are observed for total mesozooplankton, copepods, hydromeduses, siphonophores, cladocerans and bivalve larvae. Spring maxima without clear later secordary peaks are observed in appendicularians and cirriped larvae. Doliolids are more abundant in late summer-early autumn, in both systems. During the 1997-2005 period, increasing trends of abundance are observed for total mesozooplankton, copepods and mollusk (gastropod and bivalve) larvae, and decreasing trends for cnidarians (hydromedusae and siphonophores). The other taxa show no clear trends or different trends between systems. The positive and negative trends of abundance observed for total zooplankton and several taxonomic categories during the 1997-2005 might be related to the general trend of temperature during the same period. The addition of new zooplankton data to the time-series in the future would be necessary to confirm zooplankton-temperature relationships to a median-term time-scale.