reached a maximum of 23 µg L-1 following the storm (26th April), remaining at an elevated concentration until the end of the study; chlorophyll levels after the storm were significantly higher compared to before storm (Mann-Whitney U = 0, n=106, p<< 0.01), and so was cell concentration (U = 0, n= 13, p< 0.05). Phaeopigment levels were low during the pre-storm period (range: <0.1 to 0.7 µg L-1) and increased by ca. one order of magnitude during the storm (5.5 µg L-1); however, on the first sampling day after the storm phaeopigments had disappeared from the water column except near the bottom at 8 and 9 m depth, and remained at very low values until the end of the study. Stratification index S correlated negatively with wind speed averaged over 12 h before sampling when the whole period was considered (Spearman r = -0.57, n = 13, p < 0.05, Fig. 5A), but strongly peaked immediately after the storm (on the 26th April, Fig. 5B). Chlorophyl stratification on 26th April seemed to diverge from the general correlation trend of S vs wind intensity; if that date is not considered the correlation becomes highly significant (Spearman r = -0.68, n = 12, p = 0.01).
Figure 2. Time series of wind velocity recorded at Coliumo Bay during the study period. E-W denotes the East-West (above) and N-S indicates the North-South (below) components of the h o r i s o n t a l w i n d v e c t o r . I n s i t u r e c o r d e n d s o n A p r i l 2 3 r d ( b l a c k d o t s ) d u e t o t e c h n i c a l p r o b l e m s . F r o m A p r i l 2 3 r d t o 3 0 t h v a were estimated according to Calliari & Alfaro (1997) (triangles). l u e s
During the first 9 d wind from the S prevalied, following a daily cycle of low velocities in the morning, highest during midday and afternoon (up to ca. 19 km h-1), decreasing during evening and night to ca. 0 km h-1 (Fig. 2). On April 20th that pattern changed toward N winds with sustained speeds ca 40 km h-1 and gusts up to 82 km h-1 (storm event). After April 24th wind pattern returned to before-storm conditions until the end of the study period. Direct observation of wave length and water colour inside the Bay during the storm indicated a
mixed water column and strong of bottom sediments. Temperature and
Figure 3. Temperature (ºC) and salinity (practical units) measured at Coliumo Bay during the pre-storm period.
profiles evidenced a fairly with a weak temporary
mixed water stratification
between April 13th and 15th (Fig.3)
levels 5 µg
characterized the pre-storm period (Fig. 4). During the storm (22nd April) chl raised to ca. 18 µg L-1, and
The microplankton assemblage was strongly dominated by diatoms and dinoflagellates to a lesser extent during the whole study period (Table 1). However, before storm samples on one side, and samples taken during and after the storm on the other side were much more similar within each group and. formed clearly separated clusters (Fig. 6).