Maintenance of coastal surface blooms by surface temperature stratification and wind drift

Abstract

Algae blooms are an increasingly recurrent phenomenon of potentially socio-economic impact in coastal waters globally and in the coastal upwelling region off northern Baja California, Mexico. In coastal upwelling areas the diurnal wind pattern is directed towards the coast during the day. We regularly found positive Near Surface Temperature Stratification (NSTS), the resulting density stratification is expected to reduce the frictional coupling of the surface layer from deeper waters and allow for its more efficient wind transport. We propose that the net transport of the top layer of approximately 2.7 kilometers per day towards the coast helps maintain surface blooms of slow growing dinoflagellate such as Lingulodinium polyedrum. We measured: near surface stratification with a free-rising CTD profiler, trajectories of drifter buoys with attached thermographs, wind speed and direction, velocity profiles via an Acoustic Doppler Current Profiler, Chlorophyll and cell concentration from water samples and vertical migration using sediment traps. The ADCP and drifter data agree and show noticeable current shear within the first meters of the surface where temperature stratification and high cell densities of L. polyedrum were found during the day. Drifters with 1m depth drogue moved towards the shore, whereas drifters at 3 and 5 m depth showed trajectories parallel or away from shore. A small part of the surface population migrated down to the sea floor during night thus reducing horizontal dispersion. The persistent transport of the surface bloom population towards shore should help maintain the bloom in favorable environmental conditions with high nutrients, but also increasing the potential socioeconomic impact of the blooms. The coast wise transport is not limited to blooms but includes all dissolved and particulate constituents in surface waters.

Figure 1. Schematic representation of different thermal stratification depth scales.

Wind forcing (yellow arrow) induces the movement of the near-surface layer (big arrow within the water column).

Figure 2. Study site, Todos Santos Bay.

Baja California, Mexico (31° 40′ to 31° 56′N and 116° 36′ to 116° 50′W).

Figure 3. Schematic representations of CODE-type drifters.

Each drifter consisted of flag, GPS attached above floatation (yellow ovals), kite-type drogues of 1 m² in two directions (blue diamond), and 3 thermographs at depths 1, 3 and 5 m.

Figure 4. Lingulodinium polyedrum cells from sediment traps.

Vegetative cells from sediment traps deployed on October 5, 2011. A. Vegetative cells with red chlorophyll autofluorencese (Ex. 450 nm, Em. 680 nm). B. Cyst cell with green autofluorecence (Ex.495 nm, Em.520 nm), objective 20X. Scale bar: 10 µm.

Figure 5. CTD profiles taken on October 4, 5, 6, 11, 12, and 18, 2011 during a dense algal bloom in Todos Santos Bay, Mexico.

Temperature profiles (A) and chlorophyll profiles (B) Profiles of the different days are offset as indicated at the bottom below the dates.

Figure 6. Virtual displacement from ADCP (Aquadopp, Nortek) and CODE-type drifter trajectories on September 21, 2011.

Depths are indicated at the end of the trajectories. ADCP (blue), Drifters (red). ADCP trajectories are for each 0.5 m interval. The dominant wind direction is indicated by the grey arrow.

Figure 7. Wind pattern during October 2011.

Average values (black) +/− standard deviation (grey). 12∶00 of local solar time represent the time of minimum zenith angle. (A) Wind direction and (B) Wind speed [m s⁻¹].

Figure 8. Drifter and wind vectors.

Averages are from 13 to 15 hrs on October 4, 5, 6, 11, 12 and 18, 2011. Drifter velocities (1 m, red; 5m, blue) differ in scale from wind velocities (broken black). North is in the ordinate direction, and East in the abscissa.

Figure 9. 1m drifter and wind velocity components.

Time series of the longitudinal (u) and latitudinal (v) components, with scales as indicated on ordinates. Wind (dashed lines), drifters (solid lines), longitudinal u (black), latitudinal v (red).

Figure 10. Schematic representation of continuous temperature stratification in the first meters and near surface shear flow.

Temperature profile from 12-Oct-2011, current vectors are hypothetical.