After a long and wet winter, sunny weather finally returned by the end of April. At the same time, north/northwest winds began to blow which signals upwelling favorable wind conditions. Wind stress measurements at Newport indicate that the spring transition occurred on April 26. Coastal upwelling brings high levels of nutrients from the deep water on to the continental shelf, which in turn fuels large diatom blooms during the spring and summer in the northern California Current.
During our cruise on April 27, we saw an immediate response of phytoplankton species to the high nutrient concentrations and the relatively cloud-free days. The diatom bloom we observed (Figure 1) was comprised of a variety of dominant diatoms, species that typically occur offshore Newport, such as Skeletonema costatum.
During this spring transition period, we were concerned that the influx of nutrients would fuel a potentially toxic bloom of Pseudo-nitzschia (PN) species. Compared to the low PN cell abundance in early April, PN species grew fast and reached far beyond typical bloom thresholds (10,000 cells/L). Since upwelling favorable winds have been dominant, it is very likely that PN species have been abundant in the coastal waters since the onset of the spring transition.
We recently headed out on May 10 to sample the NH line, but unfortunately, the hydraulics on the boat malfunctioned after the first station, preventing us to sample offshore. However, we did collect a surface water sample very nearshore (1 nautical mile offshore Newport) which showed a decaying large diatom bloom. However, in that bloom, PN species were still active and dominated the phytoplankton assemblage, albeit in reduced cell abundance compared to April 27 (Figure 2).
However, this very nearshore sample may not be representative of the phytoplankton community and PN abundance in the mid-outer shelf off Newport. The strong north winds and offshore Ekman transport over the last four/five days might have advected the PN bloom center further offshore, leading to an underestimation of the bloom level from this nearshore sample. Whether the observed PN bloom is causing significant toxicity to the coastal shellfish and crab is still too early to be determined. We board the NOAA ship Bell Shimada on May 22 for a broad scale research cruise, and we will continue monitoring the PN bloom, so stay tuned for more updates in the following weeks.
It would be remiss of us to not take note of the blue-hued visitors that have been washing ashore the past few months. I’m talking about the aptly named hydrozoan, By-the-wind sailor, or Velella velella. Velella are cosmopolitan hydroid colonies that use a gas-filled float and chitinous sail to float at the ocean’s surface. It feeds on zooplankton by extending short tentacles below the elliptical float.
For the past few years, people beachcombing along the Oregon coast have been encountering lines of these animals washing up along the shore. The densities vary, with stormy days bringing mass strandings where it can be hard to estimate abundances. If you’ve been observant the past few months, you’ve probably also noticed the Velella are getting larger. These observations are consistent with work in the Eastern Pacific commenting on the seasonal appearance of Velella. According to Bieri (1977), observations show small individuals, less than 10 mm, are seen December and January (Newportal Post) with the maximum size, about 80 mm, reached in late April with another peak in September. Interestingly, along Agate Beach this past week there was a large stranding that included size ranges from 2 mm to 65 mm!
As I continued my beachcombing, I noticed a dominant ‘handedness’ to the colonies. Velella sails are characterized as either left-handed or right-handed. In order to check for handedness, orient the colony vertically and examine the angle of the sail. If left-handed, the sail will start in the upper left and end in the lower right. Most individuals in the Eastern Pacific, whether washed ashore or at sea, are left-handed (Bieri, 1959 ; personal observations). The significance of this dimorphism is still debated and is often discussed in terms of zonal differences or prevailing winds. Can you find any right-handed Velella?
Thinking about size ranges and dimorphism calls into question what we know about Velella natural history. For such a charismatic species, their opportunistic nature makes them difficult to study. We still have much to learn about distribution, feeding, and reproductive strategies. Stay tuned as we continue to explore these mysterious sailors.
Bieri, R. (1959). Dimorphism and size distribution in Velella and Physalia. Nature, 184, 1333–1334
Bieri, R. (1977). The ecological significance of seasonal occurrence and growth rate of Velella (Hydrozoa). Publ.Seto Mar. Biol. Lab., 24, 63-67