Bad weather for Sailors, but good for Diatoms

Beach One was the location of a mass stranding of Velella lata, more commonly known as by-the-wind-sailors. These little blue creatures are no more than 5cm long and are related to jellyfishes and anemones. Their natural habitat is on the surface of the open ocean, where the Velella feed on plankton.



They move across the ocean with their distinctive transparent "sail" catching the wind, visible in this first picture.

Every now and again a change of wind direction (like the cold blast we are currently experiencing) blows thousands of Velella helplessly towards the shore.
This second picture shows the underside of the Velella where its feeding tentacles are visible.


The same sort of weather that was responsible for the stranding of the Velella brought waters rich in nutrients that has allowed the Diatoms to bloom along the surf of Beach One.

This picture taken this morning at Beach One showing the characteristic browny-green "stain" of a Diatom bloom.

Surf Diatom Blooms

The cold weather often bring extensive surf diatom blooms to the waves along Venus Bay. The strong winds have been whipping up plenty of foam to help the blooms spread. The surf diatom and a very large number of small crustaceans washed up overnight are likely to make great food source for the grazing shellfish under the sand. That is if there are enough pipis left!

Plenty of Diatom Blooms

There are a number of patches (5-15meters wide) where ever I looked along the Venus Bay beach today. Whilst the foam and waves where heavily stained, the amount of staining on the beach at low tide (early morning) is consistent with the “urban legend” that these blooms are only a day light phenomena. The characteristic circular patterns as the diatoms slide off bubbles in the surf foam are still the easiest way to identify these surf diatom blooms.

Surf Diatom Bloom at Beach 1

Seperate bloom of Surf Diatom At Beach 5

What scum is that?

The recent heavy rains have all the low lying areas and drain permanently full of water, and a lot of different slimes, algaes and mosses growing rampantly (mainly in bright shades of green). Whilst most are not harmful it is important to realise that the relative shallow depth or Andersen inlet makes it a high risk for damaging algal blooms, such as the toxic blue-green algae. Some blooms such as diatoms (more likely to be a greeny brown) can be benefical.  The NSW office of water, give details of how to obtain a copy of “What scum is that?” an aid to identify Algal blooms and other prolific plant growths and a simplified field guide called Keys to blooms.

Small Blooms in the Surf

There were two small blooms of  surf diatom between beach one and beach two today. It was definitely cold with and on-shore wind.

Those brown sludges

Those brown sludges that re-occur from time to time (more often in the cooler months april to july) along the clean sand beach at Venus Bay are normally due to "blooms" of surf diatoms, and they are not due to pollution. These diatoms are not toxic, although they may cause some skin irritation. So it is advisable to avoid swimming in dense patches or at least shower after swimming or surfing.

These diatoms are small marine plants that have a silica casing (and are therefore heavier than water). The diatoms mainly live in the sand but use the foam in the surf zone as a means to stay afloat and spread. As the foam washes ashore, the diatoms slide off and/or the bubbles of foam burst leaving a characteristic ring pattern in the sand. Large accumulations of diatoms are naturally occurring and the diatom themselves are a major food source for prawns, fish and filter-feeding molluscs (such as pipis).

The EPA (see link below) suggest these "Blooms" of surf diatoms may be related to weather conditions, particularly onshore winds. Similar blooms on the pacific coast of USA are linked to upwelling in cold water currents (the suggestion is that the change of temperature precipates iron from the sea water and feeds the bloom).

So a nice side observation project for us is to record when the blooms are visible and how extensive they are, also the wind conditions and temperature (hot, warm, cool, cold, very cold is probably sufficient for now, but there is a link to the Pound Creek weather station over on the right-hand column of this blog). The most important observation is to distinguish them from other sludge/pollutants such as oil spills, untreated sewerage or other effluents.

• They have a distinct golden coffee colour
• They do not smell (of oil fumes or in fact any distinct smell)
• No iridescent (rainbow-coloured) slick on the water surface
• They are not sticky and if you collect them just on the stained surf foam it feels gritty
• They do not form blobs or lumps in the sand

Much of this information comes from the EPA's Surf Diatom Page