We have several kinds of "green tide" algae proliferating right now simultaneously. One is Ulva aka Sea Lettuce (soundwaterstewards.org/... ) the translucent vivid green sheets piling up on rocks as in the title photo above, and in drifts along the shoreline (photo below).
Sea Lettuce’s bright green sheets photosynthesize vigorously, absorbing dissolved
carbon dioxide from the water converting it into oxygen which both dissolves into the sea (benefiting aquatic animals) and bubbles up into the atmosphere (benefiting terrestrial creatures): three valuable ecological roles. You can see the bubbles of oxygen trapped under the top layer of Sea lettuce in the closeup. This algae is also a source of food for herbivorous invertebrates like snails, chitons, sea urchins and amphipods. Some dabbling ducks, like Wigeons, are very fond of Sea lettuce. The masses of algae also provide shelter and habitat for small invertebrates.
So what’s not to like? A clue is in the color of the deep pile by the shore. No longer green, the dead sea lettuce is decomposing, and like anything that rots, this mass uses up oxygen, releasing carbon dioxide. The proximate effect is to deoxygenate the shallow water, suffocating aquatic animals who need to absorb dissolved oxygen through their gills or skin. To the otter poking its head up through the mass the depleted oxygen is no big deal, otters being air-breathers, but if this continues, otter will find fewer prey in this bay, as crabs and fish die or move elsewhere.
This shallow quiet bay hosts a big eelgrass bed. We’ve heard before how valuable eelgrass meadows are ecologically (see RonK’s excellent Buckets describing their function and current threats: www.dailykos.com/…, www.dailykos.com/...). Eelgrass ecosystems are far more productive than any forests, sequestering carbon dioxide, generating oxygen, feeding a whole range of critters from sea slugs to herons, and acting as a safe nursery for young invertebrates and fish. In recognition of their value, many protections have been adopted, such as requiring grating on all new docks instead of a solid deck that blocks light to the eelgrass below. In this bay, the Tulalip Tribes replaced their old fishing dock with a new one a few years ago on the same footprint that now allows light; eelgrass can grow underneath it for the first time in a century.
This summer though, the light coming through the dock and everywhere else in
the bay has created another "green tide” problem, this one for the eelgrass: a vast increase in the usual modest population of epiphytic green algae that uses the eelgrass blades as a substrate. Some species that in most years lightly fur the blades, such as Cladophora and Gracilaria (www.faculty.virginia.edu/...) have formed such big clumps that much of the eelgrass is smothered.
Epiphytic algae on eelgrass is an important food source for herbivores (such as sea urchins, snails, limpets, and chitons) and detritivores (like sea cucumbers, crabs, amphipods, and isopods) but these massive clumps have overwhelmed their ability to keep up.
In fact the texture and sheer volume appears to be deterring them. I’ve seen very little invertebrate life crawling on the eelgrass, such as the very pretty Taylor’s Sea hare.
Nor have I seen much spawning activity this spring and summer. In previous years the eelgrass blades were covered with nudibranch egg cases and I’ve watched Hooded Nudibranchs and Bubble Snails mating and depositing them in vast numbers.
Some eggs hatch, but most become high-quality food for fish, ducks and other animals in the bay.
.
.
.
What’s different this year in the bay?
We’ve had an unusually wet winter and spring (w2.weather.gov/...) in the Pacific Northwest followed by an unusually warm and sunny past two months. Heavy rainfall washed lots of sediment into the bay from nearby fields and residential yards. Animal manure, agricultural activity and leaking septic systems add nutrients to the water, especially nitrates. Canada Geese staying the winter also contribute nutrients. Add the recent warmth and light and you have optimal conditions for ephemeral algae to proliferate explosively, smothering perennial eelgrass and the shoreline (file.dnr.wa.gov/...).
The Sea Lettuce has begun to die off, and will wash away by fall leaving a clean substrate. I can’t be as sure the eelgrass will fully recover though. It’s starving right now. If the plants had a chance to grow enough new shoots down below the canopy earlier in the spring those may take off later once the epiphytic algae breaks down and washes away in tidal currents. For this season, the eelgrass is on hold. Our sunny weather started early this year; historically the sun comes out after the 4th of July and stays mostly clear and warm until October. The algae season will likely be longer this year.
I can’t help but wonder about the longer term too. Climate change in the Pacific Northwest is predicted to bring heavier winter precipitation and warmer temperatures year round, conditions we are seeing this year. I hope our eelgrass meadows and aquatic nearshore ecosystems can handle the change.
What’s happening in your natural neighborhood this summer’s day?