Size matters — What can we learn from biomass and size classification?

Eyes Under Puget Sound

For the first time, benthic invertebrate biomass is being studied on a large scale in Puget Sound. What does this mean? Sediment-dwelling benthic invertebrates (benthos) live in the mud or sand at the bottom of Puget Sound; their abundance and diversity can help us assess the health of the sediment ecosystem. As it turns out, the size of the animals is also critical when evaluating ecosystem health. Recording these biomass trends over time will help us learn more about how organisms respond to environmental stressors linked to climate change.

Three photos, scientists lifting Van Veen grabber, one person using hose to rinse sediment, a bulbous critter with dull red and yellow strip

LEFT: A sediment sample is collected using a double Van Veen grab. CENTER: Angela rinses a sample though a mesh screen. RIGHT: A sample containing Molpadia intermedia, the sweet potato sea cucumber.

Making it count

Our Marine Sediment Monitoring Program has been identifying and counting Puget Sound benthos since 1989. Although a few studies have examined the biomass of particular invertebrate species on a smaller scale, large-scale monitoring wasn’t put in place until 2016, when we added a biomass and size-classification component to our program. This is important because we want to know how organisms respond to environmental stressors associated with climate change and nutrient enrichment. These stressors include changes in temperature and salinity, decreasing dissolved oxygen levels, and changes in food quality.

That’s about the size of it

Biomass is a measure of the amount of living biological tissue in a system, which can tell us about productivity, or how animals use and cycle nutrients as they grow. Calculating biomass is no small feat. It means weighing many individual animals in the lab, most of them very tiny. We have weighed over 6,500 creatures so far! After an animal is weighed, we categorize it into one of five species-specific size classes (small, medium, large, extra-large, and megafauna) based on its length. Megafauna are large animals like sea cucumber and crabs that weigh 2 grams or more; when we analyze our data, we look at these animals separately, since just one of them can greatly outweigh all the small animals in a sample.

Three: a hand with tweezers placing a worm on a scale, tweezers hold another critter in front of a ruler, 5 snail shells of different sizes.

LEFT: A marine worm is weighed to the nearest thousandth of a gram. CENTER: A tiny ice cream cone worm is measured under a microscope. RIGHT: Size series of Astyris gausapata, the Shaggy Dovesnail.


Sneak peek

Total benthic invertebrate biomass

Total benthic invertebrate biomass

Although it will take a few years before we can look at trends in biomass and size class over time in Puget Sound, we’ve already observed some interesting results from our 2016 data. These preliminary results were presented as a poster at the 2018 Salish Sea Conference in Seattle.
Some of the major findings include higher biomass in shallow areas like Bellingham Bay and terminal inlets, and generally lower biomass in deep basins (see map).
Communities with high biomass tended to have lower numbers of individuals and low diversity (meaning that a few large animals dominated, but there were not a lot of different species). Communities with the lowest biomass had the greatest numbers of individuals and were the most diverse (meaning there were many smaller animals of many different species). This shows that high biomass alone is not necessarily an indicator of a healthy benthic community. Overall, the top contributors to total biomass were stress-tolerant species of marine worms and small clams.
The sizes of animals and how they change over time may shed light on the effects that stressors have on the development of individual organisms, and how that relates to the long-term stability of benthic communities. We certainly have much more to learn from this exciting new dataset, so stay tuned!