By Maggie Dutch and Valerie Partridge with the Marine Sediment Monitoring team
The communities of small invertebrates, also known as benthos, living in the sand and mud at the bottom of Bellingham Bay are showing signs of stress.
We conducted a bay-wide survey in 2010 to characterize the sediment quality at the bottom of Bellingham Bay. As part of the survey, we collected sediment from 30 locations, then counted and identified the critters in each one.
As benthic ecologists - scientists who study the sea floor and the animals that live there - we examined the number and types of species present, and compared them with other communities throughout Puget Sound.
Using the Benthic Index we developed, we classified the benthic communities at each of the 30 stations in Bellingham Bay as "adversely affected." The communities have been impacted in some way by either natural or human-influenced environmental stressors.
It's unusual that every station in the study was adversely affected.
Our ratings were based on consideration of the following indicators:
- Total abundance: high
- This is the number of animals in each sample.
- Taxa richness: low
- This is the number of unique types of animals, or species, in a sample.
- Dominance: low
- This is the number of species making up 75 percent of the sample.
- Stress-tolerant species: common and highly abundant
- These are the animals that can survive or thrive in environments that are polluted or have harsh conditions.
- Stress-sensitive species: rare or absent
- These are the animals that need pristine conditions to survive.
What do the indicators mean?
Although high total abundance is often considered a sign of a robust community, it's not in this case. When high abundance is due to only one or a few species being present in high numbers, it can be a sign of imbalance.
These conditions, when combined with abundant stress-tolerant and few stress-sensitive species, indicate that the benthic community is not healthy.
Marine wormsThe high total abundance values were due to very high numbers of a few stress-tolerant polychaetes, or marine worms, called Aphelochaeta spp. They belong to the family of worms named Cirratulidae, which are sometimes called “spaghetti worms” for their masses of long, thin feeding tentacles that stretch across the surface of the sediment.
Aphelochaeta spp are often present in low numbers in benthic communities throughout Puget Sound, but can increase exponentially in number in areas that are impacted by natural or human-caused stressors. They survive and thrive in these locations, while more stress-sensitive species cannot. In such conditions, taxa richness and dominance values decline.
Dominant speciesAphelochaeta spp were the dominant benthic invertebrates at every station sampled, often with well over 1,000 individuals per 0.1 square meter (about one square foot) collected from each location.
Several other polychaete species were also dominant in Bellingham Bay, including Owenia johnsoni and stress-tolerant Bipalponephtys cornuta and Heteromastus filobranchus. Together, polychaetes contributed an average of 91 percent of the total benthos abundance in Bellingham Bay, an unusually high value for Puget Sound.
Some stress-sensitive species, which had been present in higher numbers during 1999 and 2006 regional sediment surveys were noticeably reduced in abundance or absent from the benthos in 2010. These include arthropods such as the amphipod (sand flea) Protomedeia spp, the ostracod (seed shrimp) Euphilomedes carcharodonta, the pea crab Pinnixa schmitti, and echinoderms such as the brittle star Amphiodia spp.
Marine monitoring website and infoMore details about the sediment quality and benthos in Bellingham Bay examined in 1997, 2006 and 2010 can be found on the Marine Sediment Monitoring Team’s website, along with information on the design and implementation of the Puget Sound Ecosystem Monitoring Program (PSEMP) sediment monitoring component.