by Simon C Brown
Fish and crabs need oxygen to stay alive just like you!
That’s why measuring the dissolved oxygen (DO) in the water can tell us a lot about the health of Spa Creek as a suitable habitat for aquatic living resources. The figure below shows the dissolved oxygen (DO) levels sampled by Spa Creek Conservancy in 2016 at three stations (Ego Alley, Capital SUP in Eastport, and Hawkins Cove — click for a map of water collection sites).
Sampling occurred from June through October and included multiple water depths (surface, middle, bottom). Colors indicate levels of DO that are unsuitable for various aquatic living resources. While blue crabs need >3 mg/L of DO to survive, fish like White Perch and Striped Bass need >5 mg/L of DO. For the Creek to act as spawning and nursery habitat for fish the DO needs to remain >6 mg/L until June.
Looking across the data set we can see several things:
● Seasonally, as water temperatures increase and then peak in August the DO noticeably declines. This trend is especially apparent at the Ego Alley station across all three depth layers. In September and October, water temperatures began to cool and DO levels gradually rise again. This seasonal trend is due to the solubility of oxygen in water decreasing with increasing temperature (i.e. warm water holds less DO) and can exacerbate DO conditions in hot summer months.
● At some point over the entire sampling period, all stations experienced various levels of degraded DO conditions from moderate (<5 mg/L) to severe hypoxia (<2 mg/L).
● The DO measured at Hawkins Cove (surface and bottom layers) and Capital SUP (bottom layer) during July and August reached severely hypoxic levels (<2 mg/L) indicating poor conditions for benthic dwelling animals like crabs.
● While the DO was generally low in the bottom layer of the Capital SUP station, the surface layer showed good DO levels (>5 mg/L) in almost 90% of the dates that it was measured.
We have work to do so that crabs, fish and subaquatic vegetation can survive in Spa Creek.