Salem Sound Water Quality
Overall, Salem Sound is a Healthy Estuarine Environment
Understanding the threats to the waters of Salem Sound and finding solutions that benefit the natural resources of our ocean and coast have always been at the heart of Salem Sound Coastwatch’s work. Salem Sound is well mixed due to tidal action with approximately 70% of its total water volume exchanged with each tidal cycle. But at certain times of the year, people ask Salem Sound Coastwatch about the murky, brown water. Monitoring the water chemistry is important for understanding the condition of our natural resources.
Establishing Salem Sound’s Baseline Conditions
In its early years as Salem Sound 2000, volunteers and staff assisted Massachusetts Division of Marine (DMF) with a year-long study of Salem Sound, which documented the status of marine resources and water quality, last studied by DMF in 1965 (Jerome et al. 1967). These two documents form the baseline for future comparisons.
In 2010-2011, Salem Sound Coastwatch worked with Professor Brad Hubeny of the Salem State University Geological Sciences Department to replicate DMF’s 1997 water quality testing. Results showed a measurable improvement in water quality since South Essex Sewer District (SESD) was upgraded to secondary sewage treatment in 1998, which reduced the number of nutrients and organic matter entering the Sound.
Decline in Water Clarity from 1997 to 2011
Questioning whether the eelgrass decline in Salem Harbor was linked with the decrease in water clarity (I.e. increase in turbidity), the partnership between SSU and Salem Sound Coastwatch continued with the Salem Harbor Turbidity Project (2012-2014). Temperature, conductivity, and turbidity were recorded every 15 minutes in Salem Harbor by four buoy suspended sondes. Sediment and water samples were taken from Salem Harbor, South and Forest Rivers, and stormwater outfalls. The most striking finding was that phytoplankton dominated the suspended particulate matter in the water.
During this time, SSU also took sediment cores, sampling the mud at the bottom of the Sound to piece together the historic record and the geological sedimentary layers of human impacts through land use changes, sewage disposal practices, and industry from pre-European contact to the present. The sedimentary record showed improvements over the past decades since the Clean Water Act of 1972 with reductions in the amount of organic matter in Salem Sound.
Both projects were published in the peer reviewed journals:
Understanding the Role of Phytoplankton in Water Quality
The Turbidity Project led to more research into the nature of the suspended organic matter in the Harbor by SSCW and SSU with Mass Bays Partnership’s Healthy Estuary grant funding (2018). This study confirmed the murky water is correlated with phytoplankton blooms, particularly dinoflagellates (Heterocapsa spp). Blooms occur during the summer when Nitrogen (N) is limited, and Phosphorous (P) is in excess. The highest turbidity and phytoplankton abundance occurs in the inner harbor near Forest River. High turbidity events occurred 60% of the time when N was limited, while only 10% when P was limited.
More questions arise from this research: what is causing the N/P imbalance in Salem Harbor, and how can it be remediated? Nutrients are necessary to all ecosystems, but in excess, they can increase algal production to potentially harmful levels.
What’s Causing the Murky Water in Salem Harbor?
Results were explained by principal investigator, Renee Veresh, at the Underwater in Salem Sound Lecture.
Phosphorus Driving Phytoplankton Blooms
To further understand the conditions of ecosystems in Salem Sound, more data was collected in the summers of 2019 and 2020. This research was funded by EPA (2019) and by NOAA Multipurpose Grant (2020). Salem Sound Coastwatch with Mass Bays Partnership and SSU collected data on water quality and seafloor condition to establish how nutrient loads affect the water quality and habitats of Salem Sound. In 2019, an autonomous vehicle, SeaTrac – a Marblehead company, was equipped with a sonde to collect temperature, salinity, dissolved oxygen, turbidity, pH, and chlorophyll (chl-a) continuously (every 10s) over space (transect) and time (tidal). In addition, grab samples of water were taken during both summers and analyzed. The data clearly show the difference in water chemistry from the freshwater of the Danvers River to Beverly Harbor and then the Sound. Phosphorus was highest in the Danvers River, near shore and at the bottom of Salem Harbor. Once again, data indicate that P is driving phytoplankton blooms (high chl-a) and is the essential nutrient for plankton growth in Salem Sound.
The study of the seafloor condition analyzed sediment grain size, which was found to be dependent on its relationship to shore. Stations with coarse material are closest to shore, probably due to wave action that pulls the finer sediments offshore into the Sound. When the benthic invertebrates living in the sediments were examined, we found that shallower areas with waters richer in nutrients and finer sediments had different species from the Salem Sound stations located further offshore with coarser sediment. Abundant important prey for bottom-feeding fishes such as winter flounder, oligochaete worms, and the amphipod – Ampelisca were abundant. These species are subtidal to intertidal species with tolerance to wide temperature and salinity ranges in estuarine waters.
Our work continues to understand water conditions to protect habitats and marine species. The nature of tidally driven upwellings from the bottom of the Sound to the surface and the source of phosphorus in Salem Harbor are next topics to be studied.
The water quality, nutrients, and phytoplankton data generated here on trends and conditions in Salem Harbor have filled a critical data gap in the Salem Sound embayment. With this knowledge of phosphorus nutrient loading, associated Heterocapsa activity, and the negative effects of these on the clarity of Salem Harbor’s waters, specific remediation strategies need to be developed for the improvement of Salem Sound’s water.