Total suspended solids are particles suspended in the water column measured as the dry weight of particles filtered from a known volume of water. They can consist of phytoplankton or pieces of plant matter, but most total suspended solids are generally made up of inorganic particles, such as sediment. Sources of suspended solids include erosion from stream banks, salt marshes, and the upland portion of the watershed. Surface water inflows, stormwater runoff, and wastewater treatment effluent all can deliver total suspended solids to estuaries. In addition to external sources of suspended solids, they can also originate from resuspension within an estuary. Increasing suspended sediments reduces water clarity and light availability for primary producers such as eelgrass, seaweeds, and phytoplankton. High total suspended solid values can also negatively impact oyster feeding and the aesthetic value of our estuaries.

No increasing trends for total suspended solids.

The Adams Point Station has the longest time series dating back to 1989. Since that time, total suspended solids have statistically increased, but concentrations appear to be decreasing in recent years (Figure 13.2). Median annual low measurements at Adams Point over the last three years (2019 – 2021) are all lower than the six preceding years. The Upper Piscataqua River Station has a similar pattern, with recent years showing a plateau in total suspended solids concentrations (Figure 13.3).

Broadly, Table 13.1 demonstrates clearly that total suspended solid concentrations have increased around the Great Bay Estuary over the past two decades, with most of the stations indicating statistically increasing trends. For the Hampton-Seabrook Estuary, data on total suspended solids has only been collected for the period 2018 through 2021. During that time, values have been comparable to the Great Bay Station, ranging between 18.3 and 23.2 mg/L (Figure 13.4). At the Great Bay Station, since 2002, suspended solids have remained relatively stable, with a low tide median concentration of 17.1 mg/L for the entire record. The observed statistical increase in suspended solids at the Great Bay Station is likely driven by the eight measurements that exceed 50 mg/L since 2015. Despite the increasing trend, the median concentration decreased in 2021 to 18.3 mg/L (Figure 13.5). The median concentration in 2021 exceeded that of Adams Point (17.9 mg/L).

For context, consider that values over 15 mg/L are often cited as being very challenging for the health of eelgrass.41 When concentrations are regularly above 15 mg/L, light is blocked and eelgrass cannot photosynthesize, while seaweeds and phytoplankton continue to thrive since they have lower photosynthesis requirements. For more on total suspended solids and eelgrass, see “Light.”

High suspended solids concentrations also have the potential to harm oysters. Oyster monitoring efforts indicate that oyster reefs that are not built high enough above the estuary floor can be smothered by sediment deposits.

As is often the case with estuarine science, there is a range of conditions that are optimal for ecosystems. For example, some light scattering by total suspended solids helps phytoplankton suspended in the water column receive sufficient light to grow. On the other hand, if the water is clearer, more light reaches the bottom, which benefits eelgrass. Thus, there is a “sweet spot” that balances the light available to phytoplankton — critical food for oysters and fish — with the light for benthic primary producers like seagrass.

Chemists, ecologists, hydrologists, and oceanographers are working together to understand the connections between the source and transport of sediments in our estuaries. For example, decreases in eelgrass and oyster habitats lead to greater resuspension of sediments, but sediments may also be added to the estuary from the surface water inflows or the estuary shores. At the same time, hydrologic and hydrodynamic drivers (e.g., changing climate patterns such as drought or record rainfall) can influence the delivery of total suspended solids from the surrounding watershed and the flushing of total suspended solids from the estuary. Finally, it is important to acknowledge that a certain amount of sediment transport from the watershed is necessary to maintain salt marsh elevations, and so is a key factor in determining salt marsh resilience to rising sea-level.

Acknowledgments and Credit

Anna Mikulis (UNH) with contributions from Miguel Leon (UNH), Kalle Matso (PREP), Easton White (UNH), Lara Martin (UNH), and Tom Gregory (UNH).

Total Suspended Solids – Extended Report
Includes methods, data, and additional discussion not included in the Printed Edition.