This brief article introduces a basic comparative analysis of the inter-annual variation in tide in two locations in Narragansett Bay and how they affect environmental indicators (such as salinity, chlorophyll, temperature, and dissolved oxygen). The data was collected from two stations: Phillipsdale (station 1), which is situated in a shallow water region and exposed to greater levels of human activity, and Conimicut Lighthouse (station 2), which is farther away from the urban area and preserves more natural conditions. In this talk, we investigated the possible impact of the annual variations in tidal elevations on environmental indicators, primarily focusing on salinity. The findings reveal that the average monthly tide heights are notably higher during early fall (August to October) than early spring (February to April). This further leads to increased horizontal flow, vertical upwelling, and turbulent mixing and results in salinity increases and chlorophyll blooms in both stations. As we advance, similar analyses will allow us to characterize the tidal nature of other estuaries of a similar kind, such as the Chesapeake Bay in Virginia.
Snapshot taken from Google map (02/14/2023) over Narragansett Bay, including some of the Rhode Island Sound. The boxed area shows our stations.
An estuary is a semi-enclosed coastal body of water typically formed where freshwater from rivers and streams mixes with saltwater from the ocean. According to Raposa, Narragansett Bay is a drowned river valley estuary and it is the largest estuary in New England, with approximately 146 square miles in size and is fed by several rivers, including the Seekonk, Blackstone, and Providence Rivers. The Bay is home to many coastal communities and a significant economic and recreational resource for Rhode Island. Several factors influence the Bay’s ecosystem, including freshwater inputs, tidal exchange, and pollutants. Despite these challenges, the Bay supports a diverse array of marine life, including numerous fish species, shellfish, and migratory birds.
From an ecological and environmental perspective, tides can have a significant impact on the salinity levels of coastal waters. The salinity of an estuary can be strongly affected by several tidal factors, including the amount of freshwater exchanging the system. When tides are high, saltwater can travel further into estuaries and rivers, increasing the salinity levels in those areas. Conversely, when tidal waves are low, fresh water from rivers and other sources can mix with saltwater, lowering the salinity levels. Tides can also influence the overall mixing in different scales and the circulation of water in a coastal area. When tides are strong, they can help to distribute salt and other dissolved substances evenly throughout the water column, helping to maintain a consistent salinity profile. However, when tides are weak, this mixing may not occur, allowing for the buildup of areas with high or low salinity. In addition, tides can also affect the exchange of water between the ocean and the atmosphere, which can further influence the salinity levels in coastal waters. For example, during periods of high tide, water can be trapped in lagoons or bays, leading to increased evaporation and, therefore, salinity. Overall, tides play a crucial role in determining the salinity levels of coastal waters and are essential to consider when studying an estuary like Narragansett Bay and its dynamic processes.
The relationship between tides and salinity in estuaries is complex. It can vary depending on several factors, including the size and shape of the estuary, the intensity and timing of tides, and the freshwater inputs from rivers and other sources. Estuaries are generally characterized by their dynamic and changing salinity levels, primarily driven by the interplay between ocean tides and freshwater inputs. In addition, other factors such as the intensity and timing of freshwater inputs, the overall size and shape of the estuary, and the presence of physical barriers can also impact the salinity levels in the estuary. For example, suppose an estuary is large and has limited physical barriers. In that case, tides may be less effective at maintaining consistent salinity levels, and the system may be more susceptible to changes in freshwater inputs. So, the relationship between tides and salinity in estuaries is complex and dynamic, and it is essential to consider each estuary’s unique characteristics and conditions when studying its salinity patterns. Therefore, in this study, we evaluated two stations with distinct geological features to understand the minute dissimilarities in the inter-annual average tide heights and their possible impacts on the Bay.
Study area and data
The first station in our study area (PH) is located at 41.8422°N, -71.3719°W, near the Seekonk River. The second station (CL) is situated at 41.7430°N, -71.3514°W, in the Providence River. Tide data for the study were collected from NOAA’s Tides and Currents. Salinity and other environmental parameters, such as chlorophyll, were obtained from Narragansett Bay Commission’s Snapshot. Both tide and salinity data were sampled at an hourly frequency. While tide data was available for both stations throughout 2020, environmental proxy data for Station 2 was only available from May to November. As a result, the inter-annual variation analysis for these parameters was conducted within this time frame. The units for the data are provided in Table 1.
Result and discussion
Tides
To analyze the tidal characteristics in Narragansett Bay, we plotted the hourly tidal variations for the entire year of 2020 (Fig. 2, lower panel). The data revealed a semi-diurnal tidal pattern, indicating two high and two low tides each day. High tides generally occur around midnight and noon, while low tides dominate during dawn and evening, consistent with climatological patterns. This predictability provides a clear understanding of when tide heights are likely to peak throughout the day.
When examining monthly average tidal data (Fig. 3, panel a), it was observed that tide heights in early fall (August to October) are higher on average than those in early spring (February to April). Both stations showed a similar monthly trend, but station 1 recorded lower averages during the winter (January to March). This discrepancy is likely due to station 1’s location in a shallow water region, where winter shorelines experience lower average water levels relative to the reference depth.
The histogram analysis (Fig. 3, panels b and c) revealed that low tides are more frequent than high tides across the bay, with a similar distribution pattern at both stations. This insight underscores the dominance of low tides in the tidal regime.
Table 1 details the units used for environmental proxies analyzed in this study:
- Salinity: Measured in parts per thousand (ppt).
- Temperature: Recorded in degrees Celsius (°C).
- Dissolved Oxygen: Expressed in milligrams per liter (mg/L).
- Chlorophyll: Reported in micrograms per liter (µg/L).
These parameters help interpret the interactions between tidal patterns and the bay’s ecological conditions, providing valuable insights into its dynamic processes.