Water Temperature and Dissolved Oxygen Levels Impact Crustacean Populations in Connecticut Waterways

Harbor Watch is a nonprofit organization that aims to improve water quality and ecosystem health in the Long Island Sound and other waterways around Connecticut. The group has been collecting data on water temperature, dissolved oxygen levels, and salinity in Norwalk Harbor since 1987. They also began conducting trawling surveys to measure the populations of aquatic species in 1990, and found that the population levels of many fish species are declining. “However, crustaceans were largely forgotten, and they wanted to fill this gap,” Roland Van Duine of Central Connecticut State University said.

Van Duine and Aidan Kieft (also of Central Connecticut State University) worked together to compare the catch totals of crustaceans over time to water conditions based on the Harbor Watch data. They discussed their work during a two-part minisymposium session of presentations from participants in the Preparation for Industrial Careers in Mathematical Sciences (PIC Math) program at the 2024 SIAM Annual Meeting, which is taking place this week in Spokane, Wash.

The team first pared down the catalog of crustaceans that live in Norwalk Harbor to only those that had a sufficiently sizable amount of available quantitative data, resulting in a list of seven species: blue crabs (see Figure 1), horseshoe crabs, long-clawed hermit crabs, mud crabs, sand shrimp, shore shrimp, and spider crabs. They also checked on trends in water conditions; based on Harbor Watch’s observations, dissolved oxygen has been decreasing while water temperature has been increasing over time. The collaborators used residual plots, Q-Q plots, and Shapiro-Wilk tests to test the statistical validity. 

Van Duine and Kieft then undertook a principal component analysis to reveal the relationships between the different aspects of water quality. “We used a linear combination of our original three variables—water temperature, dissolved oxygen, and salinity—to create the principal components,” Kieft said. The collaborators found that the water temperature and salinity have a significant positive correlation, and one can very accurately predict the other. These early steps helped guide the further direction of the research, leading to a sole focus on temperature and dissolved oxygen.

Their analysis demonstrated that blue crab population levels increase with temperature, for example, while the sand shrimp population is larger when there is more dissolved oxygen in the water. This comports with the yearly average trends from trawling, which indicate that the population of most species (including sand shrimp) has been decreasing over time as the level of dissolved oxygen falls (see Figure 2). However, a few species—such as the blue crab—are indeed experiencing population growth as water temperatures rise.

“We thought that there was more nuance to be found, so we then switched to a monthly format,” Van Duine said. When the collaborators included data points from the months of May through September—rather than yearly averages—the amount of available data was multiplied by five. This additional information, along with the use of regression to fill in missing data, helped improve the predictions.

“The trend is maintained when we go from yearly to monthly, but with more focus,” Van Duine said. While the slope of the number of blue crabs versus the water temperature was lower when they included the monthly data, there was a higher significance to the fit, with a higher p-value (see Figure 3). On the other hand, there were fewer outliers in the monthly sand shrimp data than in the yearly averages, leading to a steeper slope of their population versus dissolved oxygen.

“We saw in general that temperature has a high impact on the data’s variability,” Kieft said. In particular, blue crabs had the most significant p-values in regard to temperature out of all the crustaceans in their study. Their further analysis also confirmed that the population size of many species—including the sand shrimp, long-clawed hermit crab, mud crab, and shore shrimp—increases when more dissolved oxygen is present in the water. 

Overall, Van Duine and Kieft predicted that the blue crab population in Norwalk Harbor will grow as the water continues to warm, while the populations of many other crustaceans will decrease due to the impact of falling dissolved oxygen levels. As the total number of crustaceans that are caught by trawling in the harbor is decreasing, the duo recommended that local fishers shift away from the threatened species and towards blue crabs.

Acknowledgements

The speakers acknowledged Harbor Watch and their liaison Kasey Burns, as well as their advisor Viktoria Savatorova.