Ecosystem Targets and Supporting Indicators

Water Temperature

Long-term annual water temperatures from 1960-2023, measured from multiple locations throughout the Long Island Sound.

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Long-Term Annual Soundwide Surface Water Temperature Trend (Fahrenheit)
Year	Soundwide	Slope (Fahrenheit)
1991	58.19	60.70
1992	53.97	60.72
1993	59.49	60.74
1994	65.27	60.76
1995	52.06	60.78
1996	61.98	60.80
1997	61.15	60.82
1998	64.05	60.84
1999	62.84	60.86
2000	61.94	60.88
2001	63.83	60.89
2002	58.84	60.91
2003	60.21	60.93
2004	62.66	61.95
2005	63.50	60.97
2006	65.60	60.99
2007	62.21	61.01
2008	61.63	61.03
2009	58.76	61.05
2010	61.48	61.07
2011	62.16	61.08
2012	63.83	61.10
2013	62.85	61.12
2014	61.35	61.14
2015	62.38	61.16
2016	63.09	61.18
2017	64.93	61.20
2018	57.69	61.22
2019	56.89	61.24
2020	62.42	61.26
2021	61.98	61.27
2022	58.75	61.29
2023	60.33	61.31
2024	59.18	61.33
2025	58.42	61.35

Long-Term Annual Embayment Surface Water Temperature Trend (Fahrenheit)
Year	Milford	Niantic Bay	Woods Hole
1960	52.72
1961
1962	52.44
1963	52.53
1964	52.33		52.3
1965	50.47		51.3
1966	51.73		51.7
1967	51.69		50.1
1968	53.26		51.2
1969	53.11		52.5
1970			52.2
1971	52.96		51.9
1972	52.02		51.8
1973	52.27		53.1
1974	51.58		52.8
1975	50.95		53.5
1976	52.66	51.62	51.8
1977	52.18	51.51	51.7
1978	53.92	51.15	51.2
1979	53.39	52.83	52.8
1980	52.97	51.98	52.2
1981	52.59	51.82	52.0
1982	53.39	52.03	51.6
1983	53.93	53.56	53.6
1984	53.32	53.55	52.9
1985	54.41	53.56	53.3
1986	54.07	53.40	52.4
1987	51.93	53.20	52.0
1988	53.15	52.02	52.2
1989		52.41	52.5
1990	53.74	53.73	53.6
1991	53.30	54.63	54.5
1992	52.78	52.61	52.2
1993	54.35	53.04	53.1
1994	53.31	52.88	52.9
1995	55.93	54.50	53.6
1996		51.04	52.2
1997		51.62	54.2
1998	56.03	53.85	54.5
1999	56.09	55.26	54.1
2000	54.03	54.00	51.6
2001	54.08	54.55	52.9
2002	55.40	54.81	53.4
2003	52.69	52.63	51.2
2004	54.36	53.10	51.7
2005	55.64	53.26	51.3
2006	55.43	55.47
2007	55.08	54.48
2008	55.43	54.77
2009	54.58	53.44
2010	55.57	54.25
2011	55.80	53.78
2012	57.49	57.22
2013	55.99	54.90
2014	54.62	53.67
2015	55.56	54.63
2016	55.33	55.60
2017	54.73	54.77
2018	54.77	54.32
2019	54.56	55.24
2020	57.33	56.59
2021	NA	56.48
2022	54.73	57.07
2023	55.85	56.34
2024	56.41	55.96
2025	55.75

Long-Term Winter Soundwide and Embayments Surface Water Temperature Trend (Fahrenheit)
Year	Soundwide	Milford	Niantic Bay
1960		35.20
1961
1962		34.86
1963		33.26
1964		35.36
1965		34.29
1966		32.66
1967		35.72
1968		34.50
1969		34.84
1970
1971		33.55
1972		36.65
1973		37.24
1974		34.86
1975		37.63
1976		38.64	39.06
1977		35.23	34.33
1978		34.08	35.46
1979		35.87	37.62
1980		36.89	38.20
1981		34.23	36.23
1982		35.76	36.08
1983		38.82	40.32
1984		36.12	39.70
1985		36.78	38.53
1986		36.97	39.27
1987		37.71	39.85
1988		37.14	37.89
1989		37.59	38.84
1990		38.57	39.70
1991	38.68	40.10	41.65
1992	38.46	38.39	39.98
1993	39.31	37.58	38.79
1994	33.58	35.88	36.53
1995	37.54	39.05	41.51
1996	33.46	35.59	37.12
1997	37.76		38.65
1998	39.87	41.07	40.35
1999	38.57	39.35	40.92
2000	37.83	37.84	39.48
2001	35.62	37.15	39.65
2002	40.71	41.36	42.18
2003	32.55	35.76	36.26
2004	33.97	35.82	37.70
2005	36.10	37.25	37.76
2006	36.05	40.11	42.25
2007	38.44	37.83	40.88
2008	38.71	39.33	41.64
2009	35.39	37.28	39.27
2010	36.27	38.86	38.82
2011	35.84	37.39	37.38
2012	41.75	44.04	44.96
2013	38.48	40.15	40.32
2014	34.10	36.27	38.16
2015	33.86	35.31	35.91
2016	41.32	42.31	42.76
2017	39.07	40.43	41.19
2018	36.01	37.93	38.83
2019	37.84	38.00	40.80
2020	40.59	40.53	43.59
2021	36.98	NA	42.58
2022	37.09	NA	42.52
2023	41.00	40.84	43.63
2024	41.49	41.36	42.91
2025	37.29	37.13

Soundwide Seasonal Bottom Water Temperature Trend (Fahrenheit)
Year	Winter	Spring	Summer	Fall	Annual Average
1991	38.56	49.36	67.96	60.42	54.08
1992	38.65	48.05	64.97	54.68	51.69
1993	36.46	48.76	66.20	57.45	52.22
1994	33.60	43.62	65.59	55.44	49.56
1995	37.45	48.57	67.08	58.14	52.81
1996	33.58	48.66	65.41	56.66	51.08
1997	37.73	47.25	66.74	56.69	52.10
1998	39.70	52.16	66.64	58.23	54.18
1999	38.62	50.45	68.50	59.14	54.18
2000	37.80	50.25	66.88	56.30	52.81
2001	35.70	48.76	67.51	58.59	52.64
2002	40.78	50.73	67.99	55.89	53.85
2003	32.52	46.50	64.34	55.93	49.82
2004	33.76	48.77	66.56	58.30	51.85
2005	36.28	47.15	66.39	56.66	51.62
2006	35.93	51.36	66.38	55.99	52.41
2007	38.77	47.39	67.04	58.32	52.88
2008	38.68	49.05	67.05	56.22	52.75
2009	35.59	48.69	65.54	58.41	52.06
2010	36.34	48.86	68.50	58.47	53.04
2011	35.57	48.88	67.18	59.88	52.88
2012	42.29	52.60	69.17	57.35	55.35
2013	38.55	49.27	67.82	58.00	53.41
2014	34.43	47.35	67.59	57.01	51.60
2015	34.10	44.42	68.65	59.32	51.62
2016	41.14	51.63	69.78	59.24	55.45
2017	39.07	49.85	67.80	60.18	54.23
2018	36.04	47.51	68.16	58.73	52.61
2019	38.12	49.10	66.69	58.62	53.13
2020	40.63	57.36	69.06	61.99	57.26
2021	36.51	49.91	68.63	60.35	53.85
2022	37.08	50.13	69.36	58.34	53.73
2023	41.20	52.69	68.11	58.65	55.16
2024*	41.40	49.95	67.07		52.81
2025*	37.03	49.09	67.46		51.19

WHAT IS THE WATER TEMPERATURE INDICATOR?

We use several water temperature datasets, located throughout the Sound, to improve our understanding of decadal and long-term changes in water temperature – Niantic Bay by Millstone Environmental Lab, Milford by NOAA Milford Lab, and Woods Hole by Woods Hole Oceanographic Institute, and an average of 17 stations collected throughout the Sound by CT DEEP (i.e., herein referred to as Soundwide). It is important to track water temperature as changes can significantly impact biological, chemical, and physical components in Long Island Sound. For example, increases in surface water temperatures have been linked to observed changes in the fish community – as cold-adapted fish (e.g., alewife, Atlantic cod, and winter flounder) have been observed less frequently in recent years, while warm-adapted fish (e.g., striped bass, black sea bass, and summer flounder) have been observed more frequently. Furthermore, increases in bottom water temperature also have an affect on the benthic community. For example, American lobsters cannot maintain their metabolisms in waters above 20°C for long periods of time and therefore have declined significantly in recent years. Additionally, these changes in fish community can have cascading effects up the food web, where the prey of these species are changing in assemblages, as well. Seasonal increases in water temperature can also impact different life stages of fish, shellfish, crustaceans, and zooplankton, and therefore may hinder their success in growth.

Here we report on both long-term annual and winter water temperatures to show us relative temperature change at several stations throughout the region over time. It helps us see the decade scale-up and downswings in the regional data, as well as the overall year to year variability. We also included winter temperatures because it is less impacted by daily weather patterns and station locations, and therefore stays pretty uniform temperature throughout the Sound. The other dataset shows Soundwide seasonal bottom averages. This dataset is much shorter, but higher resolution. By clicking the individual seasons on or off in the legend, you can more clearly see the patterns within each season.

HOW IS THIS TEMPERATURE DATA COLLECTED?

The long-term annual and winter graphs show several longer datasets from throughout the region. In most cases, these measurements are taken at least weekly from near-surface inshore waters in the vicinity of various laboratory facilities. The seasonal bottom water temperature dataset is collected by CT DEEP from 17 stations located throughout the Sound through the Long Island Sound Water Quality and Hypoxia Monitoring Program.

STATUS

When all stations are averaged, from 1991-2025, the Soundwide’s annual surface water temperatures has increased by 1.1, with a difference of 0.65°F;  whereas when all embayments are averaged, from 1960-2025, the annual surface water temperatures has increased by 8.6, with a difference of 4.4°F. Using the winter surface water temperature can be a stronger indicator as it removes fluctuations caused by storm disturbances – the Soundwide’s winter surface water temperatures have increased by 4.6 percent, whereas the embayment’s winter surface water temperatures have increased by 19 percent. Moreover, the rate of increase for the winter surface water temperatures in the Sound is approximately 1°F/decade, which is currently above the global average of 0.36°F/decade (NOAA 2025). Seasonal bottom water temperatures have been slowly but steadily increasing throughout the Sound. Because 2025 was a colder year compared to others, from 1991-2025, each season experienced a percent decrease – winter by 4 percent, spring by 0.5 percent, and summer by 0.7 percent (note: data for fall is not available). The overall mean from 1991-2025 is 37.03°F for winter, 49.09°F for spring, and 67.46°F for summer.