Monthly Kachemak Bay report, August 2025
Summary
Seasonal warming is now expanding into mid-water layers. A warm-water anomaly that started at the beginning of the year, is ongoing throughout the water column. A low salinity anomaly that also started this year, was also ongoing and may now be the longest since 2012. Chlorophyll concentrations were highest in the outer bay, but stayed below the seasonal mean.
Conditions
This month, Karyn, Martin, and Ross conducted the regular survey, which included T9 across the bay from the Homer Spit and the AlongBay transect on 13 August. Sea state during this survey varied from Beaufort 0 to Beaufort 3. Air temperatures at Homer Airport were above the seasonal mean for the four weeks leading up to this survey, whereas rainfall over this period, normally increasing at this time of the year, was minimal, not even in the 10th percentile compared to other years. We did not hear of further reports of sick or dying seabirds or marine mammals.
A complete set of up-to-date graphs can be found on the Google Drive: section plots and time sections.
Physical oceanography
Temperature
Recorded water temperatures during this survey ranged from a minimum of 9.0 °C at station AlongBay-12 (70 m depth) near Bear Cove to a maximum of 12.6 °C at AlongBay-7 (1 m depth) in the inner bay, not far from Homer Spit. In contrast to the previous month, there was a gradual warming of midwater layers down to about 40 m depth. This was likely due to thermal diffusion rather than mixing, since salinity did not show this pattern, but rather a sharply demarcated surface layer of fresher water. Waters near the mouth of the bay were more uniform in both salinity, and temperature, indicating a mixed water column (Figure 1 (a)). Even those mixed waters were warmer than the deep waters in the main channel, indicating that waters entering the bay and inlet had also warmed up,
As last month, satellite observations showed an area of cooler surface water from lower Cook Inlet to Kodiak Island, which is indicative of mixing or upwelling in this area (Figure 2). Compared to last month, sea surface have increased in much of the gulf, as should be expected in August. The pool of cool water in lower Cook Inlet had diminished in size, but its temperature did not increase.
Since the beginning of the year, water temperatures have been warmer than the seasonal mean, and this anomaly continued in August, but was diminished compared to previous months (Figure 3). The warm anomaly was also seen at the Homer SWMP station, operated by the KBNERR (Figure 4), and most pronounced above the pycnocline (Figure 5). The anomaly about 1 °C above the long-term average, and remained near the 90th percentile.
Salinity
Salinity values ranged from 21.8 PSU at AlongBay-10 (1m depth) near Bear Cove to 31.4 at AlongBay-3 (94m depth) in the outer bay. As last month, typical summertime patterns were found along the estuary with a pronounced layer of fresher water in the inner bay, setting up strong vertical stratification over the more saline deeper waters (Figure 1 (a), Figure 1 (b)).
Since the beginning of the year, salinities at the mid-bay station T9-6 have been lower than the seasonal mean (Figure 6), particularly in waters below 25 m (Figure 5). This anomaly was now amongst the strongest and longest-lasting salinity anomalies during the entire span of the time series and was particularly pronounced in the mid and deep waters (Figure 5). The persistence of fresher and warmer than average conditions throughout 2025 was similar to patterns observed during the 2014-2016 North Pacific marine heat wave, although current temperature anomalies were less intense (Figure 5).
Turbidity
Areas of high turbidity from glacial outflow were noticed at several stations in the inner bay. The turbidity sensor on the CTD indicated that high turbidity was restricted to the uppermost water layers, whereas mid and deep waters were clear (Figure 1 (a)).
Biological conditions
Phytoplankton
Surface waters were noticeably green to the eye. Chlorophyll concentrations observed with the CTD fluorometer were near the seasonal mean for most stations (e.g. Figure 7). Highest concentrations were observed in the inner bay in a thin layer immediately below the pycnocline (Figure 1 (a)).
Zooplankton
Even though several stations in the outer bay showed moderate to strong returns on the echo sounder, a five minute oblique tow through the area of strongest echo returns yielded a few copepods and chaetognaths, but not the expected abundance of zooplankton.
Wildlife
A group of two or three harbor porpoises were noted in the inner bay. In contrast to July, several shearwaters (sooty and short-tailed) were observed in the outer bay. These species are present in the Gulf of Alaska throughout the summer and fall, but for unknown reasons only rarely ventures into Kachemak Bay before mid-summer.
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