Monthly Kachemak Bay report, June 2025
Summary
The warm-water anomaly that started at the beginning of the year was much diminished this June, likely a consequence of the cool air temperatures. A low salinity anomaly of deep waters, however, was ongoing. Driven by freshwater influx, strong stratification of surface waters extended from the inner into the outer bay, which created a stable water column resistant to mixing. Chlorophyll concentrations in the inner bay had dropped from their peak last month and areas of highest concentration had shifted into the outer bay. Nutrient levels in the inner bay were likely depleted from the bloom last month with stratification preventing replenishment. Weaker stratification and proximity to upwelling in Cook Inlet were likely factors driving high chlorophyll concentrations in the outer bay through an ongoing supply of nutrients.
Conditions
On 12 Jun 2025, Chris, Hans, Martin, and Ross conducted our monthly survey. While within normal limits, it has been a cool and wet June so far. Average air temperature at Homer Airport were still below 10 °C [50 °F]. Conditions during the survey were choppy in the outer bay ( < beaufort 5, with short steep waves up to 1.2 m, leading us to skip station AlongBay 1), but were predictably calmer in the inner bay. 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 6.1 °C at station AlongBay-12 (69 m depth) in the inner bay to a maximum of 11.7 °C at AlongBay-12 (1 m depth). Throughout the bay, even in the outer bay, thermal stratification was evident, with a thin layer of warmer surface waters covering colder mid- and deep waters. In contrast to winter, differences between the inner and outer bay were minor: surface layers were warmer in the inner than in the outer bay, whereas mid- and deep waters were warmer in the outer than in the inner bay (Figure 1 (a)). Only minor temperature differences were apparent between the north and south end of the mid-bay Transect 9 (Figure 1 (b)), showing a gradual warming from deep to shallow waters, with fastest changes near the surface.
Satellite observations showed that farther out in Cook Inlet, sea surface temperatures (SST) were substantially cooler (3 K) than in the bay, and even cooler than in much of coastal Gulf of Alaska (Figure 2).
Since the beginning of the year, water temperatures in the middle of the bay, at station T9-6, have been warmer than the seasonal mean. This anomaly continues in a much diminished form in deep waters (Figure 3), but in line with the cool air temperatures, this anomaly is no longer evident at the surface (Figure 4, Figure 5).
Salinity
Salinity values ranged from 26.2 PSU at AlongBay-12 (1m depth) to 31.4 at AlongBay-2 (69m depth), near Bear Cove and Seldovia, respectively. A substantial layer of fresher water, from the surface down to about 25 m depth was present in all but the outermost parts of the bay (Figure 1). A defining feature of the summer months, this stratification now extended well past the more sheltered waters of the inner bay (Figure 1 (a)). As indicated by the Brunt–Väisälä frequency (bvf, Figure 1 (a)), even the far reaches of the outer bay showed some degree of stratification, however, water column stability there was much weaker than in the inner bay. Even though the survey this month did not extend into lower Cook Inlet, the satellite SST data showed a pocket of cool water in lower Cook Inlet, surrounded by warmer waters (Figure 2). The most likely cause is local upwelling (likely driven by the strong tidal currents in that area), which would preclude stratification.
Since the beginning of the year, salinity at T9-6 had been lower than the seasonal mean (Figure 6). In contrast to the temperature anomaly, the salinity anomaly was ongoing. The magnitude of this anomaly was now the third-largest such event in thirteen years of observations (Figure 6).
Turbidity
We did encounter visually highly turbid surface waters, indicative of glacial run-off, at several stations in the inner bay, but also on the southern part of T9. Both transects showed two layers of turbid waters: at the surface, and near the bottom (Figure 1), likely caused by a combination of terrestrial and glacial run-off (surface and bottom) and phytoplankton activity (surface).
Biological conditions
Phytoplankton
Surface waters were noticeably green to the eye. High concentrations of chlorophyll were observed with the CTD fluorometer between the surface and 30 m throughout the bay. In contrast to the previous month, highest concentrations were now found in the outer bay (Figure 1 (a)). At T9-6, off the Homer Spit, chlorophyll concentrations peaked last month and now dropped substantially below seasonal mean (Figure 7).
Strong stratification of the water column in the inner bay was an effective barrier to vertical mixing. This was likely the reason for the observed drop in chlorophyll concentrations. The plankton bloom last month depleted nutrient concentrations and stratification prevented their replenishment. High chlorophyll concentrations in the outer bay were likely due to resupply from Cook Inlet and weaker vertical stratification forming only recently.
Zooplankton
There were technical issues with the echo sounder. No samples were taken.
Wildlife
No marine mammals or birds beyond the common expected species were noted.
Mailing list
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