Monthly Kachemak Bay Report, September 2026

Summary

While almost all of the North Pacific is experiencing unusually warm conditions, all of Kachemak Bay was colder than normal, by over 2.5 standard deviations in some places. The usual early-fall warming of deeper waters was evident, but to a much lower degree than usual. The water colum was still vertically stratified, but other than in the surface layer, salinity was about two standard deviations higher than normal. Chlorophyll concentrations were highest near the surface in the far outer bay and near the head of the bay. Those concentrations were high for the season but not lower than during summer.

A complete set of up-to-date graphs can be found on this Google Drive: section plots and time sections (request access from martin.renner@noaa.gov).

Conditions

Air temperatures leading up to this survey close normal for the season (Figure 1). September is usually the wettest month of the year. In 2026, precipitation peaked in July and slightly below-average amounts in the month leading up to this survey. The mountains near the head of the bay had some fresh snow near the peaks.

Results are shown for the Kasitsna Bay Lab oceanographic survey conducted on 24 September 2026 by Jen, Kris and Martin in calm conditions.

Physical oceanography

Temperature

Water temperatures during the September survey ranged from a minimum of 8.3 °C at station AlongBay-5 (161 m depth), in the outer bay, to a maximum of 10.5 °C at station AlongBay-11 (1 m depth) in the inner bay. Vertical stratification was observed throughout the bay, with a layer of warmer, fresher surface water over cooler, more saline waters below (Figure 2 (a), top-left panel). In contrast to earlier in the summer, there was no well demarkated thermocline, but a more gradual cooling with depth. Unusually, the much shorter cross-bay transect T9 showed more horizontal pattern this month than the AlongBay transect. T9 patterns were complex. On the south surface waters were warmer than at the same depth on the north side, whereas at depth below 50 m, waters at the same depth were colder on the south than on the north side.

Satellite sea surface temperature data show Kachemak Bay and lower Cook Inlet as a pool of cool 9-10 °C waters, surrounded by warmer water in the upper inlet and the Gulf of Alaska upstream (Figure 3).

(a) AlongBay Transect from Kachemak Bay entrance (left) to Bear Cove (right).

Figure 3: Sea surface temperature in the Gulf of Alaska, based on satellite remote sensing data

Figure 4: Surface water temperatures (0-20 m) at station T9-6, off the Homer Spit, compared to the seasonal means

The bay was colder than normal at all depth and locations other than near the surface on the south side of the bay. Deep waters were up to a remarkable 2.5 standard deviations colder than normal (Figure 2, top-right panel, Figure 6, Figure 8). Most of Kachemak Bay was about 1 °C colder than normal. This cold spell has persisted now since December 2025 (Figure 4, Figure 5, Figure 6). Surface waters of the Gulf of Alaska, and almost anywhere else in the North Pacific, were warmer normal in September 2026, as shown by satellite sea temperature anomaly data, making Kachemak Bay a rare exception to the larger scale situation (Figure 7).

Figure 5: Water temperature at the Homer Harbor SWMP station, operated by the KBNERR, at approximately 7 m depth (This is the deeper of two sensors, but is still located in the surface layer).

Figure 6: Recent bottom water temperatures in outer Kachemak Bay (station AlongBay-5, 172 m depth) compared to the long-term mean at this location.

Figure 7: Global sea surface temperature anomaly, based on satellite remote sensing.

Salinity

Salinity values during the September survey ranged from 26.6 PSU at station 9-8 (1m depth) off the Homer Spit to 31.6 PSU at station AlongBay-5 (161m depth) in the outer bay. The water column was vertically stratified at all estations. The north side of the T9 cross-bay transect had a noticeably thicker layer of fresher surface water, indicative of freshwater outflow from glaciers and streams exiting Kachemak Bay (Figure 2 (b)).

All the deep- and mid-waters of Kachemak Bay were substantially saltier than normal, at depth by as much as two standard deviations (Figure 2, Figure 8). Surface waters in the outer bay were slightly fresher than normal, showing that the layer of fresh surface waters extended particularly far (Figure 2 (a)).

Figure 9: Variability of deep-water salinity at station T9-6 off the Homer Spit, compared to the seasonal means.

Biological conditions

Phytoplankton

Chlorophyll concentrations were highest in the outer bay and near the head of the bay as well as off the Homer Spit (Figure 2). Most parts of the water column had below-normal chlorophyll concentrations. However, concentrations in these hotspots were substantially above normal, in places by over standard deviations (Figure 2 (a)). Integrated over the entire water column, chlorophyll concentrations experienced a seasonal drop compared to last month, but were also low compared to previous years (Figure 10). By contrast, surface plankton samples in the outer and inner bay were extremely dense (awaiting analysis). Our partners from the Kachemak Bay National Estuarine Research Reserve detected potentially toxic Pseudo-nitschia spp. as “present” in most samples. (Kim Schuster, Kachemak Bay NERR, The Phyto Times, 24 September 2026).

Figure 10: Seasonal patterns of chlorophyll concentrations at outer-bay station AlongBay-5 (integrated over the full water column).)

Wildlife

Two orcas were seen off the Homer Spit at T9-6. A red-necked phalarope in the outer bay was late for the season. Notably, no shearwaters were seen, usually common in the bay at this time of year. 3 ancient murrelets in the outer bay were uncommon, but not unexpected for this month.

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