Monthly Kachemak Bay report, September 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
On 24 and 25 September, Hans, Kris, Martin, and Ross conducted the extended survey, which included T9 across the bay from the Homer Spit and the AlongBay transect. Sea state during this survey varied from Beaufort 1 to Beaufort 4. Air temperatures on the Homer Spit were above the seasonal mean, approaching the 90th percentile, for much of this summer, including early September. Temperatures have now dropped not only seasonally, but are also approaching the long-term mean again. Precipitation has been well below the long-term mean since mid-June. While September has been wetter than August, precipitation is still below the long-term mean. On a larger scale, much of the North Pacific has been in heat wave conditions this month, reminiscent of the ‘blob’ of 2014-16, but bigger; in fact only a few areas on the globe experience negative sea surface anomaly conditions at the moment.
Kachemak Bay National Estuarine Research Reserve (KBNERR) detected an Alexandrium bloom in early September, a potentially harmful dinoflagellate. Testing of shellfish has since revealed toxin levels up to twice the regulatory limit for human consumption (AOOS). The last time similar densities of Alexandrium were seen in Kachemak Bay was in 2016, during the extreme North Pacific marine heat wave.
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 10.1 °C at station AlongBay-5 (164 m depth), in the outer bay near Homer Spit, to a maximum of 11.7 °C at 6-3 (1 m depth) in Cook Inlet, off Nanwalek. The falling air temperatures left their mark. Surface water temperatures in the inner bay began to equalize with midwater temperatures, which still carried heat from the summer. Even the coldest waters at the bottom of Kachemak Bay were significantly above the 8.5 °C thought to be necessary for sustaining an Alexandrium bloom.
A temperature (and salinity) front, was found in lower Cook Inlet, between Flat Island and Nanwalek, reaching from the surface all the way to the bottom (Figure 1 (a)). The same front is visible on satellite data, showing a tongue of warm water from coastal waters, extending across Kennedy Entrance towards Cape Douglas (Figure 2). Interesting temperature patterns were apparent along T9 and T4 (Figure 1). While the bottom half of the water column was still layered from cool bottom waters to warmer mid-waters, surface waters on T9 and the south end of T4 had cooled to be already colder than mid-water layers, the latter typically reaching their annual peak in September.
The warm water anomaly that started here at the beginning of the year, had diminished in surface waters, as indicated by the SWMP station operated by the KBNERR in Homer Harbor (Figure 3). Temperatures there are now back to at the long-term mean. The warm anomaly persists, however, in mid and deep waters (Figure 6, Figure 4). Deep waters in the inner bay were about 0.8 °C, or 1 standard deviation warmer than normal (Figure 5). Compared to previous warm water events here, the current anomaly is less severe but has now been going on for a long time (Figure 4).
Salinity
Salinity values ranged from 28.4 PSU at 9-10 (1m depth) in the inner bay to 31.2 at AlongBay-5 (164m depth) in the outer bay near Homer Spit. While the surface waters have cooled, compared to last month, a salinity gradient maintains a stable, stratified water column in the inner, and parts of the outer bay (Figure 1 (b), Figure 1 (c)).
The across-transects T9 and, especially, T4 show a gradient of this stratification with a thicker layer of fresher water near the northern shore. This is consistent with freshwater exiting Kachemak Bay in a narrow band along the north shore, following the general counter-clockwise currents of Kachemak Bay.
Since the beginning of the year, salinities at the mid-bay station T9-6 have been lower than the seasonal mean (Figure 7), particularly in waters below 25 m (Figure 6). This anomaly has come to an end, at least for now, as we saw a slight reversal (Figure 7, Figure 6).
Turbidity
While no areas of high turbidity were encountered during the survey, waters in the outer bay were noticeably clear. The turbidity signal differed markedly from the spatial patterns of temperature and salinity. Turbidity was low throughout the outer bay and into Cook Inlet. Highest turbidity values were found near the surface in the inner bay as well as near the bottom of the inner bay (Figure 1 (a)).
Biological conditions
Phytoplankton
There were two areas of high chlorophyll concentrations: in surface waters of the inner bay, as is common during summer. A second, more unusual area of high activity was at the southwestern end of the extended AlongBay transect, in Cook Inlet off Nanwalek (Figure 1 (a)), coinciding with warmer temperatures and lower salinity. Interestingly, a plankton sample from this station contained Pseudo-nitzschia, which was abundant in Kachemak Bay in July, but was not expected this late in the year (Rose Masui, KBERR, pers. com.). Overall highest chlorophyll concentration was at station 9-9 ( 1.2 mg m-3 at 3 m depth). Chlorophyll concentrations observed with the CTD fluorometer were below the seasonal mean for most stations (e.g. Figure 8).
Zooplankton
Echo sounder returns were weak to moderate at a few stations. No samples were taken.
Wildlife
A humpback whale was seen in the outer bay. About forty red-necked phalaropes were seen foraging on small items at the surface along convergence lines. This common migratory species is usually already farther south at this time of the year. Fork-tailed storm-petrels were seen both in the inner and the outer bay. About 200 short-tailed shearwater were seen in the tide-rips off Point Pogibshi, with another 150 throughout the outer bay. Five horned and one tufted puffin, the latter only rarely seen in Kachemak Bay during summer, were also off Point Pogibshi. A parasitic jaeger was seen north of Flat Island.
Mailing list
Please email Martin at martin.renner@noaa.gov if you would like to be added to or removed from this mailing list.