The seasonal variation of bacterial production (BP) in a shallow, eutrophic Lake Kasumigaura was clarified from 2012 to 2016. During the studied period, BP fluctuated from 1.9 to 138 μg C L−1 d−1. There were no significant correlations between BP and bacterial abundance in any season, suggesting a strong top‐down regulation on BP throughout the year. On the other hand, BP was also related to bottom‐up regulation factors such as water temperature, phosphorus, and primary production (PP) annually. During winter, BP was positively correlated with chlorophyll a concentration, suggesting that autochthonous substrates were relatively important for BP. Moreover, BP was positively correlated with heterotrophic nanoflagellates, ciliates, and copepods, suggesting higher availability of substrates for BP. In summer, although there was no significant correlation between BP and PP, rainfall amount showed significant negative correlations with both BP and PP, suggesting depressed PP from relatively lower solar irradiance coupled with unfavorable weather conditions that decreased the substrate supply for bacteria. These results suggest that temporal variation of BP was regulated not by allochthonous, but by autochthonous substrates during both the highest (summer) and lowest (winter) productive seasons, even in a shallow, eutrophic lake. PP in autumn was approximately half that of spring due to lower solar irradiance, although water temperatures during both seasons were similar and nutrient concentrations during autumn were higher. On the other hand, BP in autumn was comparable with that in spring, and the bacterial carbon demand (= BP + bacterial respiration; 1.12 ± 0.79 g C m−2 d−1) was comparable to PP (1.16 ± 0.53 g C m−2 d−1), suggesting the relative importance of higher allochthonous substrates relative to other seasons.

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The sea surface microlayer (SML), less than 1,000-μm uppermost layer of the ocean water column, is located at the air-sea interface and plays critical roles in global biogeochemical cycles and climate change through air-sea gas exchange. To clarify the significance of the dynamics of carbon monoxide (CO) in the SML, where active photochemical and biological processes are expected, in air-sea CO exchange, the production and consumption of CO in the SML and its sea-air emission were investigated in temperate coastal waters. In the SML, the light-normalized photochemical CO production rate was relatively high from spring to autumn (median: 2.57 nM [kWh m −2 ] −1 ) when relatively high absorbance of chromophoric dissolved organic matter (0.55 m −1 ) was observed. Biological CO consumption rate constant in the SML showed relatively high values from spring to autumn (mean ± standard deviation: 0.060 ± 0.010 h −1 ) during the period of relatively high water temperature (22.3 ± 2.7°C). The calculated sea-air CO flux (F) varied similarly to CO concentration in the subsurface water. Comparison among the production, consumption, and sea-air emission of CO in the SML suggests that biological consumption in the SML can be ignored in air-sea CO exchange throughout the year whereas photochemical production in the SML enhances F during summer under intense light, active biological production, and weak wind conditions. Further, seawater warming experiments found the tendency of the stimulation of biological CO consumption by water temperature increase from spring to autumn, which suggests negative but insignificant feedback on global warming. Plain Language SummaryThe uppermost layer of the ocean water column is called the sea surface microlayer (SML) whose thickness is less than 1 mm. Although the SML is critical in climate change due to its roles in air-sea gas exchange because the SML is located at the interface between the atmosphere and the ocean, the information on the effect of chemical and biological processes in the SML on air-sea gas exchange remains limited. In this study, we investigated the photochemical production and biological consumption of carbon monoxide (CO), an indirect greenhouse gas, in the SML of temperate coastal waters. By comparing with CO emission to the atmosphere, we revealed that CO consumption in the SML would not be significant enough to affect air-sea CO exchange throughout the year. On the other hand, CO production in the SML potentially enhances the emissions of CO from the ocean to the atmosphere during summer. Further, we experimentally examined the effect of the increase in water temperature on CO production and consumption in the SML assuming global warming and found that water temperature increase tends to stimulate CO consumption in the SML from spring to autumn. However, the feedback on global warming can be ignored.