Nutrient co‐limitation in the subtropical Northwest Pacific

Abstract

Constraining which nutrients limit phytoplankton growth is important for understanding ocean productivity, its response to climate change, and providing a benchmark on the accuracy of ocean biogeochemical models. The extensive subtropical Northwest Pacific is assumed to be nitrogen limited, but this is based mostly on indirect evidence. We conducted experiments along an~8000 km cruise track showing a geographic switch from sites that were nitrogen limited and those that were nitrogen-iron co-limited. Co-limited sites showed much larger responses to nutrient supply, due to blooming of diatoms that were initially undetectable. Because diatom growth leads to higher biomass accumulation and more efficient energy transfer up food chains, we hypothesize that this gradient could be important for regulating matching gradients in predatory tuna.

Satellite Detection of a Massive Phytoplankton Bloom Following the 2022 Submarine Eruption of the Hunga Tonga‐Hunga Haʻapai Volcano

“…If we assume that phytoplankton growth started after the main eruption (4 a.m. on 15 January), the implied exponential rate of chlorophyll a increase would be 2.2 day −1 (using a growth period of 1.06 days based on the average hours of illumination from the eruption to 16 January GMT). Even though such fast rates of chlorophyll a accumulation have recently been observed during incubations with added nutrients (Browning et al, 2022), most observations report rates of chlorophyll a accumulation ≤1.0 day −1 in conditions of presumed nutrient sufficiency (Figure 1 of Behrenfeld & Boss, 2014;Mahaffey et al, 2012;McAndrew et al, 2007). Furthermore, if chlorophyll a accumulation was driven by a phytoplankton species that was not dominant prior to the eruption, then its growth rate had to be greater than the bulk chlorophyll accumulation rate.…”

Biodiversity and Biogeography of Abundant and Rare Microbial Assemblages in the Western Subtropical Pacific Ocean

“…The chlorophyll a and nutrient concentrations of surface waters in the western subtropical Pacific Ocean are among the lowest globally ( Longhurst, 2010 ). Despite the prevalence of oligotrophic characteristics, this region exhibits significant spatial heterogeneity in terms of biological oceanography ( Browning et al, 2021 ), which is mainly constrained by strong and complex western boundary current systems and the warm pool ( Barber and Chavez, 1991 ). Therefore, the western subtropical Pacific Ocean is an ideal area to investigate the relationship between the environment and the biogeography of plankton communities ( Rowe et al, 2012 ).…”

Phytoplankton responses to changing temperature and nutrient availability are consistent across the tropical and subtropical Atlantic

“…These results suggest that thermal response curves obtained in the laboratory, perhaps due to limited genetic variability in cultured strains, may underestimate the potential of phytoplankton to acclimate to sudden environmental changes. Ecosystem nutrient status may be a more relevant factor than temperature to predict the ability of phytoplankton to respond to increased nutrient availability, given that the largest responses to added nutrients were observed in the least oligotrophic locations, as reported before both for the tropical Atlantic 41 and the tropical Pacific 58 . This pattern may result from higher cellular nutrient quotas in regions with less severe oligotrophy 78 , and also from the fact that those regions harbour more fast-growing species with the potential to respond to increased nutrient availability 58 .…”

“…While this conclusion may be appropriate in more productive regions, our observations in the tropical Atlantic strongly suggest that phytoplankton inhabiting oligotrophic waters require time scales longer than 24 h to respond with a net biomass increase to enhanced nutrient availability. This delayed response may be due to both ecological and physiological constraints: the tight trophic coupling between phytoplankton growth and protist grazing in oligotrophic waters contributes to moderate the increase of photoautotroph biomass 56 , low phytoplankton cell abundance makes it less likely that fast-growing taxa are present in sufficient numbers to cause a biomass increase 57,58 , and heavily depleted internal nutrient quotas mean that a longer time is required for cells to upregulate their biosynthetic machinery 59 .…”