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Abstract Islands in oligotrophic oceans act as local sources of nutrients. These nutrients originate from land and from deep oceanic nutrients introduced to the photic zone by tides, currents, and internal waves interacting with island bathymetry. These processes create the island mass effect (IME), in which increased chlorophylla(Chla) is found near islands compared to oceanic waters. The IME has been described via satellite observations, but the effects on phytoplankton community structure are not well documented. From 2013 to 2020, chlorophyll, nutrient, and picoplankton samples were collected from multiple depths on quarterly cruises at two sites south of O'ahu, Hawai'i.Prochlorococcus,Synechococcus, picoeukaryotes, and heterotrophic bacteria were enumerated using flow cytometry. We compared nearshore results to Sta. ALOHA, 100 km from O'ahu. Consistent with the expected IME, Chlaconcentrations were significantly enhanced at both nearshore sites compared to Sta. ALOHA.Prochlorococcusconcentrations increased with greater distance from shore, particularly below 50 m; mixed layer concentrations ofSynechococcusand picoeukaryotes significantly decreased with greater distance from shore, as did concentrations of nitrate and phosphate below the mixed layer. Heterotrophic bacteria concentrations did not show a spatial trend. Carbon‐based biomass estimates of the picoplankton population indicated that the IME‐associated Chlaincreases near the island are likely driven by larger phytoplankton classes. This study describes the IME‐associated shift in the picophytoplankton community distribution, which has implications for nutrient cycling, food web dynamics and fisheries in oligotrophic island ecosystems, and adds to the understanding of spatial heterogeneity in carbon fixation in the ocean.
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Phytoplankton biomass responses to a marine heat wave align with altered nitracline depth
Abstract The 2014–2015 warm anomaly (aka “the Blob”), the largest of periodic and intensifying marine heat wave (MHW) perturbations in the northeast Pacific, may provide some insight about the future warmer ocean. Here, we use mixed‐layer carbon estimates for total phytoplankton, major size classes and functional groups from 45 CalCOFI cruises to: (1) compare 2014–2015 MHW impacts in the southern California Current System to baseline estimates from 2004 to 2013 and (2) to test a space‐for‐time exchange hypothesis that links biomass structure to variability of nitracline depth (NCD). Seasonal and inshore‐offshore analyses from nine stations revealed almost uniform 2°C MHW warming extending 700 km seaward, fourfold to sixfold declines in nitrate concentration and 18‐m deeper NCDs. Phytoplankton C decreased 16–21% compared to 45–65% for Chla, with the threefold difference due to altered C : Chla. Among size classes, percent composition of nanoplankton decreased and picophytoplankton increased, driven by higherProchlorococcusbiomass, whileSynechococcusand picoeukaryotes generally declined. Diatom and dinoflagellate C decreased in both onshore and offshore waters. Seasonally, the MHW delayed the normal winter refresh of surface nitrate, resulting in depressed stocks of total phytoplankton and nanoplankton,Synechococcusand picoeukaryotes during winter. Consistent with the space‐for‐time hypothesis, biomass variations for baseline and MHW cruises followed similar (not significantly different) slope relationships to NCD. All biomass components, exceptProchlorococcus, were negatively related to NCD, and community biomass structure realigned according to regression slopes differences with NCD variability. Empirically derived biomass‐NCD relationships could be useful for calibrating models that explore future food‐web impacts in this coastal upwelling system.
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- Award ID(s):
- 2224726
- PAR ID:
- 10520643
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 69
- Issue:
- 8
- ISSN:
- 0024-3590
- Format(s):
- Medium: X Size: p. 1683-1694
- Size(s):
- p. 1683-1694
- Sponsoring Org:
- National Science Foundation
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