Seasonal and interannual phytoplankton dynamics and forcing mechanisms in the Northern Benguela upwelling system

Louw, Deon C.; Plas, Anja K. van der; Mohrholz, Volker; Wasmund, Norbert; Junker, Tim; Eggert, Anja

doi:10.1016/j.jmarsys.2016.01.009

Cited by 41 publications

(37 citation statements)

References 41 publications

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“…Nonetheless, the overall good agreement between the platforms, particularly in the SB and AB regions, is encouraging and supports the merging of the various data sets, similar to previous global studies (Brewin et al, 2014;Mélin et al, 2017). The use of a GLM to reconstruct the time series over the SeaWiFS period effectively removed the systematic biases between the SeaWiFS and MODIS-Aqua Chla While surface Chla measurements are considered to be reasonably representative of phytoplankton biomass in the euphotic zone of the NB and SB shelf regions (Barlow et al, 2005Louw et al, 2016), subsurface Chla maxima commonly occur on the AB shelf Probyn, 1992;Probyn et al, 1994), as well as in the open ocean domains of each of these regions . Thus, the variations of satellite Chla and size structure presented in this study are considered to represent the surface layer only.…”

Section: 1029/2018jc014334supporting

confidence: 81%

“…This is also likely to be true for the BUS, where intense upwelling events are common and the associated rapid offshore advection of newly upwelled water under persistently strong upwelling‐favorable winds, as observed in the NB shelf region (Figure ; Lamont, García‐Reyes, et al, ), could result in the Chla retrievals over large areas being impacted. The NB, SB, and AB shelf regions of the BUS are all subject to occasional very high biomass phytoplankton blooms (Louw et al, ; van der Lingen et al, ; Verheye et al, ), and the intensity of absorption (or scattering in some cases) by these blooms can often be problematic for accurate atmospheric correction and result in poor Chla retrieval (Gordon & Wang, ; M. Wang, ).…”

Section: Discussionmentioning

confidence: 99%

“…While surface Chla measurements are considered to be reasonably representative of phytoplankton biomass in the euphotic zone of the NB and SB shelf regions (Barlow et al, , ; Brown et al, ; Lamont, Brewin, et al, ; Louw et al, ), subsurface Chla maxima commonly occur on the AB shelf (Brown & Cochrane, ; Brown et al, ; Probyn, ; Probyn et al, ), as well as in the open ocean domains of each of these regions (Lamont, Brewin, et al, ). Thus, the variations of satellite Chla and size structure presented in this study are considered to represent the surface layer only.…”

Section: Discussionmentioning

confidence: 99%

“…;Lamont, García-Reyes, et al, 2018), could result in the Chla retrievals over large areas being impacted. The NB, SB, and AB shelf regions of the BUS are all subject to occasional very high biomass phytoplankton blooms(Louw et al, 2016; …”

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confidence: 99%

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Long‐Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System

2019

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The Benguela Upwelling System (BUS) is among the most productive ecosystems globally, supporting numerous fisheries and ecosystem services in Southern Africa. Sea‐viewing Wide Field‐of‐view Sensor and Moderate‐resolution Imaging Spectroradiometer‐Aqua chlorophyll a (Chla) concentrations between September 1997 and February 2018 were used to investigate long‐term trends in phytoplankton biomass and size structure (microphytoplankton [>20 μm], nanophytoplankton [2–20 μm], and picophytoplankton [<2 μm]) in the Northern Benguela, Southern Benguela (SB), and Agulhas Bank (AB) shelf and open ocean regions of the BUS. Trends in upwelling and correlations with Chla and size structure were examined. Increasing Chla and microphytoplankton trends occurred in the Northern Benguela shelf and open ocean, while decreases were evident on the SB shelf in all seasons. In the SB open ocean, small increases occurred during austral winter, with a decrease in spring. On the AB shelf, increases in Chla and microphytoplankton occurred in summer with decreases during the other seasons. Patterns differed in the AB open ocean, with increases in winter and spring and decreases in summer and autumn. Although R2 values indicated that linear trends accounted for a reasonable portion of the variance, and most trends were statistically significant, they showed only small changes on the shelf domains and little to no change in the open ocean. Strong correlations between upwelling, Chla, and the size classes were observed, but distinct seasonal differences occurred in each region. This is the first 20‐year analysis of phytoplankton biomass and community structure in the BUS and provides a baseline against which future changes can be monitored.

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Section: 1029/2018jc014334supporting

confidence: 81%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

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Long‐Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System

2019

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“…Hansen et al (2014) measured chla concentrations of 0.9-1.8 µg/L in rather freshly upwelled water in the northern Namibian coastal region. An extensive long-term study on chla concentrations in front of Walvis Bay is published by Louw et al (2016).…”

Section: Phytoplankton Biomass Composition and Growth Ratesmentioning

confidence: 99%

Phytoplankton Stimulation in Frontal Regions of Benguela Upwelling Filaments by Internal Factors

Wasmund

Siegel

Bohata³

et al. 2016

Front. Mar. Sci.

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Filaments are intrusions of upwelling water into the sea, separated from the surrounding water by fronts. Current knowledge explains the enhanced primary production and phytoplankton growth found in frontal areas by external factors like nutrient input. The question is whether this enhancement is also caused by intrinsic factors, i.e., simple mixing without external forcing. In order to study the direct effect of frontal mixing on organisms, disturbing external influx has to be excluded. Therefore, mixing was simulated by joining waters originating from "inside" and "outside" the filament in mesocosms ("tanks"). These experiments were conducted during two cruises in the northern Benguela upwelling system in September 2013 and January 2014. The mixed waters reached a much higher net primary production and chlorophyll a (chla) concentration than the original waters already 2-3 days after their merging. The peak in phytoplankton biomass stays longer than the chla peak. After their maxima, primary production rates decreased quickly due to depletion of the nutrients. The increase in colored dissolved organic matter (CDOM) may indicate excretion and degradation. Zooplankton is not quickly reacting on the changed conditions. We conclude that already simple mixing of two water bodies, which occurs generally at fronts between upwelled and ambient water, leads to a short-term stimulation of the phytoplankton growth. However, after the exhaustion of the nutrient stock, external nutrient supply is necessary to maintain the enhanced phytoplankton growth in the frontal area. Based on these data, some generally important ecological factors are discussed as for example nutrient ratios and limitations, silicate requirements and growth rates.

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Volatile halocarbon measurements in the marine boundary layer at Cape Point, South Africa

Kuyper

Lesch

Labuschagne

et al. 2019

Atmospheric Environment

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Volatile organic halocarbons play a significant role in the biogeochemical cycling of halogens in the atmosphere. High frequency, in situ measurements of methyl iodide (CH3I), bromoform (CHBr3), dibromomethane (CH2Br2) and methyl bromide (CH3Br) were measured in the marine boundary layer at Cape Point, South Africa by an automated Absorption Desorption System-Gas Chromatography-Mass Spectrometer (ADS-GC-MS) between January and November 2017. These are the first multi-halocarbon measurements recorded at Cape Point and represent the longest time series to date. Baseline wind conditions were established using both wind direction and radon ( 222 Rn) concentration as markers of clean air. The baseline mixing ratios observed were 0.61 ± 0.29, 2.02 ± 0.89, 1.08 ± 0.17 and 6.09 ± 0.50 ppt, for CH3I, CHBr3, CH2Br2, and CH3Br, respectively. A statistically significant difference in short-lived halocarbon occurrence was established between anthropogenically affected and clean marine air masses for CH3Br at the Cape Point site. The CHBr3 and CH2Br2 mixing ratios were not statistically different (p > 0.05) when comparing the anthropogenically affected and clean marine air masses.The mixing ratios of CH3I suggested a strong seasonal variation with higher production in austral Summer-Autumn months than in the austral Winter-Spring months. A general chemical decay line was calculated for baseline CH2Br2 and CHBr3 measurements at Cape Point. An analysis of the ln([CH2Br2]/[CHBr3]):ln([CHBr3]) ratio for the Cape Point dataset exhibited a slight deflection to the right of the general chemical decay line, suggesting the influence of greater [CHBr3]:[CH2Br2] source ratios rather than their dilution.

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Seasonal and interannual phytoplankton dynamics and forcing mechanisms in the Northern Benguela upwelling system

Cited by 41 publications

References 41 publications

Long‐Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System

Long‐Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System

Phytoplankton Stimulation in Frontal Regions of Benguela Upwelling Filaments by Internal Factors

Volatile halocarbon measurements in the marine boundary layer at Cape Point, South Africa

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