Elsevier

Continental Shelf Research

Volume 102, 1 July 2015, Pages 33-46
Continental Shelf Research

Research papers
Study of photosynthetic productivity in the Northern Gulf of Mexico: Importance of diel cycles and light penetration

https://doi.org/10.1016/j.csr.2015.04.014Get rights and content

Highlights

  • The sub-pycnocline primary production is very important in the NGOM.

  • Diel cycles of phytoplankton photosynthesis with depths exist in the NGOM.

  • Phytoplankton in the NGOM adapt to light conditions by their own strategies.

  • Phytoplankton community structure influences the measured photosynthetic parameters.

Abstract

Based on 14C uptake assays, in vivo chlorophyll (chl) a fluorescence and HPLC pigment analysis, phytoplankton photosynthetic physiology and productivity in 24-h diel cycles were characterized at three stations in April and August 2012 in the Northern Gulf of Mexico. The results indicated the sub-pycnocline primary production accounted for 5–48% of the total integrated primary production, emphasizing the important influence of euphotic zone in shallow coastal areas. During the diel cycles, chl a-specific light-saturated photosynthesis (PmaxB) as measured with photosynthesis versus irradiance curves (P–I) and the photoprotective pigment pool (diadinoxanthin, diatoxanthin, chl a) showed phytoplankton acclimation to be strongly influenced by water column structure (mixing versus stratification). Changes in chl a fluorescence and transformations between photoprotective pigments were most recognizable in surface samples. The dominate phytoplankton groups (diatoms and cyanobacteria in April and August respectively) also influenced the measured photosynthetic parameters. The Northern Gulf of Mexico is a typical coastal ecosystem with high variability of nutrients, light (intensity and attenuation) and mixing. Our study provided evidence that phytoplankton in this area are adapted to changing environmental conditions by means of fast responses as well as long-term photoacclimation strategies. Understanding the major drivers could help us to improve models involving the calculation of primary productivity, such as those focused towards understanding mechanisms controlling hypoxia.

Introduction

Phytoplankton dynamics are the outcome of nutrient availability, light conditions and physical mixing. The influence of light in diel rhythms on phytoplankton is observed in many aspects such as cell divisions, photosynthesis, chl a fluorescence and gene expression (Suzuki and Johnson, 2001, Ohi et al., 2005, John et al., 2012). Both laboratory and field studies have found that diel rhythms are predictable (Vaulot and Marie, 1999, Litaker et al., 2002, Bruyant et al., 2005, Brunet et al., 2008, Quigg et al., 2012, McInnes et al., 2014). Phytoplankton carbon fixation rates usually exhibit peak values early in the morning or around noon (Harding et al., 1981, Prézelin, 1992); without consideration of this variability errors in the calculation of total integrated primary production occur (Harding et al., 1981, Harding et al., 1982). For light reactions, chl a fluorescence parameters such as minimum fluorescence (F0) and the maximum quantum yield of photosystem (PS) II (Fv/Fm) show quenching in the daytime caused by photoinhibition under excess light stress and recovery at night (Falkowski and Raven, 2007). Photoprotection involves an enzymatic-controlled epoxidation and de-epoxidation of pigment conversion, in order to dissipate extra energy as nonphotochemical quenching (NPQ) before the damage of light reaction centers (Falkowski and LaRoche, 1991). This process of pigment conversion was named the xanthophyll cycle (XC) (Long et al., 1994). In chromophyte algae, XC involves the transformation from diadinoxanthin (Dd) to diatoxanthin (Dt) (Lavaud et al., 2004, Falkowski and Raven, 2007). In cyanobacteria, photoprotection involves zeaxanthin and decoupling of phycobilisomes (Falkowski and Raven, 2007).

The diel “bio-clock” in phytoplankton shows variability in terms of frequencies and amplitudes in field studies. Physical mixing causes vertical movement of phytoplankton cells, which changes the irradiance experienced at different depths. Claustre et al. (1994) indicated the effect of mixing could diminish the difference in the proportion of Dt between day and night. At a 50 m deep coastal site with day–night alternations of thermal induced stratification and mixing, Brunet et al. (2008) found the sinusoidal diel patterns and exponential vertical patterns of Dt/ chl a, Dt/(Dt+Dd) (DES) and ∆F/Fm (effective quantum yield of fluorescence). Doblin et al. (2011) found homogenous chl a fluorescence and photoprotective pigments within the mixed layer, but a lack of diel rhythm with carbon fixation rates in the Sub-Antarctic and Polar Front Zones.

As the largest river in the North America, the Mississippi River drains 40% of the area of the United States (Dagg et al., 2007). With the influence of riverine nutrients, the Northern Gulf of Mexico (NGOM) fuels high phytoplankton biomass and primary productivity, contributing to organic matter export and the complex food web. In the NGOM, the role of nutrients is frequently investigated, particularly along the Louisiana shelf (e.g., Quigg et al., 2011; Laurent et al., 2012; Turner and Rabalais, 2013), but the importance of light is less often examined (e.g., Lohrenz et al., 1994; Lehrter et al., 2009; Nunnally et al., 2014). John et al. (2012) found diel patterns of Rubisco (rbcL) mRNA and the chl a-specific light-saturated photosynthetic rate (PmaxB) in four different size classes of phytoplankton in the Mississippi and Orinoco River plumes, but did not consider the influence of hydrographic factors like mixing and depth on the amplitudes of the diel patterns.

Here, we examined diel patterns of primary productivity and photosynthetic physiology at a range of depths above and below the pycnocline, across the shelf at three stations, and during two very different time periods (April and August, 2012). Multiple techniques were used in our study, such as the 14C method, Fluorescence Induction and Relaxation (FIRe) System and High Performance Liquid Chromatography (HPLC) pigments analysis, which were also common for the investigations in other field studies (Qian et al., 2003, Suggett et al., 2009a, Sylvan et al., 2011). Collectively this study provides information on the magnitude of productivity across a range of spatial and temporal scales.

Section snippets

Sample collections and hydrographic conditions

Two research cruises were conducted in April and August 2012 on the R/V Pelican. In each cruise, three 24 h stations along the 20 m isobath in the NGOM were studied (Fig. 1). Station A (29.07 °N, 89.93 °W) was in the Mississippi River plume while the other two stations were located further west on the Louisiana shelf area (B: 28.60 °N, 90.53 °W and C: 29.00 °N, 92.00 °W). A CTD rosette with 12 Niskin bottles and shipboard calibrated sensors was deployed overboard every 2 h to measure vertical

Hydrographic conditions

We summarized the major hydrographic parameters measured and calculated for the water column at the three stations in Table 2. Station A, located in the Mississippi River plume, showed the lowest surface salinity (25.8±0.38) in August, and the highest phytoplankton biomass (measured as chl a; 3.41±0.4 µg l−1), the highest light attenuation coefficient (kd=0.29) but the shallowest euphotic zone (Zeu, 15.75 m) in April. Stations B and C were less influenced by the Mississippi River with higher

Discussion

While it is well known that primary productivity varies with diel cycles of light, and consequently, with depth, these two factors are rarely examined simultaneously in field situations. This may be partly due to the logistics and expense of such a sampling scheme, but it is also in part due to the challenges associated with interpreting the findings given the range of abiotic and biotic factors which influence the outcomes. As part of determining the mechanisms controlling hypoxia, we were

Conclusions

Our study supported the findings of Lehrter et al. (2009) and others who identified light penetration to the bottom of the euphotic zone as an important driver of primary productivity on the Louisiana shelf along with nutrients and mixing (Lohrenz et al., 1997, Lohrenz et al., 1999, Fennel et al., 2011, Quigg et al., 2011, Laurent et al., 2012, Turner and Rabalais, 2013, Nunnally et al., 2014), and emphasized the importance of sub-pcynocline primary production. Mostly important, we provided

Acknowledgments

We thank the scientific parties from the project ‘Mechanisms Controlling Hypoxia' (especially Steve DiMarco – Lead PI; Texas A&M University, College Station), two technicians (Paul Clark and Eric Quinoz) from the Geochemical and Environmental Research Group, and the crew of the R/V Pelican for all their help. We thank Allison McInnes and Tyra Booe for help with the shipboard measurements, and Bo Li for the calculations of hydrographic parameters. This work was supported by the National Oceanic

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