Volume 162, 5 September 2015, Pages 76–87
Special Issue: Global Patterns of Phytoplankton Dynamics in Coastal Ecosystems
Edited By Riina Klais, James E. Cloern and Paul J. Harrison
Long-term changes of the phytoplankton community and biomass in the subtropical shallow Patos Lagoon Estuary, Brazil
- a Institute of Oceanography, Federal University of Rio Grande (FURG), Brazil
- b Department of Bioscience, Aarhus University, Denmark
- Received 14 July 2014, Accepted 3 March 2015, Available online 11 March 2015
Highlights
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Salinity variability is the main driver of water quality and phytoplankton variation
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Diatoms are the main phytoplankton group and determine chlorophyll a seasonality.
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Salinity fluctuations were filtered out by a novel decomposition tool, revealing:
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Strong inter annual variations, but no trends of chlorophyll a and diatoms.
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Increasing trends of N:P, cyanobacteria and dinoflagellates.
Abstract
Seasonal and interannual changes (1993–2012) of water temperature and transparency, river discharge, salinity, water quality properties, chlorophyll a (chl-a) and the carbon biomass of the main taxonomical phytoplankton groups were evaluated at a shallow station (∼2 m) in the subtropical Patos Lagoon Estuary (PLE), Brazil. Large variations in salinity (0–35), due to a complex balance between Patos Lagoon outflow and oceanic inflows, affected significantly other water quality variables and phytoplankton dynamics, masking seasonal and interannual variability. Therefore, salinity effect was filtered out by means of a Generalized Additive Model (GAM). River discharge and salinity had a significant negative relation, with river discharge being highest and salinity lowest during July to October. Diatoms comprised the dominant phytoplankton group, contributing substantially to the seasonal cycle of chl-a showing higher values in austral spring/summer (September to April) and lowest in autumn/winter (May to August). PLE is a nutrient-rich estuary and the phytoplankton seasonal cycle was largely driven by light availability, with few exceptions in winter. Most variables exhibited large interannual variability. When varying salinity effect was accounted for, chl-a concentration and diatom biomass showed less irregularity over time, and significant increasing trends emerged for dinoflagellates and cyanobacteria. Long-term changes in phytoplankton and water quality were strongly related to variations in salinity, largely driven by freshwater discharge influenced by climatic variability, most pronounced for ENSO events. However, the significant increasing trend of the N:P ratio indicates that important environmental changes related to anthropogenic effects are undergoing, in addition to the hydrology in the PLE.
Keywords
- hydrology;
- salinity;
- eutrophication;
- chlorophyll;
- phytoplankton composition
Geographic coordinates
- 32°1′35″S;
- 52°6′21″W
Regional index terms
- Brazil;
- Rio Grande do Sul;
- Patos Lagoon;
- South Atlantic Ocean
1. Introduction
Phytoplankton play an important role in aquatic environments as primary producers and the species constitute a diverse phylogenetic group with a broad range of ecological strategies (Katz et al., 2004). Functional and physiological characteristics, including morphology, photopigmentation, growth rate and behaviour are important factors governing which species are selected under given environmental setting (Ptacnik et al., 2008, Schmidtke et al., 2010 and Lewandowska et al., 2012). Phytoplankton photosynthesis and cell growth require availability of light and inorganic nutrients and hence, phytoplankton can significantly affect and be affected by biogeochemical cycles of carbon, nitrogen, phosphorus and silicate, from local to global scale (Redfield et al., 1963 and Arrigo, 2005).
Estuaries with high turbulence and high nutrient concentrations combined with short-term variability (hours, days) generally favour the predominance of small diatoms and flagellates (Margalef, 1978, Kilham and Kilham, 1980 and Levasseur et al., 1984). However, site-specific hydrological characteristics of estuaries may shape seasonal and interannual changes in phytoplankton biomass (Malone et al., 1988, Harding, 1994, Cloern, 2001 and Hall et al., 2013). Thus, in these shallow transitional aquatic systems, the identification of seasonal and long-term signals is often a challenge. These trends can be masked by various drivers operating at different time scales as well as complex links between pelagic and benthic systems (Cloern and Jassby, 2008 and Winder and Cloern, 2010).
The response of the phytoplankton community to hydrology over time may be superimposed by human impacts on coastal ecosystems (Domingues et al., 2005 and Winder and Cloern, 2010). Due to the proximity to land, these systems are more vulnerable to anthropogenic activities, like enhanced input of nutrients, which stimulate primary producers growth (Smith, 2006 and Carstensen et al., 2011). The effects of nutrient enrichment can exceed that of hydrological changes, as exemplified with biomass peaks related to sewage input shown for the Swan River Estuary (Australia) (Chan et al., 2002). Although, in some cases, estuaries experiencing nutrient enrichment do not exhibit biomass increase due to high turbidity and limited light availability (Cloern, 1999 and Cloern, 2001), strong grazing control (Cloern et al., 2007 and Chen et al., 2009) or low water residence time that prevents the accumulation of cells (Odebrecht et al. this issue).
Most of our knowledge on phytoplankton responses in estuaries is derived from studies of temperate ecosystems in the northern hemisphere, whereas studies from subtropical and tropical ecosystems are few. This is a strong caveat in developing global understanding of the coastal ecosystems because seasonal variations in physical factors and nutrients differ between these regions. For example, temperate ecosystems are usually N-limited, while tropical ones often exhibit P-limitation (Downing et al., 1999). Of course, even apparently similar ecosystems within the same coastal region may possess local attributes of physical, chemical and biological characteristics, which modulate phytoplankton dynamics (Cloern, 2001), and must be considered for interpreting phytoplankton responses (Carstensen and Henriksen, 2009).
The Patos Lagoon Estuary (PLE; Fig. 1) in southern Brazil is a shallow environment where phytoplankton growth is largely light limited and affected by the retention time of the water (Abreu et al., 2010, Odebrecht et al., 2005 and Odebrecht et al., 2015). The hydrology of the PLE is highly variable due to the action of winds, generating episodic inflows of oceanic water that mixes with the low salinity outflowing water from the Patos Lagoon. As a consequence, salinity can display large variations within periods of days to hours (Möller et al., 2001). Sampled water properties in the PLE are affected by these strong fluctuations in salinity, making the comparison of monitoring data over time difficult as the samples likely represent different water masses having different properties for both water quality and phytoplankton variables. It is necessary to account for this source of variation in order to assess any seasonal and long-term trends above these generated by inflow and outflow regime.
