Volume 162, 5 September 2015, Pages 35–44

Special Issue: Global Patterns of Phytoplankton Dynamics in Coastal Ecosystems

Edited By Riina Klais, James E. Cloern and Paul J. Harrison

Retention time generates short-term phytoplankton blooms in a shallow microtidal subtropical estuary

  • a Institute of Oceanography, Federal University of Rio Grande (FURG), Brazil
  • b Department of Bioscience, Aarhus University, Denmark


Phytoplankton blooms are observed in mesohaline waters leaving the Patos Lagoon Estuary.

The inflow of marine water due to southerly winds increase water retention time in the PLE.

Higher PLE retention time allows the development of phytoplankton blooms.

Short- and long-term processes are connected and explain phytoplankton blooms in PLE.


In this study it was hypothesised that increasing water retention time promotes phytoplankton blooms in the shallow microtidal Patos Lagoon estuary (PLE). This hypothesis was tested using salinity variation as a proxy of water retention time and chlorophyll a for phytoplankton biomass. Submersible sensors fixed at 5 m depth near the mouth of PLE continuously measured water temperature, salinity and pigments fluorescence (calibrated to chlorophyll a) between March 2010 and 12th of December 2011, with some gaps. Salinity variations were used to separate alternating patterns of outflow of lagoon water (salinity <8; 46% of the time) and inflow of marine water (salinity >24; 35% of the time). The two transition phases represented a rapid change from lagoon water outflow to marine water inflow and a more gradually declining salinity between the dominating inflow and outflow conditions. During the latter of these, a significant chlorophyll a increase relative to that expected from a linear mixing relationship was observed at intermediate salinities (10–20). The increase in chlorophyll a was positively related to the duration of the prior coastal water inflow in the PLE. Moreover, chlorophyll a increase was significantly higher during austral spring-summer than autumn-winter, probably due to higher light and nutrient availability in the former. Moreover, the retention time process operating on time scales of days influences the long-term phytoplankton variability in this ecosystem. Comparing these results with monthly data from a nearby long-term water quality monitoring station (1993–2011) support the hypothesis that chlorophyll a accumulations occur after marine inflow events, whereas phytoplankton does not accumulate during high water outflow, when the water residence time is short. These results suggest that changing hydrological pattern is the most important mechanism underlying phytoplankton blooms in the PLE.


  • saltwater inflow;
  • light limitation;
  • biomass accumulation;
  • choked lagoon;
  • sediment resuspension

Geographic coordinates

  • 32°08′10.00″ S;
  • 52°06′09.00″ W

Regional index terms

  • Brazil;
  • Rio Grande do Sul;
  • Patos Lagoon;
  • South Atlantic ocean

1. Introduction

Phytoplankton variability in coastal ecosystems is determined by diverse range of factors and complex interacting site-specific processes (Cloern, 2001, Cloern and Jassby, 2010 and Gallegos and Neale, 2015). Differences among ecosystems may result from physical, geomorphological and hydrodynamic characteristics, but also nutrient enrichment, climatology and human disturbances. In many estuaries and coastal lagoons, phytoplankton biomass and species composition variability are strongly associated with hydrodynamics, when basic growth requirements (light and nutrients) are plenty (Peierls et al., 2012 and Thompson et al., 2015).

Coastal lagoons are shallow, dynamic and highly productive ecosystems separated from the ocean by a sand barrier that is penetrated by one or several channels allowing water exchange with the ocean. Coastal lagoons are classified as choked, restricted or leaky according to their degree of water exchange with the ocean (Kjerfve, 1986). Due to the restricted water exchange, choked lagoons have comparatively long water retention time and high phytoplankton biomass (Knoppers et al., 1991 and Roselli et al., 2013).

The Patos Lagoon in Southern Brazil (Fig. 1), is the largest (10,360 km2) choked coastal lagoon in the world (Kjerfve, 1986). The connection with the coastal ocean in the southern area of the Patos Lagoon has typical estuarine conditions, which influence the growth and distribution of all biota from primary producers to fishes (Seeliger et al., 1997 and Odebrecht et al., 2010). In the Patos Lagoon Estuary (PLE), phytoplankton growth and dynamics are strongly light-limited and only partially influenced by nutrients (Abreu et al., 1994a, Abreu et al., 1995 and Haraguchi et al., 2015) and grazing (Abreu et al., 1994b). However, hydrology is considered the key forcing function of phytoplankton variability at both shorter and longer time scales (Abreu et al., 2010).

Geographical location showing the watershed of the lagoons Patos and Mirim in ...
Fig. 1. 

Geographical location showing the watershed of the lagoons Patos and Mirim in the Southwest Atlantic Ocean and the sampling station located at the mouth of the Patos Lagoon Estuary near the city of Rio Grande.

This strong connection to the hydrodynamics in the PLE makes phytoplankton variability in this estuary highly unpredictable on shorter time scales. However, peaks of chlorophyll a were frequently observed in oligohaline-mesohaline waters leaving the estuary after a certain period of coastal water entering the lagoon ( Fujita and Odebrecht, 2007 and Abreu et al., 2010), and these authors suggested that elevated chlorophyll a concentrations in this salinity range were likely the result of phytoplankton accumulation as a response to increasing water retention time.

In this study it is hypothesised that the occurrence of short-term phytoplankton blooms in the shallow microtidal PLE are linked to the increasing water retention time. To test this hypothesis a statistical approach was applied to identify such periods with enhanced retention time from continuous time series of salinity and chlorophyll a monitored in the main navigation channel of the PLE for almost two years. In this study a phytoplankton bloom is defined as a significant increment of chlorophyll a measured in the short time scale (hours – days), hypothesised to result from phytoplankton biomass accumulation generated by increasing estuarine water retention time. The duration of coastal water intrusion into the PLE, characterised by high salinity in the navigation channel, was used as a proxy of water residence time. The aim of the study was to investigate if the magnitude of the phytoplankton bloom increased with the duration of coastal water inflow. This relatively short-term, although high-resolution, investigation was compared with data from a long-term (21 years) monitoring program to assess the general applicability of the results.

2. Study area

The Patos Lagoon is a choked and shallow system (average depth 5 m) with a watershed of approximately 200,000 km2 extending 250 km along the microtidal coastline of the southern Brazilian plain (10,360 km2; 30°12′- 32°12′S; 50°40′- 52°15′W) (Fig. 1). The Patos Lagoon is mainly oligohaline, whereas large salinity variability characterises the Patos Lagoon Estuary (PLE; about 1000 km2). In 1998, the PLE became a monitoring site of the Brazilian Long-Term Ecological Research Program (BR-PELD), and water quality and phytoplankton have been continuously sampled since 1993 (Abreu et al., 2010 and Odebrecht et al., 2010).