Effects of freshwater inputs on the lower trophic levels of a temperate estuary: physical, physiological or trophic forcing?

Abstract

The aim of this six year study was to assess whether freshwater inputs (rainfall and dam discharges) were acting as physical, physiological or trophic forcing factors on phytoplankton pigment concentrations and the dominant mysids of a temperate estuary (Guadalquivir estuary; SW, Spain). The effects of natural and human-controlled freshwater inputs modified the physicochemical conditions and consequently biological production (bottom up control). Nutrient (nitrogen hypernutrification), suspended particulate matter and allochthonous photosynthetic pigment importations linked to freshwater inputs from adjacent habitats were observed, as well as light-limited autochthonous primary production. Seasonal and/or spatial patterns were shown by all studied variables, including mysid species. Freshwater management effects on dominant mysids differed depending on the species' salinity tolerances (physiological forcing) and preferred prey availability (trophic forcing); moreover high inorganic matter content had a negative effect on the density of Mesopodopsis slabberi (physical forcing), which led to an increased detritivory/herbivory ratio (Neomysis integer/M. slabberi ratio). In conclusion, freshwater input appeared to affect estuarine lower trophic levels via a combination of different forcing mechanisms. Although several general patterns can be derived, the response of the system to freshwater input was not always univocal.

Human pressures on two estuaries of the Iberian Peninsula are reflected in food web structure

“…Rapid urban and agricultural development is the major factor contributing to wetland loss and the deterioration of water quality in these coastal areas 8,9 . Nutrient load inputs to estuaries are directly related to intensive agriculture and large populations 10 and have the potential to alter nutrient dynamics, in turn modifying the functioning and structure of estuarine ecosystems 11 .…”

Patterns of larval fish assemblages along the direction of freshwater input within the southern branch of the Yangtze Estuary, China: implications for conservation

“…The rhythm and magnitude of freshwater input can cause major changes in estuarine physical, biogeochemical, and biological attributes (Lez-Ortegon and Drake 2012; Human et al 2016). Species compositions and abundances vary along the gradient in estuarine systems under different temporal and spatial scales (Neves et al 2011;Lez-Ortegon and Drake 2012;dos Passos et al 2013).…”

Anthropogenic shift of planktonic food web structure in a coastal lagoon by freshwater flow regulation

“…This would also reduce the magnitude of stress related to water quality change. Similarly, several authors have argued that increased water discharge could contribute to balance of processes at spatial and temporal scales in systems affected by altered flow regimes, but that each system is likely to behave differently 42 43 44 45 46 . However, a higher monthly minimum water release volume would imply an increase in the total volume of water available for the Coorong, which is possibly harder to achieve due to increasing human demands.…”