Co-Limitation of Phytoplankton by N and P in a Shallow Coastal Lagoon (Ria Formosa): Implications for Eutrophication Evaluation

Abstract

The Ria Formosa coastal lagoon is a highly productive shallow ecosystem in southern Portugal, subjected to nutrient inputs from anthropogenic and natural sources. Nutrients are major abiotic drivers of phytoplankton in this system, but their effects on phytoplankton assemblages and the occurrence of nutrient limitation are still poorly understood. The main goal of this study was, thus, to evaluate the occurrence, type, and effects of nutrient limitation on phytoplankton community and specific functional groups in the Ria Formosa coastal lagoon. We conducted nutrient enrichment experiments with factorial additions of nitrogen (N) and phosphorus (P) using natural phytoplankton assemblages from distinct locations in the Ria Formosa, throughout a yearly cycle. Phytoplankton composition and abundance were evaluated using inverted and epifluorescence microscopies, and spectrophotometric methods were used for biomass. Limitation was defined as higher phytoplankton growth following enrichment with a particular nutrient in relation to the non-enriched control. The most common type of phytoplankton limitation was simultaneous co-limitation by N and P; diatoms, as r-strategists, were the most frequently limited group. Single N and P limitation, and serial P limitation were also observed, as well as negative responses to nutrient enrichment. Group-specific responses to nutrient enrichment were not reflected in the relative abundance of phytoplankton groups within the whole assemblage, due to the numerical dominance of pico-sized groups (cyanobacteria and eukaryotic picophytoplankton). Ambient nutrient ratios and concentrations did not predict phytoplankton nutrient limitation, given the different nutrient utilisation traits among phytoplankton functional groups. Therefore, nutrient ratios should not be used as indicators of nutrient limitation in eutrophication assessment.

Patterns and Predictors of Phytoplankton Assemblage Structure in a Coastal Lagoon: Species-Specific Analysis Needed to Disentangle Anthropogenic Pressures from Ocean Processes

“…The relative increases in cryptophyceans during spring followed by planktonic diatoms in summer, detected at all stations, together with the picophytoplankton maxima, represent the annual phytoplankton succession pattern previously reported for the western sector of the RF lagoon, unaffected by WWTP discharges [30,54,67]. The biomass of cryptophytes is top-down controlled by microzooplankton grazing, whereas diatoms appear to be controlled by metazooplankton and/or benthic filter feeders (see [54,66]).…”

“…Picophytoplankton is a key functional group in lagoonal systems, usually promoted by regenerated nutrients and dissolved organic matter, and sometimes associated with harmful, ecosystem-disruptive blooms (e.g., [24,27,121]). Indeed, in the western sector of the RF lagoon, Synechococcus-like picocyanobacteria and picoeukaryotes numerically dominate phytoplankton [54,67], but represent a more modest contribution to the total phytoplankton biomass (ca. 7-14%; [54]), as reported for other coastal lagoons (e.g., [122]).…”

“…The in situ growth rates of cryptophyceans and diatoms in the RF lagoon are limited by light availability and/or temperature during the autumn-winter period (see [54,64,66]). However, nitrogen and/or phosphorus limitation has also been observed during the autumn-summer period, especially for diatoms [63][64][65]67], although not with sufficient severity to cause a reduction in Chl-a and diatoms during summer. The classical diatom-dinoflagellate succession, reported for some lagoons (e.g., [117,155,156]), was not observed in the RF lagoon, probably due to high vertical mixing and relatively high nutrient availability (see Section 4.2).…”

“…In our study, for example, the strong association between high pH values and some summer samples at the innermost stations (see canonical axis 1) probably reflected the consequence of the intense photosynthetic uptake of CO 2 and relative diurnal water basification during summer blooms, and not an effect of pH on the phytoplankton growth rate (e.g., [19]). Conversely, considering the experimentally determined effects of changes in temperature and nutrient availability on the phytoplankton growth rate in the RF lagoon [65][66][67], associations with these abiotic variables may imply a higher degree of causality. Regardless of their causes, the significant associations detected between phytoplankton assemblage structure, temperature, and nutrient concentrations may have practical implications for environmental management in this and other confined coastal systems affected by WWTP disposal.…”

“…However, RF is exposed to various anthropogenic stressors [45,46], including discharges from wastewater treatment plants [30,47,48], and is located in an area particularly vulnerable to climate change [49]. Previous studies of phytoplankton in the RF lagoon, focusing on the western sector of the lagoon, have addressed the spatial-temporal dynamics of chlorophyll-a (e.g., [42,50,51]), assemblage structure [30,[52][53][54][55][56], production [57,58], specific HAB-forming taxa [59][60][61], and the effects of physico-chemical variables (temperature, light, CO 2 , nutrients, ultraviolet radiation) on phytoplankton growth and composition (e.g., [62][63][64][65][66][67]). However, no information on phytoplankton is currently available for the eastern sector of the lagoon, which is more affected by freshwater discharges [47] and where water quality is worst [46,51].…”