Species-specific variation in the phosphorus nutritional sources by microphytoplankton in a Mediterranean estuary

Marly C Martínez-Soto,Esther Garcés,Gotzon Basterretxea,Antonio Tovar-Sánchez,Sílvia Anglès,Antoni Jordi

doi:10.3389/fmars.2015.00054

Abstract

We investigated the species-specific phosphorus (P) nutrition sources in the microphytoplankton community in the Mahon estuary (Minorca, Western Mediterranean) in 2011, under two contrasting hydrographic scenarios. Estuarine flow, nutrient concentrations, phytoplankton community composition and enzyme-labeled fluorescence (ELF) were measured in June and October, corresponding to the beginning and the end of summer. Dissolved inorganic nitrogen (DIN) and inorganic phosphate (Pi) exhibited enhanced concentrations in the inner estuary where N:P molar ratios suggested P-limitation in both surveys. Pi was low and variable (0.09±0.02 μmol•l-1 in June and 0.06±0.02 μmol•l-1 in October), whereas organic phosphorus remained a more reliable P source. Even though ambient Pi concentrations were slightly higher on June, when the microphytoplankton assemblage was dominated by dinoflagellates, the percentage of cells expressing ELF labeling was notably higher (65% of total cells) than in October (12%), when the presence of diatoms characterized the microphytoplankton community. ELF was mainly expressed by dinoflagellate taxa, whereas diatoms only expressed significant AP in the inner estuary during the June survey. A P-addition bioassay in which response of AP to Pi enrichment was evaluated showed remarkable reduction in AP with increasing Pi. However, some dinoflagellate species maintained AP even when Pi was supplied in excess. We suggest that in the case of some dinoflagellate species AP is not as tightly controlled by ambient Pi as previously believed. AP activity in these species could indicate selective use of organic phosphorus, or slow metabolic response to changes in P forms, rather than physiological stress to low Pi availability. We emphasize the importance of identifying the links between the different P sources and the species-specific requirements, in order to understand the ecological response to anthropogenic biogeochemical perturbations.

Highlights

The escalating input of nutrients from anthropogenic activities near the coast is considered as one of the greatest threats to worldwide coastal ecosystems
Atmospheric temperatures were in the range of 23–25◦C in June, and declined to approximately 20◦C by October (Figure 2A)
Currents displayed a clear vertical pattern with two layers at the inner current profiler (ADCP1), whereas a more complex pattern occurred at ADCP2

Summary

Introduction

The escalating input of nutrients from anthropogenic activities near the coast is considered as one of the greatest threats to worldwide coastal ecosystems The consequences of this process include changes in species composition, reduction in aquatic biodiversity, alteration of food webs, uneven growth of primary producers, including algal blooms, with an increase of respiration rates in response to the rapid production of organic matter (Nixon, 1995; Cloern, 2001). These effects are pervasive in coastal aquatic systems, and reveal a major influence of nutrient availability on the distribution of marine microbial communities. It is widely accepted that despite the preferential use of orthophosphate, many phytoplankton taxa have the capability to utilize dissolved organic phosphorus (DOP) and other Pi forms such as polyphosphate (Currie and Kalff, 1984; Cotner and Wetzel, 1992; Nicholson et al, 2006)

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Conclusion