Abstract
Increased oligotrophication is expected for oligotrophic areas as a consequence of ocean warming, which reduces diffusive vertical nutrient supply due to strengthened stratification. Evidence of ocean oligotrophication has been, thus far, reported for the open ocean. Here we reported oligotrophication and associated changes in plankton community metabolism with warming in a pristine, oligotrophic Mediterranean coastal area (Cap Salines, Mallorca Island, Spain) during a 10 years time series. As a temperate area, there were seasonal patterns associated to changes in the broad temperature range (12.0 - 28.4 oC), with a primary phytoplankton bloom in late winter and a secondary one in the fall. Community respiration (R) rates peaked during summers and showed higher rates relative to gross primary production (GPP) with a prevalence of heterotrophic metabolism (2/3’s of net community production (NCP) estimates). Chlorophyll a concentration significantly decreased with increasing water temperature in the coastal site at a rate of 0.014 ± 0.003 µg Chla L−1 oC−1 (P< 0.0001). The study revealed a significant decrease with time in Chlorophyll a concentration and nutrients concentration, indicating oligotrophication during the last decade. Community productivity consistently decreased with time as both GPP and R showed a significant decline. Warming of the Mediterranean Sea is expected to increase plankton metabolic rates, but the results indicated that the associated oligotrophication must lead to a slowing down of the community metabolism.
Highlights
The plankton community was characterized by higher respiration rate relative to gross primary production (GPP) and, a prevalence of heterotrophic metabolism (2/3’s of NCP estimates < 0, Table 1)
Community respiration rates (R) and GPP increased with increasing temperature, with a higher activation energy for R (0.68 ±0.15 eV, P < 0.0001) compared to GPP (0.45 ± 0.17 eV, P = 0.013, Figure 2)
The plankton community at Cap Ses Salines showed a seasonal pattern characteristic of the Mediterranean coastal zone, characterized by a late winter bloom followed by a secondary bloom in the fall, low chlorophyll a concentration and primary production in the summer along with high respiration rates (e.g., Duarte et al, 2004; Navarro et al, 2004)
Summary
Evidence of recent oligotrophication of the ocean, reflected in decreased chlorophyll a concentration (Antoine et al, 2005; O’Malley et al, 2009; Boyce et al, 2010; Gregg and Rousseaux, 2014; Signorini et al, 2015), reduced primary production (Behrenfeld et al, 2006; Signorini et al, 2015), and an expansion of the area occupied by the subtropical gyres (McClain et al, 2004; Polovina et al, 2008; Irwin and Oliver, 2009; Signorini et al, 2015) has raised interest on the drivers of this process. To-date, evidence of marine oligotrophication is largely restricted to the open-ocean, with many areas of the coastal ocean exhibiting an opposite eutrophication pattern, due to increased nutrient inputs (Duarte, 2014; Paerl et al, 2014) together with disruption of food webs (Cloern et al, 2016; Östman et al, 2016). (2010) reported oligotrophication of the Northern Adriactic Sea involving a decline of chlorophyll a concentration of, on average, about −0.11 mg m−3 year−1 along the period 19702007 attributed to reduced nutrient input from the Po river. Nishikawa et al (2010) reported oligotrophication of the Eastern Seto Inland Sea, which they attributed to reduced nutrient inputs and warming resulting in increased stratification and reduced nutrient supply Mozeticet al. (2010) reported oligotrophication of the Northern Adriactic Sea involving a decline of chlorophyll a concentration of, on average, about −0.11 mg m−3 year−1 along the period 19702007 attributed to reduced nutrient input from the Po river. Nishikawa et al (2010) reported oligotrophication of the Eastern Seto Inland Sea, which they attributed to reduced nutrient inputs and warming resulting in increased stratification and reduced nutrient supply
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