Our review of the literature has revealed Southern Ocean subsurface chlorophyll-a maxima (SCMs) to be an annually recurrent feature throughout the basin. Most of these SCMs are different to the "typical" SCMs observed in the tropics, which are maintained by the nutrient-light co-limitation of phytoplankton growth. Rather, we have found that SCMs are formed by other processes including diatom aggregation, sea-ice retreat, eddies, subduction events and photo-acclimation. At a local scale, these SCMs can facilitate increased downward carbon export, primary production and food availability for higher trophic levels. A large proportion of Southern Ocean SCMs appear to be sustained by aggregates of large diatoms that form under severe iron limitation in the seasonal mixed layer. The ability of large diatoms to regulate their buoyancy must play a role in the development of these SCMs as they appear to increase buoyancy at the SCM and thus avoid further sinking with the decline of the spring bloom or naturally iron fertilized blooms. These SCMs remain largely unobserved by satellites and it seems that ship-based sampling may not be able to fully capture their biomass. In the context of the Marine Ecosystem Assessment of the Southern Ocean it is important to consider that this phenomenon is missing in our current understanding of Southern Ocean ecology and future climate scenarios. The broader implications of SCMs for Southern Ocean ecology will only be revealed through basin-wide observations. This can only be achieved through an integrated observation system that is able to harness the detailed information encapsulated in ship-based sampling, with the increased observational capacity of fluorometers on autonomous platforms such as those in the biogeochemical Argo (BGC-Argo) and the Marine Mammals Exploring the Ocean Pole to pole (MEOP) programs. The main challenge toward achieving this is the uncertainties associated with translating fluorescence to chlorophyll-a concentrations. Until this translation is resolved, the reporting of subsurface fluorescence maxima (SFMs) in place of SCMs could still yield valuable insights with careful interpretation.

The Red Sea is characterized by its high seawater temperature and salinity, and the resilience of its coastal ecosystems to global warming is of growing interest. This high salinity and temperature might also render the Red Sea a favorable ecosystem for calcification and therefore resistant to ocean acidification. However, there is a lack of survey data on the CO 2 system of Red Sea coastal ecosystems. A 1-year survey of the CO 2 system was performed in a seagrass lagoon, a mangrove forest, and a coral reef in the central Red Sea, including fortnight seawater sampling and high-frequency pH T monitoring. In the coral reef, the CO 2 system mean and variability over the measurement period are within the range of other world's reefs with pH T , dissolved inorganic carbon (DIC), total alkalinity (TA), pCO 2, and Ω arag of 8.016±0.077, 2061±58 μmol/kg, 2415±34 μmol/kg, 461±39 μatm, and 3.9±0.4, respectively. Here, comparisons with an offshore site highlight dominance of calcification and photosynthesis in summer-autumn, and dissolution and heterotrophy in winter-spring. In the seagrass meadow, the pH T , DIC, TA, pCO 2 , and Ω arag were 8.00±0.09, 1986±68 μmol/kg, 2352±49 μmol/kg, 411±66 μatm, and 4.0±0.3, respectively. The seagrass meadow TA and DIC were consistently lower than offshore water. The mangrove forest showed the highest amplitudes of variation, with pH T , DIC, TA, pCO 2 , and Ω arag , were 7.95±0.26, 2069±132 μmol/kg, 2438±91 μmol/kg, 493 ±178 μatm, and 4.1±0.6, respectively. We highlight the need for more research on sources and sinks of DIC and TA in coastal ecosystems.Plain Language Summary Global warming and ocean acidification are consequences of increased CO 2 emissions to the atmosphere by humankind and are major threats to marine ecosystems. The Red Sea waters are naturally warm and saline. The resilience of its coral reefs, mangrove forests, and seagrass meadows is of growing interest for the scientific community in the context of global warming. The high temperature and salinity might render the Red Sea quite resistant to ocean acidification as well as an environment chemically very favorable for calcification, notably by corals. Calcification is a process dampened by the acidity (pH) of water, which depends on the chemistry of CO 2 in seawater. Warm and saline water naturally tend to have a more basic pH and then be less corrosive to calcareous skeletons. However, the chemistry of the CO 2 and acidity baselines and variability in the Red Sea are poorly documented. We conducted a year-round survey of the CO 2 chemistry of seawater in a seagrass meadow, mangrove forest, and coral reef ecosystem, involving discrete water sampling and highfrequency measurements.

Mangroves are important coastal ecosystems of warm climatic regions that often grow in shallow saline or brackish waters of estuaries and river mouths which are affected by wide tidal intervals and receive abundant nutrient supply. However, mangroves also occur in areas of little tidal influence and devoid of riverine inputs, where they can develop a stunted plant form. Here we report that Avicennia marina trees in the fringe of the Red Sea have maximum heights toward the lower range of that reported elsewhere (average maximum canopy height of 4.95 m), especially in the central region, where mangroves are stunted with an average tree height of 2.7 m. Maximum tree height and chlorophyll a concentration correlated positively with nitrogen concentration in the leaves of A. marina. We conclude that the stunted nature of mangrove trees in the central Red Sea is likely driven by nitrogen limitation.

Tropical seagrass meadows are highly productive ecosystems that thrive in oligotrophic environments. The Red Sea is characterized by strong N-S latitudinal nutrient and temperature gradients, which constrain pelagic productivity. To date, the influence of these natural gradients have not been assessed in metabolic rates for local seagrass communities. Here we report metabolic rates [gross primary production (GPP), respiration (R), and net community production (NCP)] in four common species of seagrass (Halodule uninervis, Halophila ovalis, Halophila stipulacea, and Thalassia hemprichii) along latitudinal and thermal gradients in the Red Sea. In addition, we quantified leaf nutrient concentration (nitrogen, phosphorous, and iron), and correlate this with latitude. Our results show that average metabolic rates and aboveground biomass of seagrass meadows in the Red Sea were generally in the lower range when compared to global values reported for the same species elsewhere. The optimum temperature of Red Sea seagrass meadows varied among species with increases along the sequence: H. stipulacea < T. hemprichii < H. uninervis ∼ H. ovalis. GPP for H. uninervis -a seagrass thermophile -was lowest in higher latitudes and increased toward lower latitudes during the summer months. While temperature was identified as a strong driver of metabolic rates across seagrass meadows, leaf concentration of phosphorous and iron (but not nitrogen) was below nutrient sufficiency thresholds, indicating these two elements might be limiting for seagrass meadows in the Red Sea.