Continuous long-term observations of the carbonate system dynamics in the water column of a temperate fjord

Measuring the trophic status of ponds: Relationships between summer rate of periphytic net primary productivity and water physico-chemistry

Reply to comment by J. Marra on “Measuring primary production rates in the ocean: Enigmatic results between incubation and non‐incubation methods at Station ALOHA”

The riverine supply of the globally limiting nutrient, phosphorus, to the ocean accounts for only a few percent of nutrient supply to photosynthetic organisms in surface waters. Recycling of marine organic matter by heterotrophic organisms provides almost all of the phosphorus that drives net primary production in the modern ocean. In the low-oxygen environments of the Proterozoic, the lack of free oxygen would have limited rates of oxic respiration, slowing the recycling of nutrients and thus limiting global rates of photosynthesis. A series of steady-state mass balance calculations suggest that the rate of net primary production in the ocean was no more than 10% of its modern value during the Proterozoic eon, and possibly less than 1%. The supply of nutrients in such a world would be dominated by river input, rather than recycling within the water column, leading to a small marine biosphere found primarily within estuarine environments.

Understanding changes in vegetation productivity and carbon use efficiency and their responses to climate change is significant to accurately assess and predict regional carbon budget in Northeast forest area, a region being an important carbon sink and sensitive to global change. Based on MODIS monitoring data and vegetation type distribution data, I analyzed the spatiotemporal varia-tions of ecosystem productivity (net primary productivity (NPP), gross primary productivity (GPP)) and carbon use efficiency (NPP/GPP) of Northeast forest from 2000 to 2015. Results showed that the average NPP and GPP were 346.4 and 773 g C·m-2·a-1, respectively, and the average NPP/GPP was 0.45 during 2000 and 2015. NPP and GPP of different forest types were following the order: coniferous and broad-leaved mixed forests > deciduous broad-leaved forests > coniferous forests, while the difference in NPP/GPP was not significant among different forest types. NPP and GPP were high in Southeast part and low in Northwest part. From 2000 to 2015, the NPP, GPP and NPP/GPP of Northeast forest showed a fluctuating increase, suggesting the carbon sequestration capacity was gradually enhanced. However, the trends and rates of NPP, GPP and NPP/GPP showed spatial variation. NPP, GPP and NPP/GPP increased significantly in the southern part of the Daxing'anling while decreased significantly in the northern part of the Daxing'anling, and showed a weak increasing trend in the rest of Northeast forest. The increase of annual precipitation was the main factor driving the fluctuating increase of NPP, GPP and NPP/GPP in Northeast forest.

New production (New P, the rate of net primary production (NPP) supported by exogenously supplied limiting nutrients) and net community production (NCP, gross primary production not consumed by community respiration) are closely related but mechanistically distinct processes. They set the carbon balance in the upper ocean and define an upper limit for export from the system. The relationships, relative magnitudes and variability of New P (from 15NO3- uptake), O2 : argon-based NCP and sinking particle export (based on the 238U : 234Th disequilibrium) are increasingly well documented but still not clearly understood. This is especially true in remote regions such as polar marginal ice zones. Here we present a 3-year dataset of simultaneous measurements made at approximately 50 stations along the Western Antarctic Peninsula (WAP) continental shelf in midsummer (January) 2012-2014. Net seasonal-scale changes in water column inventories (0-150 m) of nitrate and iodide were also estimated at the same stations. The average daily rates based on inventory changes exceeded the shorter-term rate measurements. A major uncertainty in the relative magnitude of the inventory estimates is specifying the start of the growing season following sea-ice retreat. New P and NCP(O2) did not differ significantly. New P and NCP(O2) were significantly greater than sinking particle export from thorium-234. We suggest this is a persistent and systematic imbalance and that other processes such as vertical mixing and advection of suspended particles are important export pathways.This article is part of the theme issue 'The marine system of the west Antarctic Peninsula: status and strategy for progress in a region of rapid change'.

Nitrogen uptake and primary productivity rates in the Mid-Atlantic Bight (MAB)

Throughout the last century, the Swedish coasts, have been exposed to climatic variation and human influence, and this has altered the biota in some areas. Diatoms and silicoflagellates in a periodically-laminated sediment core from Koljo Fjord (Swedish west coast) were examined to assess recent changes in the microalgal community. The most notable changes in the plankton occurred at the beginning of a long period of water column stratification in the fjord that extended from 1930 to 1980. The planktonic flora changed from a community dominated byBacterosira bathyomphala andThalassionema nitzschioides during the 1800s to a community dominated byDetonula confervacea, T. nitzschioides, andThalassiosira spp. after 1930. Silicoflagellates, were more abundant after 1940. Planktonic variations corresponded to oscillations in climate, hydrography, and weather, which determine water column stability in the fjord. The tychopelagic species,Paralia sulcata, was more abundant in unlaminated sections of the core, indicating preference for a vertically mixed water column. No direct effects of increased nutrient loading on the plankton could be established. Epiphytic diatoms show a period of decline from the 1950s to 1990s. This trend probably follows a shift in the macroalgal community to less-suitable host species. To what extent this pattern has been influenced or reinforced by humans cannot be determined at the present time. The results from Koljo Fjord, in particular the exploration of meteorological and physical oceanographic influences on algal dynamics, emphasize the importance of distinguishing between natural and humaninduced changes in the environment.

Major changes in diatom abundance, productivity, and net community metabolism in a windier and dryer coastal climate in the southern Humboldt Current

The main hypothesis of this work is that Trapa natans L. and similar floating leaved macrophytes are only temporary sinks of atmospheric carbon dioxide and that they favour water hypoxia and large methane efflux from sediment to the atmosphere, due to their shading effect and scarce ability to transfer oxygen to submerged tissues. For this purpose, from April to August 2005, T. natans production, dissolved O2, CO2 and CH4 concentrations in the water column and CO2 and CH4 fluxes across the wateratmosphere interface were measured in an oxbow lake (Lanca di Po, Northern Italy) where a monospecific floating mat of water chestnut develops. Net primary production by T. natans was determined via biomass harvesting while gas fluxes were determined via short-term incubations of light and dark floating chambers. From July onwards, when the water surface of the oxbow lake was entirely colonized by the plant, the dense canopy resulted in a physical barrier for light and water reareation. As a consequence of sediment and plant respiration, persistent hypoxia and often anoxia, and CO2 and CH4 supersaturation occurred in the water column. Net primary production of T. natans, calculated at peak biomass, was 13.05 ± 0.32 mol CO2 m-2. The T. natans mat was a net sink for atmospheric CO2 from mid June to mid August, with an uptake peak measured at the beginning of July (229 mmol m-2 d-1); estimated net ecosystem metabolism was ≤10.09 ± 1.90 mol CO2 m-2. Contextually, during the vegetative period of T. natans, the oxbow lake was a net source of methane (9.52 ± 2.10 mol m-2), and the resulting CH4 to CO2 flux ratio across the water-atmosphere interface was ≥0.94. The large methane release was probably due to the persistent hypoxia and anoxia induced by the T. natans meadow, which uncoupled methane production from methane oxidation.

Tropical forests have the highest rates of net primary productivity (NPP) in terrestrial ecosystems and, therefore, may contribute significantly to the mitigation of global climate change. Although NPP is influenced by soil fertility, and recently, in some regions, mining activity in forest ecosystems has intensified. Little is known about how soils determine the restoration of NPP in forests degraded by mining. We evaluated the influence of soil nutrients on wood NPP of post-mining forests in the biogeographic Chocó region (Colombia), with emphasis on the effects of nitrogen (N) and phosphorus (P) limitations in post-mining forests under successional stages of 12-15 or 30-35 years. For this, permanent plots were established in secondary post-mining forests in Jigualito (Colombian Pacific), the wood NPP (accumulated and current) was evaluated, and it was related to soil properties such as organic matter (OM), acidity, Al, total N, available P, magnesium (Mg), potassium (K), calcium (Ca), and texture. An accumulated wood NPP of 0.72 t ha -1 yr -1 was recorded in post-mining forests 12-15 years old. Meanwhile, in post-mining forests of 30-35 years, the accumulated wood NPP was 6.52 t ha -1 yr -1 . The current wood NPP was 4.25 t ha -1 yr -1 in post-mining forests with 30-35 years of recovery. Accumulated NPP positively correlated with soil OM, total N, Ca, Mg, and effective cation exchange capacity–ECEC in post-mining forests. In post-mining forests, a slow recovery of the wood NPP was denoted in the first years. Soil nutrients determined the wood NPP, and a multiple limitation of nutrients with the succession was observed, which corroborates the need to restore the degraded ecosystem in the region.

Cycling phosphorus on the Archean Earth: Part II. Phosphorus limitation on primary production in Archean ecosystems

Temperature, dissolved oxygen and dissolved methane profiles were measured during autumn and summer, in a shallow floodplain lake in south-eastern Australia to determine the effects of water-column stability on methane and oxygen dynamics. The water column was well mixed in autumn. Strong thermal stratification developed in the late afternoon in summer, with top-to-bottom temperature differences of up to ∼6 °C. Methane concentrations in surface waters varied over a daily cycle by an 18-fold range in summer, but only by a 2-fold range in autumn. The implication of short-term temporal variation is that static chambers deployed on the water surface for short times (less than a day) in summer will significantly underestimate the diffusive component of methane emissions across the water–atmosphere interface. There was a marked diel variation in dissolved oxygen concentrations in summer, with the highest oxygen values (commonly 5–8 mg l−1) occurring in the surface waters in late afternoon; the bottom waters were then devoid of oxygen (< 0.2 mg l−1). Because of high respiratory demands, even the surface water layers could be nearly anoxic by morning in summer. The concentration of dissolved oxygen in the surface waters was always less than the equilibrium value. When the water column became thermally stratified in summer, the dissolved oxygen and methane maxima were spatially separated, and planktonic methanotrophy would be limited to a moving zone, at variable depth, in the water column. In summer the whole-wetland rates of oxygen production and respiration, calculated from long-term (∼5 h) shifts in dissolved oxygen concentrations over a diel period, were approximately 6–10 and 3–6 mmol m−3 h−1, respectively. These values correspond to net and gross primary production rates of ∼0.7–1.2 and ∼1.0–1.9 g C m−3 day−1, respectively.

Carbon, nitrogen, oxygen and sulfide budgets in the Black Sea: A biogeochemical model of the whole water column coupling the oxic and anoxic parts

biological production due to the supply of nutrients from subsurface layers to the upper layer (Wooster and Reid 1963). The difference in the partial pressure of CO2 between the sea surface and the overlying air is one of the factors controlling the air-sea CO2 flux. Compared with the partial pressure of CO2 in the air (pCO2 air), the partial pressure of CO2 in surface seawater (pCO2 sea) varies largely, both in time and space. Therefore, it is the pCO2 sea that determines the direction of the CO2 flux between the sea surface and the air. By compiling numerous pCO2 sea data, Takahashi et al. (2009) have reported that the western North Pacific (WNP) is the area where the greatest air-sea CO2 flux (oceanic CO2 uptake) occurred in the Pacific Ocean, while the eastern North Pacific (ENP) is a weak sink for atmospheric CO2. In the Kuroshio extension, Ogawa et al. (2006) confirmed a large air-sea CO2 flux in 1999 and 2000 based on the voluntary observation ship pCO2 sea data. In the area including the Kuroshio extension (30– 40∞N, 150–180∞E), Takahashi et al. (2006) reported low decadal rates of change in Temporal and Spatial Variations in Carbonate System and Air-Sea CO2 Flux in the Kuroshio and Kuroshio Extension

A major fraction of phytogenic organic carbon, produced in the surface ocean, is processed by the heterotrophic biota in the water column, whereas only a minor fraction is buried in marine sediments. The depth distribution of organic carbon mineralisation in the water column determines the time scales for carbon dioxide withdrawal from the atmosphere. The transfer of organic carbon from the surface to the deep ocean through gravitational settling and downward advection are therefore key processes in determining the efficiency of the biological pump. Sediment trap studies indicate the downward flux of particulate organic carbon to decrease exponentially with increasing water depth. The vertical export flux from the euphotic zone correlates positively with primary production up to productivities of~200 g C m-2 y-1 and attains a constant value at higher production rates. The amount and quality of organic carbon exported from the surface mixed layer strongly depends on the food web structure in the upper ocean. Some of the current perspectives on the role of grazing by micro- and mesozooplankton on retention versus export of organic carbon in the mixed layer are presented. The transformation through solubilization, respiration and fragmentation of aggregates and fecal pellets by particle associated food webs diminishes the downward flux of particulate organic carbon with increasing water depth. Calculated and measured metabolic activities of these food webs suggest turn-over times of the aggregate organic carbon in the order of weeks. The hydrodynamics of sinking particles appears crucial for the mass exchange between the particle and its surrounding water and regulates the activity of the associated community. In addition, while sinking through the water column the particle associated communities are exposed to changes in physico-chemical conditions such as temperature and oxygen concentration which, by controlling the activity of the community, may regulate the downward flux of particulate organic carbon in deeper waters. Our current understanding of biological processes, especially in the upper water column, is insufficient to predict the composition and downward fluxes of organic matter.