coastal darkening

lecture framing: carbon cycle + coastal systems

• this lecture links:

  • the marine carbon cycle

  • primary production

  • land sea coupling

  • increasing coastal darkening

• coastal ecosystems are strongly influenced by terrestrial inputs, not only atmopsheric forcing

• emphasis in light availability as a controlling factor for productivity

vertical structure for marine primary production

• light decreases with depth; nutrients increase with dpeth

• euphotic zone: layer where photosynthesis is possible

• deep chlrophyll maximum (DCM) forms where light ad nutrients overlap

• importand reminder:

  • high chlorophyll concentration ≠ high production

• mixing depth strongly affects phytoplankton growth

physical vs biological carbon pumps

• physical (solubility) pump:

  • CO2 dissolves in surface waters

  • cold dense water sink

  • operates on 1000 year timescales

• biological pump:

  • phytoplankton fix CO2 into organic carbon

  • carbon trasnferred through food webs

  • export via sinking particles, fecal pellets, and vertical migration

• both pumps interact and are necessary to explain ocean carbon storage

land-ocean carbon coupling

• carbon enters the ocean from:

  • atmosphere

  • rivers

  • estuaries

  • coastal wetlands

• continental shelves are key sites for

  • organic carbon (OC) burial

  • inorganic carbon (PIC) burial

• coastal zones are hotspots of carbon processing

catchements as drivers of coatal systems

• concept adapted from limnology:

  • the aquatic system mirrors the catchement more than the sky

• coastal water reflect:

  • soil type

  • vegetation

  • hydrology

  • land use

• dissolved organic carbon (DOC) and colour and concentration very storngly with catchement properties

boreal feedbacks and climate change

• climate warming leads to:

  • increased precipitation

  • higher terrestrial productivity

  • increased DOC export

• DOC casues:

  • ligth attenuation

  • reduced photosynthesis

  • increased respiration

• results in increased CO2 and CH4 emissions from aqautic systems

boreal forests as major carbon reservoirs

• boreal forests and tundra store more carbon than tropical forests

• climate change threatens to mobilise this stored carbon

• mobilised carbon can enter aquatic systems as DOC or particles

spatial patterns of environmental change

• maps show strong regions differences in

  • temperature change

  • carbon cycling

• boreal and arctic regions show particulary strong responses to climate forcing

organic carbon variability in nordic waters

• very large variability (>1000x) in organic carbon concentrations

• strong geographic gradients driven by:

  • precipitation

  • peatlands and bogs

  • vegetation cover

• these patterns in lakes help explain coastal DOC inputs

permafrost thaw

• releases previously frozen carbon

• carbon exported as:

  • dissolved organic carbon (DOC)

  • particulate organic matter (POM)

• strong implications for arctic and subarctic coastal waters

land coast material transport

• rivers export:

  • DOC (often coloured)

  • nutrients (N, P, Si, Fe)

  • inorganic carbon

• satellite imagery shows plumes of DOC and chlorophyll near coasts

• terrestrial inputs significantly alter coastal biogeochemistry

arctic land-coast interaction

• increased glacial melt and permafrost thaw

• enhanced delivery of: organic carbon

• particles

strong effect on arctic coastal productivyt and light climate

acidification and buffering by land inputs

• ocean acidification driven by atmospheric CO2 uptake

• terrestrial inputs of inorganic carbon can locally buffer pH

• carbonate chemistry:

  • increased CO2 → lower pH → lower carbonate ion concentration

• calcifying organisms are negatively affected

DOC, light, productivty, and fish

• DOC affects ecosystem mainly through light attenuation

• fish production shows a unimodal response to DOC:

  • low DOC → subsidy effect dominates

  • high DOC → light limitation dominates

• system depth matters: shallow systems tolerate more DOC

coastal and marine darkening

• evidence that coastal waters are becoming darker

• increase in non-phytoplankton light absorption

• potential for ecosystem regime shifts, especially in fjords

darkening and contaminant

• increased DOC affects contaminant trasnport and bioavailability

• alters microbial activity and food-web strcuture

• infleunces accumulation and transfer of pollutants

norwegian coastal current (NCC)

• major transport pathway along the norwegian coast

• roughly equal freshwater input from:

  • baltic sea

  • norwegian mainland runoff

  • key vector for DOC trasnport northward

long term coastal darkening trends

• browning signal propogates from land to coast to arctic

• synchronised changes in

  • phytoplankton phenology

  • fish phenology

• effects detectable over centennial timescales

forest → coast → fish conenctivity

• increased forest biomass → increased DOC export

• DOC reduces light → delays in phytoplankton blooms

• delayed blooms → delayed cod spawning

• direct link between terrestrial ecosystems and fisheries

long term ecosystem change

• evidence from

  • long term data sets

  • media reports

• breakpoints around 2000 suggest structural ecosystem changes

• increasing DOC and nutrient inputs overtime

impacts on kelp and benthic production

• coastal darkening reduced kelp productivyt

• loss of kelp reduces:

  • coastal carbon fixation

  • habitat complexity

• alters coastal carbon pathways

tracing terrestrial vs marine carbon

• DOC decreases with increasing salinity

• stable isotopes distinguish:

  • terrestrial carbon sources

  • marine carbon sources

• clear terrestrial signal in coastal zones

sediment core evidence

• stable isotope profiles from sediment ccores

• show increasing terrestrial carbon infleunce over recent centuries

• indicates recent intensification of land-sea coupling

ecosystem consequences of caostal darkening

• shallower compensation depth

• reduced vertical distribution of phytoplankton

• lower primary production

• higher respiration, lower oxygen

• potential shift toward jellyfish-dominated systems