Ocean circulation, transport and mixing at seamounts and biological consequences Christian Mohn and J. William Lavelle SEAMOUNTS’09 workshop Exploration, Biogeosciences and Fisheries Scripps Institution of Oceanography March 19-21, 2009

The long journey towards a detailed picture Foundation Seamount Chain, south Pacific (satellite gravimetry) etopo-5 (NGDC, NOAA, 1988) Smith and Sandwell (1997)

Global distribution of seamounts and seamounts sampled Predicted locations of seamounts (Kitchingman and Lai, 2004) Biologically sampled, but no data available Some level of data available at about 300 seamounts Data available (taxonomy, abundance, species compostion, sampling strategy) at about 50 seamounts censeam.niwa.co.nz

Physical parameter space Coriolis parameter, geographical location currents, forcing Seamount morphology f  (s,t,p) U mean U var L H hmhm Q T,Q S Stratification Vertical heat and salt flux

Flow-topography interaction: Transient response and Taylor cap formation STRONG weak flowstrong flow 122 weak inflow: Butterfly pattern strong inflow: Taylor cap circulation, cold water (dense) dome dominant in regions with strong mean flows and weak tidal variability σ Taylor cap disappears for very strong flows Transient stateSteady state

Flow-topography interaction: Tidal rectification STRONG RECTIFIED 122 σ Transient stateSteady state transient response: Rotating trapped wave pattern and tidal amplification time-mean: rectified anti-cyclonic recirculation, cold water dome dominant in regions where mean far field flows are weak (Fieberling Guyot)

Idealized flow pattern at seamounts: The steady state Bottom-intensified recirculation cell, isopycnal doming Downwelling in the seamount center BUT… Upwelling along the outer seamount flanks U mean Tidal currents Wind Ũ mean Internal waves Local turbulence Wind mixing, wind-driven currents

Seamount processes: Flow amplification at seamount summits Flow at seamount summits can be a factor of 20 (or more) higher than the surrounding far field flow! Lavelle (unpublished)

Local processes: Internal wave generation and propagation Generated by flow over topography and sea surface winds C=1: bathymetry  is considered critical, potential internal wave formation sites, associated with intense fluid dynamics Dissipation of internal waves affects large-scale circulation and plankton distribution Slope criticality: S Internal wave energy propagation: Lavelle (in prep.) C  S

Local response to long-term variability of impinging flows Sedlo Seamount, 780 m summit depth -+  = dv/dx – du/dy Sub-tidal far field flow, SW of the seamount (AVISO currents) Temporary destabilization of Taylor Cap flow at the summit correlated with disturbance in the far field (White et al., 2007;Mohn et al., 2009)

Summary I: Knowledge base: still very limited, mainly based on individual snapshots and modelling studies Each seamount is a unique environment, defined by its very own physical parameter space Idealized steady state: Taylor cap and/or tidally rectified toroid Steady state permanently exposed to concurrent/intermittent oceanic processes (sub-tidal flow variability, wind events, tidal currents, internal waves, local flow amplification)

Bio-physical interactions: A conceptual model Passive particle retention, enhanced primary productivity, vertical particle scattering Uplifting of nutrient- rich deep water Nutrient-depleted surface layer Attraction and aggregation of higher trophic levels

Enhanced levels of primary productivity? Great Meteor Seamount (summit depth: 280 m, subtropical North Atlantic), Climatology - enhanced levels of Chlorophyll over seamount But: Patchiness of same scale around seamount High inter-annual variability SeaWifs Chlorophyll-a (White et al., 2007) 7 years, August monthly mean June-August, 7 year mean summit

Restricted particle dispersal and retention? Retention above seamounts mainly influenced by physical processes? Top View – 150 m depth Great Meteor Seamount (Beckmann & Mohn, 2002) Fieberling Guyot (Mullineaux & Mills, 1997)

Porcupine Bank Rockall Bank km 320 km Mechanisms for particle displacement: Vertical scattering d (m) days after tracer release vertical horizontal Strong tidal influence on tracer displacement: up to several 100 meters (vertically) and several kilometers (horizontally) within one tidal cycle (Mohn and White, 2007)

Elevated levels of suspended material at the seabed near the location of the carbonate mounds (Mienis et al., 2007) mounds Benthic nepheloid layer (high SPM) Continental slope 500m 1000m 1500m 2000m 0m Mechanisms for particle displacement: Internal waves Large vertical movement of the 10ºC isotherm at northern Porcupine Bank slope

Passive tracers: Response to sub-tidal flow variations? Inflow: Modulation of amplitude and direction Modulation period: 30 days No tides Simulated Eulerian tracer concentrations (the last 30 days of a 90 days simulation were averaged) (Mohn and White, 2009) Initial tracer release area

Passive tracers: Response to sub-tidal flow variations - Tracer variability patterns - High variability in regions of particle loss and accumulation (not limited to the summit) Normalized tracer variance (90 day simulation period) (Mohn and White, 2009)

Patterns of seasonal variability Repeated surveys at Great Meteor Seamount, summit depth : 280 m, subtropical North Atlantic (Mourino et al., 2001): Seasonal changes in mixed layer depth and stratification translates into 2-3 fold changes in depth- integrated Chl-a and primary production rates.

Summary II: Changes in hydrographic and flow conditions (high- and low-frequency) are an important factor for shaping seamount communities (and controlling bio- communication?) Sphere of influence is not restricted to the seamount (implications for sampling strategies), but penetrates deep into the oceanic far field. Complex co-existence of physical controls and biological distribution patterns requires a more comprehensive ecosystem modelling approach (full spectrum forcing).

Outlook: Full spectrum forcing Lavelle (in prep.) Response of passive tracer patterns to mean and combined mean + tidal forcing (representing only part of the spectrum of oceanic motions) More retention when tidal flow is added

Outlook: Full spectrum forcing Current meter time series from the East Pacific Rise EPR (Lavelle, 2009)

Outlook: Full spectrum forcing Lavelle (in prep.) Tracer response to a more realistic forcing spectrum as determined from the EPR time series.

Conclusions Seamounts are very good habitats to study and understand bio-physical interactions well enough to predict abundance and distribution of marine species. Is our knowledge good enough? Adaptation of sampling and modelling strategies (sphere of influence, full-spectrum forcing and biological response, robustness of observed patterns against long-term changes) A broad, multi-disciplinary ecosystem approach is required for both research and sustainable protection/management. Intensification of exisiting networks.