Surface nutrient availability and irradiance

The major factors determining the distribution of nutrients in the English Channel and the Bay of Biscay are inputs from rivers and exchange with the open ocean, biological regenerative processes and exchange due to water advection and mixing (there will also be a minor input from the atmosphere).

River and ocean inputs: the Loire and Gironde rivers account for 80 % of fresh water inputs of nitrogen and phosphorus along the French Atlantic coast (Loyer et al., 2006). The phosphorus input from the Gironde is larger than from the Loire due to the relatively high quantity of suspended particulate matter (Gil et al., 2002). Reid et al., (1993) noted that the river Seine contributes ~85 % of the fresh water inputs of nutrients to the English Channel. The importance of the open ocean as a source (or sink) for nutrients on the continental shelf depends on surface winter gradients across the shelf break (Hydes et al., 2004). In general, it appears that, at the latitude of the southern Celtic Sea and Bay of Biscay, oceanic waters are a relatively weak source of nutrients to the shelf except locally at the shelf edge where upwelling (e.g. along the Cantabrian coast of northern Spain) or mixing due to internal tides occur (Sharples et al., 2007).

Differences in nitrate-to-phosphate ratios mean that the ocean may act as a source for one but not the other (Hydes et al., 2004).

Chapter 1 General Introduction

Biological control: Nutrient concentrations decrease in the spring and early summer as assimilation into the planktonic food chain exceeds regeneration in the water column and from bottom sediments, and then rise again in autumn and winter as

regeneration from detritus and dissolved organic matter exceeds assimilation (Reid et al., 1993). During the summer months, relatively high nutrient concentrations below the seasonal thermocline lead to a slow upward flux associated with vertical mixing

processes, which is largely utilised within the subsurface chlorophyll maximum (Pingree and Pennycuick, 1975; Sharples et al., 2001).

Advection and mixing: Horizontal advection and exchange will act as a source of nutrients along gradients associated, for example, with freshwater plumes and frontal boundaries. Tides are the main cause of vertical mixing on the shelf and affect nutrient exchange between the bottom sediments and overlying water (Trimmer et al., 1999). In the ocean upward vertical motion associated with mesoscale eddies is a source of nutrients to the surface water (Pingree and Le-Cann, 1992a, 1992b).

A summary of typical winter and summer inorganic nutrient (nitrate, phosphate and silicate) concentrations in the English Channel and Bay of Biscay is presented in Table 1.1. The spatial variability of inorganic nutrients in winter from a survey covering most of the English Channel waters was reported by Tappin et al., (1993). They showed a south to north gradient in inorganic nutrients in the western English Channel. The seasonal decline in nitrate concentration in the summer and increase in winter is clear at station E1 in the western English Channel (Armstrong et al., 1974; Pingree et al., 1977a; Jordan and Joint, 1998). Jordan and Joint, (1998) re-examined the historical nutrient data from station E1 from 1923 to 1987 and showed a high degree of variability in the nitrate: phosphate ratios. The latter analysis revealed that mid-summer values of phosphate increased for short periods of time while nitrate concentrations remained low, but there was no clear explanation (Jordan and Joint, 1998). Pingree et al., (1977a) considered that nitrate was limiting phytoplankton growth at E1. In regions where the waters are permanently well mixed by tidal action, such as the central English Channel and Ushant region, nutrients are not depleted (Wafar et al., 1983).

The Bay of Biscay is characterised by a homogeneous water mass, relatively rich in nutrients during the winter (Treguer et al., 1979). Freshwater inputs (Loire and

Chapter 1 General Introduction

Gironde rivers) contribute to the nutrients in the northern Bay of Biscay (Loyer et al., 2006). The winter nitrate supply persists throughout spring and nitrate concentrations are depleted in the summer (Loyer et al., 2006). By using a monthly time series (1993-2003) of nutrient data from the southern Bay of Biscay, all nutrients showed important seasonal variations and decreasing trend overall (Llope et al., 2007).

The regional consequences of seasonal changes in solar radiation and water column stratification on plankton distributions and nutrient utilisation have been described by Lavin et al., (2006) for the Bay of Biscay and Sharples and Holligan (2006) for the Celtic Sea and western English Channel. In the winter, phytoplankton biomass is low, as growth is light limited due to low solar radiation and deep mixing. In the northern Bay of Biscay, increased phytoplankton abundance early in the year

(February-March) has been associated with shallow haloclines in plumes of the Gironde and Loire Rivers (Pingree et al., 1986; Labry et al., 2001; Hureta et al., 2007) that carry relatively high nutrient and suspended sediment loads. The effects of these plumes occasionally extend around Ushant into the western English Channel.

In spring, with increased heating of the surface water resulting from increased daily insolation, thermal stratification occurs and combined with the increase in irradiance and the availability of nutrients in the water, leads to an increase in phytoplankton biomass in most regions in the western English Channel and Bay of Biscay. In regions where the waters are permanently mixed by tidal action, such as the central English Channel and Ushant region, a stratification-induced spring bloom does not occur and the phytoplankton population only reaches its maximum in the summer, when irradiance intensity is such that the euphotic depth becomes comparable to the water depth (Wafar et al., 1983). The balance between the effects of vertical turbulence and the increasing of spring irradiance determines the start of spring bloom (Van-Haren et al., 1998). However, when wind mixing is weak, the spring bloom can begin even before thermal stratification is established (Henson et al., 2006).

Development of spring and autumn surface phytoplankton blooms, as well as summer blooms associated with tidal fronts are well documented by ocean colour satellite imagery (Joint and Groom, 2000). Enhanced chlorophyll levels associated with internal wave activity at the Celtic Sea shelf break in summer have recently been

Chapter 1 General Introduction

investigated by Sharples et al., (2007). In all cases, increased phytoplankton biomass can be attributed to a relatively stable surface layer in which light and nutrients are sufficient for growth to exceed losses. The only feature of the annual distribution of phytoplankton that cannot be resolved by satellite is the subsurface chlorophyll maximum (SCM) associated with the seasonal thermocline (Sharples et al., 2001). It should be noted however, that the dynamics of the SCM is largely a function of mixing at the base of the thermocline and is relatively unaffected by the nutrient properties of the surface water.