7.2 Nutrient limitation and eutrophication

7.2 Nutrient Limitation and Eutrophication as a Process

Are marine phytoplankton communities responsive and even vulnerable to anthropogenic nutrient enhancement? Numerous field and experimental studies suggest that the supply of nitrogen for primary production in marine waters is the critical nutrient and process. This has led to the paradigm that marine phytoplankton growth is nitrogen-limited, unlike fresh water communities which are phosphorus-limited. Hecky and Kilham [221] have questioned whether marine coastal waters are more N-limited than P-limited, and attribute this view to experimental artifact. Nonetheless, the bulk of the experimental and field evidence favors widespread N-limitation. The notion of nutrient limitation is an old concept, extrapolated from Liebig's Law of the Minimum formulated for agricultural systems (see [118]) and applied to the sea a century ago [58]. The essence of this concept is that the amount of nutrient (i.e. nitrogen or phosphorus) available regulates phytoplankton growth and biomass through a yield-dose response. In this relationship, there is usually a linear, direct increase in phytoplankton abundance in response to nutrient supply, and which corresponds to the first (linear) segment of the hyperbolic (curvilinear) relationship shown for nutrient uptake (Figure 1). However, should nutrient loading continue to increase, there is a threshold concentration at which the population carrying capacity is reached, and above which the response to further increases in nutrient loading is similar (hyperbolic) to that in Figure 1.

The yield-dose response is one aspect of the relationship; changes in phytoplankton species composition and bloom dynamics can also accompany nutrient enrichment, will be discussed. The focus on nitrogen has tended to ignore evidence that phosphorus, silicon and iron also can limit marine phytoplankton growth. Silicon can limit the in situ growth of diatoms, and unique among the phytoplankton in their requirement for this silicon [101]. Documentation of iron limitation remains an intense line of research [447][242]. While the N limitation hypothesis underpins marine eutrophication research, it is important to recognize that eutrophication alters the total sum of macro- and micro-nutrients that affect phytoplankton, either as growth stimulants or inhibitors. As a result, there may be multiple, concurrent nutrient limitation involving N, P, Si, Fe which have different impacts on, and targets different floristic components (species, functional groups) within the phytoplankton community.

It is essential that eutrophication be recognized as a process of impact and change that progresses through various phases in response to the intensity, nature and degree of nutrient loading (see [257]); it is not a trophic state. Seki & Iwami [413] recognized four stages in the eutrophication of habitats, progressing from an oligotrophic state (believed to be the original state of natural waters) to an advanced hypereutrophic state reached after passing through intermediate mesotrophic and eutrophic stages. Transitions from one stage to the next are gated by thresholds in habitat parameters that are modified during nutrient enrichment. Seki & Iwami used the ratio of particulate to dissolved organic matter in relation to the total organic content, and produced in response to enrichment, to designate the thresholds at which transition into the next eutrophication stage occurs. The relationship between the turnover time of soluble amino acids vs. inorganic nitrogen levels was also used. Seki & Iwami emphasized that in focusing on the phytoplankton the heterotrophic microbial community must not be neglected; indeed, the metabolic contribution of the latter to nutrient-enhanced, organic cycling progressively increases with eutrophication intensity. That is, the importance of phytoplankton relative to bacteria will progressively diminish in its biochemical regulation of the saprobic water quality that accompanies increasing eutrophication. In a slightly different approach, Nixon [341] used the total annual supply of carbon from primary production within the system (autochthonous carbon) and introduced from external sources (allochthonous carbon) to classify trophic states along the eutrophication pathway. He also recognized four eutrophication stages, including a terminal hypertrophic stage when nutrient enrichment is particularly high (Table 1). The Seki & Iwami and Nixon classification schemes link the definition of eutrophication as a process to the metabolism or total supply of organic carbon, rather than to the fluxes and concentrations of specific inorganic nutrients, the parameter usually focused upon. For example, the UK Comprehensive Studies Task Team [76][77] uses winter concentration of `dissolved available inorganic nitrogen' (DAIN) to classify eutrophication status. A state of hypernutrification is considered to be present and a precursor to eutrophication when/where winter DAIN concentrations significantly exceed 12 mM m^-3 and there is at least 0.2 mM dissolved inorganic phosphorus m^-3. The CSTT considers a habitat to be eutrophic if summer chlorophyll concentrations regularly exceed 10 mg chl m^-3.

7.2 Nutrient limitation and eutrophication