Stoichiometric multitrophic networks reveal significance of land-sea interaction to ecosystem function in a subtropical nutrient-poor bight, South Africa.

Nearshore marine ecosystems can benefit from their interaction with adjacent ecosystems, especially if they alleviate nutrient limitations in nutrient poor areas. This was the case in our oligo- to mesotrophic study area, the KwaZulu-Natal Bight on the South African subtropical east coast, which is bordered by the Agulhas current. We built stoichiometric, multitrophic ecosystem networks depicting biomass and material flows of carbon, nitrogen and phosphorus in three subsystems of the bight. The networks were analysed to investigate whether the southern, middle and northern bight function similarly in terms of their productivity, transfer efficiency between trophic levels, material cycling, and nutrient limitations. The middle region of the bight was clearly influenced by nutrient additions from the Thukela River, as it had the highest ecosystem productivity, lower transfer efficiencies and degree of cycling. Most nodes in the networks were limited by phosphorus, followed by nitrogen. The middle region adjacent to the Thukela River showed a lower proportion of P limitation especially in summer. Interestingly, there was a clear distinction in sensitivities to nutrient limitations between lower and higher trophic level organisms. This is a reflection of their discrepant nutrient turnover times that are either higher, or lower, than that of the systems, and which might provide a balance to the system through this antagonistic influence. Furthermore, by tracking the stoichiometry through entire food webs it appeared how important the role of lower trophic level organisms was to regulate stoichiometry to more suitable ratios for higher trophic level requirements. Although we gained good insight into the behaviour of the three subsystems in the KZN Bight and the role of terrestrial influence on their functioning, a merged approach of incorporating data on metabolic constraints derived from experiments could further improve the representativeness of multitrophic stoichiometric ecosystem networks

Elemental content and stoichiometry of taxa analyzed during this study and in other freshwater and marine global locations.

<p>Values represent mean ± SD, unless otherwise stated.</p

Nutrient Dynamics of Estuarine Invertebrates Are Shaped by Feeding Guild Rather than Seasonal River Flow

<div><p>This study aimed to determine the variability of carbon and nitrogen elemental content, stoichiometry and diet proportions of invertebrates in two sub-tropical estuaries in South Africa experiencing seasonal changes in rainfall and river inflow. The elemental ratios and stable isotopes of abiotic sources, zooplankton and macrozoobenthos taxa were analyzed over a dry/wet seasonal cycle. Nutrient content (C, N) and stoichiometry of suspended particulate matter exhibited significant spatio-temporal variations in both estuaries, which were explained by the variability in river inflow. Sediment particulate matter (%C, %N and C:N) was also influenced by the variability in river flow but to a lesser extent. The nutrient content and ratios of the analyzed invertebrates did not significantly vary among seasons with the exception of the copepod <i>Pseudodiaptomus</i> spp. (C:N) and the tanaid <i>Apseudes digitalis</i> (%N, C:N). These changes did not track the seasonal variations of the suspended or sediment particulate matter. Our results suggest that invertebrates managed to maintain their stoichiometry independent of the seasonality in river flow. A significant variability in nitrogen content among estuarine invertebrates was recorded, with highest % N recorded from predators and lowest %N from detritivores. Due to the otherwise general lack of seasonal differences in elemental content and stoichiometry, feeding guild was a major factor shaping the nutrient dynamics of the estuarine invertebrates. The nutrient richer suspended particulate matter was the preferred food source over sediment particulate matter for most invertebrate consumers in many, but not all seasons. The most distinct preference for suspended POM as a food source was apparent from the temporarily open/closed system after the estuary had breached, highlighting the importance of river flow as a driver of invertebrate nutrient dynamics under extreme events conditions. Moreover, our data showed that estuarine invertebrates concentrated C and N between 10–100 fold from trophic level I (POM) to trophic level II (detritivores/deposit feeders) and thus highlighted their importance not only as links to higher trophic level organisms in the food web, but also as providers of a stoichiometrically homeostatic food source for such consumers. As climate change scenarios for the east coast of South Africa predict increased rainfall as a higher number of rainy days and days with higher rainfall, our results suggest that future changes in rainfall and river inflow will have measurable effects on the nutrient content and stoichiometry of food sources and possibly also in estuarine consumers.</p></div

Mean (± SD) δ¹³C and δ¹⁵N of macrozoobenthos species in the Mlalazi (right) and Mpenjati (left) estuaries.

<p>Color and shape indicate group or taxa (grey rhombus = susPOM, grey square = sedPOM, black square = Spionidae, black rhombus = <i>P</i>. <i>blephariskios</i>, black triangle = <i>A</i>. <i>parva</i>, black circle = <i>A</i>. <i>digitalis</i>, grey rhombus = <i>Glycera</i> spp., grey triangle = Cirratulidae, grey circle = <i>M</i>. <i>litoralis</i>, grey hexagon = <i>D</i>. <i>hepatica</i>, half rhombus = <i>D</i>. <i>arborifera</i>, half circle = <i>C</i>. <i>keiskama</i>, half hexagon = <i>C</i>. <i>capitata</i>).</p

Two-way ANOVA results for %C, %N and C:N ratio of suspended and sediment particulate matter in the study estuaries.

<p>Abbreviations indicate seasons (Se), stations (St).</p><p>Two-way ANOVA results for %C, %N and C:N ratio of suspended and sediment particulate matter in the study estuaries.</p

Diet proportions of zooplankton and macrozoobenthos invertebrates in the Mpenjati Estuary.

<p>Food sources are suspended (susPOM) and sediment (sedPOM) particulate organic matter. <i>Mesopodopsis</i> has additional food sources: <i>Acartia natalensis</i> and <i>Pseudodiaptomus hessei</i>.</p

Map of the study areas (Mlalazi and Mpenjati estuaries) on the east coast of South Africa.

<p>Dots represent sampling stations.</p

Mean (± SD) δ¹³C and δ¹⁵N of zooplankton species in the Mlalazi (right) and Mpenjati (left) estuaries.

<p>Color and shape indicate group or taxa (grey rhombus = susPOM, grey square = sedPOM, black circle = <i>Acartia</i> spp (a) and <i>Acartia natalensis</i> (b), black square = <i>Pseudodiaptomus</i> spp. (a) and <i>Pseudodipatomus hessei</i> (b), black triangle = <i>M</i>. <i>africana</i> (a) and <i>R</i>. <i>terranatalis</i> (b)).</p