Ian Hawes

Relationships between water level fluctuations and vegetation diversity in shallow water of New Zealand lakes

Hydrological and vegetation data from 21 New Zealand lakes were used in the study, to assess how water level fluctuations affected the diversity of a low-growing species-rich littoral plant community (here LMC). Species diversity of the LMC was affected by the water level range, the frequency of variation and the duration of low water level events. Diversity tended to increase with monthly level range, with a possible saturation at an inter-quartile range (the monthly water level fluctuation occurring between 25 and 75% of the time) of approximately 1 m. The species richness was much lower in lakes with inter-annual level variations than in lakes with intra-annual fluctuations. This indicates a relative long establishment time for a species-rich LMC. Up to a period of 1 month, increasing duration of events below median water level appeared to increase species diversity, though events longer than 2 months reduced it. To determine what hydrological conditions determine the upper depth limit of the LMC we made a series of paired sample tests of the measured upper limit of the LMC and the water levels where the mean length of dry period is 10, 30, 60, 120 and 180 days. We found that the observed upper limit best corresponded to the level where the dry period lasted between 10 and 30 days. Hence, the community was not observed where aerial exposure exceeds 30 days. Amongst the species found in the LMC, Elatine gratioloides, Glossostigmasp., Lilaeopsis ruthiana and Limosella lineataare the commonest species under most conditions, and can be considered as the most competitive species in the community. A generalised optimum habitat for a diverse LMC can thus be defined as occurring in a 1 m monthly water level range, with a mean duration of low level events lasting up to 1 month in lakes with low inter-annual water level fluctuation.

Carbon flow in the littoral food web of an oligotrophic lake.

Benthic food web dynamics and carbon flow were examined in the littoral zone of Lake Coleridge, a large deep oligotrophic lake, using radioactive and stable isotope techniques in conjunction with analyses of stomach contents of the fauna. We specifically address two hypotheses: (1) that macrophytes only contribute to the carbon flow to higher trophic levels when they have decayed; and (2) that epiphytic algae is the major source of carbon for macroinvertebrates, and thus fish, with only minor contributions from phytoplankton or terrestrial sources. Epiphytic diatoms were a major component of the stomach contents of the gastropod snail Potamopyrgus antipodarum, and of chironomids. Animal remains were also common in the diet of some chironomids, while amorphous organic matter predominated in the stomachs of oligochaetes. A variety of epiphytic algal taxa was found in trichopteran larvae. Feeding rate of P. antipodarum measured with radioactive tracers increased by 10× on decayed macrophytes (Elodea) compared with live material, while feeding rates on characean algae increased by a factor of 3 when decayed material was presented. However, assimilation rates were less than 20% on decayed material compared with 48–52% on live material. Potential carbon sources were easily distinguished based on their δ13C values, although isotopic ratios showed significant variation among sites. Epiphytic algae showed less variation among sites than macrophytes and were depleted by 4–5‰ compared with macrophytes. Detrital material, organic matter in the sediments and plankton were significantly depleted in δ13C relative to macrophytes and slightly depleted relative to epiphytic algae. Most macroinvertebrate taxa showed a similar pattern among sites to macrophytes and epiphytic algae. P. antipodarum and chironomids were slightly enriched compared with epiphytic algae. Ratios for the common bully (Gobiomorphus cotidianus) were generally consistent with a diet dominated by chironomids, while there was some evidence for terrestrial inputs for koaro (Galaxias brevipinnis) and juvenile brown trout. Epiphytic algae appear to underpin much of the production in the littoral zone of this oligotrophic lake, with trichopteran and chironomid larvae mediating carbon flows from algae to fish. Macrophytes do not make a major contribution directly to carbon flow to higher trophic levels even when decayed. The lack of a direct link between macrophytes and higher trophic levels is due to the faunal composition, including a lack of large herbivores.

ABSORPTION AND UTILIZATION OF IRRADIANCE BY CYANOBACTERIAL MATS IN TWO ICE-COVERED ANTARCTIC LAKES WITH CONTRASTING LIGHT CLIMATES

We investigated the under‐ice light climate and the efficiency with which light was absorbed and utilized by benthic algal mats in Lakes Hoare and Vanda, two perennially ice‐covered lakes in the McMurdo Dry Valleys area of Southern Victoria Land, Antarctica. The ice cover and water column of Lake Vanda were much more transparent than those of Lake Hoare (18% vs. 2% transmission though ice and attenuation coefficients for downwelling irradiance of 0.05 vs. 0.12 m−1, respectively). In both lakes the under‐ice spectra were dominated by blue‐green wavelengths. The benthic flora under perennial ice covers of both lakes comprised thick mucilaginous mats, dominated by cyanobacteria. The mats were well suited to absorb the dominant blue‐green wavelengths of the under‐ice light, with phycoerythrin being present at high concentrations. The pigment systems of the benthic mats absorbed 30%–50% of the light that reached them, varying with depth and lake. There was a tendency for the percentage of absorption to increase as ambient irradiance decreased. The efficiency of utilization of absorbed irradiance was examined by constructing absorbed irradiance/oxygen evolution curves to estimate community quantum yield. Mats from 13 m in Lake Hoare showed the highest quantum yields, approaching 1 mol of carbon fixed for every 8 mol quanta absorbed under light‐limiting conditions. Lake Vanda mats had lower quantum yields, but these increased with depth. Calculated in situ irradiance occasionally exceeded the measured saturating irradiance for oxygen evolution in both lakes, thus efficiency in situ was below the maximum at times. As in other environments, optimization strategies allowed efficient capture and utilization of the lower and middle ranges of experienced irradiance but led to a compromised capacity to use the highest irradiances encountered at each depth.

Effects of changing water clarity on characean biomass and species composition in a large oligotrophic lake

Abstract The biomass and species composition of characean meadows in oligotrophic Lake Coleridge, New Zealand, were monitored over a 2 year period during which water clarity underwent significant fluctuation. Water clarity, measured as vertical attenuation coefficient ( K d ), varied between 0.1 and 0.4 m −1 and there was a period of prolonged low clarity in the middle of the study. Characean meadows extended to a maximum depth of over 30 m at the start of the study in April 1993 but were reduced to less than 20 m by July 1995. At depths of 5, 10 and 15 m, where characean meadows persisted through to July 1995, there was a change in species composition, with an upward shift in the proportions of dominant taxa. This change was manifested as an increase in the percentage contribution to biomass of Chara globularis and Chara corallina at the expense of Chara fibrosa . Over the entire study period characean biomass remained maximal at depths of 5 and 10 m at approximately 180 g (dry weight) m −2 . At a depth of 5 m, biomass was not affected by changing water clarity and appeared to be limited by exposure to wave action and variable water level. At depths greater than 10 m, biomass declined significantly over the study period of 2 years. Reductions could be related to the underwater light field which plants experienced over the period between each sampling. Total characean biomass declined below maximal when irradiance fell below 1.0 mol m −2 day −1 . Despite an increase in water clarity in early 1995, recovery of biomass was not observed within the time scale of this study.

N2-Fixation in Cyanobacterial Mats from Ponds on the McMurdo Ice Shelf, Antarctica

We have investigated the ecological importance of N2-fixation in cyanobacterial mats, dominated by oscillatorean species, in ponds of the Bratina Island area of the McMurdo Ice Shelf, Antarctica (78°S, 166°E). Nitrogenase activity, estimated as acetylene reducing activity (ARA), was found in all the mats investigated (n = 16). The average ARA was 75.9 mmol ethylene m-2 h-1, ranging from 6 to 201 mmol ethylene m-2 h-1. Nitrogenase activity was positively correlated with dissolved reactive phosphorus concentration in pondwater and the C/N ratio of the mat, and was negatively correlated with pondwater NH4+-N concentrations and natural abundance of 15N in the mats. ARA was restricted to the upper, oxic layer of the mats. Experiments conducted to ascribe ARA to different groups of prokaryotes suggested that ARA was mainly conducted by heterocystous cyanobacteria, since no activity was found in the dark and the activity was inhibited by the photosystem II inhibitor DCMU (3-[3,4-dichlorophenyl]-1,1-dimethyl urea). In spite of 24 h of daylight, nitrogenase activity showed a diel cycle with maximum activity at midday (10-18 h) and minimal activity at early morning (6-10 h) when pond temperatures were at their minima. Light dependency of nitrogenase activity for three cyanobacterial communities showed that the irradiance required for saturating ARA was low, in every case lower than 100 mmol photon m-2s-1. Irradiance rarely fell below 100 mmol photon m-2s-1 during Antarctic summer days and ARA was likely to be light saturated for much of the time. We estimate that N2 fixation represented on average a N input into the ponds of over 1 g m-2y-1. This value appears to be the highest N input to this Antarctic ecosystem.

Environmental conditions during freezing, and response of microbial mats in ponds of the McMurdo Ice Shelf, Antarctica

Environmental conditions, both external to and within three shallow ponds of the McMurdo Ice Shelf, were measured over an annual cycle between January 1997 and January 1998. We combined this with a study of the response of the benthic microbial mat communities to the transition from summer conditions to winter freezing. Over the study period air temperature was above 0°C for a few days during summer. At this time pond temperatures were higher than air temperatures, with evidence of thermo-haline stratification. The shallow areas of ponds froze between late February and early March, with bottom waters in the deepest pond remaining unfrozen until early June. Minimum winter air temperatures were below −40°C. There was little evidence of freezing point depression due to freeze-concentration of solutes, except at the very bottom of ponds. In the most conductive pond investigated, the temperature of basal freezing was −4°C and conductivity did not exceed approximately 60 mS cm −1 immediately prior to freezing. Microbial mats remained photosynthetically active up to conductivities between 40 and 80 mS cm −1 , and were able to acclimate to lowered irradiance associated with ice formation. Although photosynthesis and respiration were reduced by 11% and 40% respectively at temperatures of −2°C compared to 1°C, there was no difference in light harvesting efficiency. Results from this study suggest that light limitation of photosynthesis, or freezing, determine the growth season for the microbial communities, depending on depth.

Penetration of solar ultraviolet radiation into New Zealand lakes: influence of dissolved organic carbon and catchment vegetation

Abstract Eleven lakes in the South Island of New Zealand were sampled in summer 1996. Water column profiles of ultraviolet radiation (UVR) at four wavelengths and photosynthetically available radiation (PAR) were obtained, along with analyses of dissolved organic carbon (DOC) concentration, total suspended solids (TSS), and catchment vegetation, including forest and natural grassland. Downward attenuation coefficients (Kd) and lake water transparency (1/Kd) for UVR were examined in relation to these variables. Consistent with other regions of the world, DOC concentration and variables related to DOC were the best predictors of UVR penetration. With our data set, we calculated ratios of water column integrals (RI) of UVR/PAR irradiance, using equations from the literature. At DOC concentrations below 4 g m−3, a progressive increase in RI shows that lakes become increasingly transparent to UVR. We also normalized chromophoric dissolved organic matter (CDOM) absorption of UVR at 380 nm (a380) to DOC concentration and found that the UVR-absorbing capacity per unit DOC increases with increasing percentage of forest in the catchment area. This indicates that not only DOC concentration but also DOC type or composition is important in determining the transparency of lake water to UVR, and that qualitative differences in DOC are dictated by the type and amount of vegetation present in the lakes catchment area.

EFFECT OF CURRENT VELOCITY ON THE DETACHMENT OF THALLI OF ULVA LACTUCA (CHLOROPHYTA) IN A NEW ZEALAND ESTUARY

Experiments were undertaken in a recirculating flume to determine the relationships among water velocity, thallus area, drag, and the probability of thallus breakage or detachment in the foliose green alga Ulva lactuca L. In all specimens tested to breaking point, thalli detached from their bivalve substrates as a result of stipe breakage rather than in midthallus or by holdfast detachment. There was no relationship between thallus size and drag at which detachment occurred. Rather, the probability of detachment was normally distributed about a mean drag of 0. 70 N (95% confidence limits 0.55–0.85 N). Average breaking stress of stipes was 345 kN.m‐2 (95% cl 250–485 kN.m‐2). Similar results were obtained in field experiments where the horizontal force required to detach thalli was measured directly as 0.93 N (95% cl 0.69–1.15 N). Drag coefficients of plants were not constant with water velocity but increased up to 0.4 m.s‐1, declining exponentially at velocities above this. Empirical relationships were established between coefficient of drag and Reynolds number and, hence, among drag, thallus area and water velocity. These relationships permitted estimation of mean water velocity at which plants of a given area would detach.

Evidence for regional climate change in the recent evolution of a high latitude pro-glacial lake

Lake Wilson, a perennially ice-capped, deep (>100 m) lake at 80°S in southern Victoria Land was investigated in January 1993. Water chemistry and physical structure showed three distinct layers; an upper c. 35 m mixed layer of low salinity, moderately turbid water; a less turbid mid layer, 20 m thick of slightly higher salinity and supersaturated with oxygen; and a deep 20 m brackish layer (conductivity c. 4000 μS cm −1 ) with anoxic conditions in the lower 5 m. Extreme supersaturation of N 2 O (up to 400 times air saturation) together with high nitrate concentration (4000 mg m −3 ) was recorded in the deep layer. Phytoplankton biomass and photosynthetic activity was confined to the upper mixed layer and the band of supersaturated dissolved oxygen located at 40–55 m appears to represent a relict layer from when the lake level was lower. The evidence from a comparison of profiles between 1975 and 1993 suggests that Lake Wilson has risen 25 m since 1975, synchronous with a period of lake level rise in the McMurdo Dry Valleys lakes to the north at 77°S. Geochemical diffusion models indicate that Lake Wilson had evaporated to a smaller brine lake about 1000 yrs BP, which also fits the pattern shown by the McMurdo Dry Valleys lakes. Climate changes influencing lake levels have thus covered a wide area of southern Victoria Land.

Effect of localised nutrient enrichment on the shallow epilithic periphyton of oligotrophic Lake Taupo, New Zealand

Abstract A localised increase in nutrient concentrations in the shallow littoral zone of Lake Taupo, New Zealand, resulted from injection of secondary‐treated, nitrified sewage effluent into the groundwater close to the lake. Coincident with this enrichment, there was an increase in periphyton abundance. This increase was restricted to the zone of enrichment and a biomass of up to 1000 mg chlorophyll a m−2 was attained. There was also a change in species composition from a diatom/heterocystous cyano‐bacteria‐dominated assemblage to one dominated by two diatoms, Synedra ulna and Gomphoneis herculeana. Interpretation of the effect of nutrient enrichment on periphyton was complicated by occasional dramatic biomass declines in response to physical variables, notably strong wind events and lake level changes.